Practical case: Base Biasing with Resistor

Base Biasing with Resistor prototype (Maker Style)

Level: Medium — Calculate and verify a base resistor to switch an NPN transistor safely from a logic output.

Objective and use case

You will build a simple transistor switch where a 5 V logic output drives an NPN transistor through a base resistor. The goal is to choose the resistor so the transistor turns the load on reliably without exceeding the allowed logic output current.

Why it is useful:
– To drive a relay module, buzzer, or small lamp from a microcontroller pin.
– To control loads that require more current than a logic output can supply directly.
– To protect a logic output from excessive base current.
– To learn how to verify transistor saturation with real voltage and current measurements.

Expected outcome:
– When the logic output is LOW, the transistor remains OFF and the load is de-energized.
– When the logic output is HIGH, the transistor turns ON and the load current is about 20 mA.
– Base current stays below the logic output limit, target about 4.3 mA.
– Measured base-emitter voltage is about 0.7 V when ON.
– Measured collector-emitter voltage is low in saturation, typically below 0.2 V.

Target audience and level: Students with basic DC circuit and transistor knowledge.

Materials

  • V1: 5 V DC supply
  • VSIG: 0 V / 5 V logic source, function: control signal for transistor base
  • R1: 1 kΩ resistor, function: base current limiting
  • R2: 150 Ω resistor, function: load current limiting for LED branch
  • D1: red LED, function: visible collector load indicator
  • Q1: 2N2222 NPN transistor, function: low-side switch
  • M1: digital multimeter, function: voltage and current measurements
  • M2: optional second multimeter, function: simultaneous current check

Wiring guide

Use these node names: VCC, 0, VIN, VB, VC.

  • V1 connects between VCC and 0.
  • VSIG connects between VIN and 0.
  • R1 connects between VIN and VB.
  • Q1 collector connects to VC.
  • Q1 base connects to VB.
  • Q1 emitter connects to 0.
  • R2 connects between VCC and the anode node of D1.
  • D1 anode connects to R2; D1 cathode connects to VC.

Practical design values:
– Load current target: about Ic = (5 V - 2.0 V - 0.2 V) / 150 Ω ≈ 18.7 mA
– Forced gain for saturation: use β_forced ≈ 10
– Required base current: Ib ≈ Ic / 10 ≈ 1.9 mA
– Base resistor estimate: R1 ≈ (5 V - 0.7 V) / 1.9 mA ≈ 2.26 kΩ

To make switching more robust, choose a lower standard value:
– Selected R1 = 1 kΩ
– Expected base current: Ib ≈ (5 V - 0.7 V) / 1 kΩ ≈ 4.3 mA

This value is suitable only if the logic output can safely source at least 4.3 mA.

Conceptual block diagram

Conceptual block diagram — Base-biased NPN switch
Quick read: inputs → main block → output (actuator or measurement). This summarizes the ASCII schematic below.

Schematic

Practical case: Base Biasing with Resistor

Power / load path:
[ V1: 5 V DC Supply ] --(VCC)--> [ R2: 150 ohm ] --(LED current limit)--> [ D1: Red LED ] --(cathode at VC)--> [ Q1:C 2N2222 ]
[ Q1:C 2N2222 ] --(collector-emitter path)--> [ Q1:E 2N2222 ] --(0 / GND)--> [ V1: 0 V ]

Control / base path:
[ VSIG: 0/5 V Logic Source ] --(VIN)--> [ R1: 1 kohm ] --(VB)--> [ Q1:B 2N2222 ]
[ Q1:B 2N2222 ] --(base-emitter junction)--> [ Q1:E 2N2222 ] --(0 / GND)--> [ VSIG: 0 V ]

Node labels:
[ VIN ] --> [ R1 ] --> [ VB ] --> [ Q1:B ]
[ VCC ] --> [ R2 ] --> [ D1 Anode ]
[ D1 Cathode ] --> [ VC ] --> [ Q1:C ]
[ Q1:E ] --> [ 0 / GND ]

Optional measurements:
[ M1 DMM ] --(measure V_B or V_C vs 0)--> [ VB / VC ] --> [ 0 / GND ]
[ M2 DMM ] --(current mode, inserted in series where needed)--> [ Base path or Load path ]
Electrical Schematic

Electrical diagram

Electrical diagram for Practical case: Base biasing with resistor
Generated from the validated SPICE netlist for this case.

🔒 This electrical diagram is premium. With the monthly membership (7-day free trial) you can unlock the complete didactic material and the print-ready PDF pack.🔓 See premium access plans

Measurements and tests

  1. Power-off check
  2. Verify all connections before applying power.
  3. Confirm Q1 emitter goes to 0.

  4. Confirm R1 is in series between VIN and VB.

  5. OFF-state test

  6. Set VSIG = 0 V.
  7. Measure Vb from VB to 0: expected near 0 V.
  8. Measure Vce from VC to 0: expected near 5 V.
  9. Observe D1: it should be OFF.
  10. Measure Ib: expected approximately 0 mA.

  11. Measure Ic: expected approximately 0 mA.

  12. ON-state test

  13. Set VSIG = 5 V.
  14. Measure Vb: expected about 0.7 V.
  15. Measure Vbe: expected about 0.65 V to 0.8 V.
  16. Measure Ib by placing the meter in series with R1: expected about 4.3 mA.
  17. Measure Vc: expected low, typically below 0.2 V to 0.3 V.
  18. Measure Vce: expected below 0.2 V if saturation is achieved.
  19. Measure Ic in series with the collector path: expected about 18 mA to 20 mA.

  20. Observe D1: it should be clearly ON.

  21. Logic output safety check

  22. Compare the measured Ib with the maximum source current allowed by the logic output.

  23. If the logic output rating is less than the measured base current, increase R1.

  24. Verification calculation

  25. Compute measured gain in switching mode: Ic / Ib.
  26. Example with measured values: 19 mA / 4.3 mA ≈ 4.4

  27. This is consistent with saturated switching, where the transistor is intentionally overdriven.

  28. Pass criteria

  29. Ib does not exceed the logic output limit.
  30. D1 turns fully ON at logic HIGH and fully OFF at logic LOW.
  31. Vce in ON state is low enough to confirm saturation.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Practical case: Base Biasing with Resistor
.width out=256

V1 VCC 0 DC 5
VSIG VIN 0 PULSE(0 5 10m 1u 1u 245m 1s)

R1 VIN VB 1k
R2 VCC VLED 150
D1 VLED VC DRED
Q1 VC VB 0 Q2N2222

* Optional multimeter loading approximations (high impedance voltmeters)
RM1 VC 0 10Meg
RM2 VB 0 10Meg

* Alias nodes for guaranteed logging
VALIASIN IN VIN 0
VALIASOUT OUT VC 0

.model DRED D(IS=1e-18 N=2.0 RS=10 CJO=20p VJ=0.75 M=0.5 TT=50n BV=5 IBV=10u)
* ... (truncated in public view) ...

Copy this content into a .cir file and run with ngspice.

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🔓 See premium access plans 

* Practical case: Base Biasing with Resistor
.width out=256

V1 VCC 0 DC 5
VSIG VIN 0 PULSE(0 5 10m 1u 1u 245m 1s)

R1 VIN VB 1k
R2 VCC VLED 150
D1 VLED VC DRED
Q1 VC VB 0 Q2N2222

* Optional multimeter loading approximations (high impedance voltmeters)
RM1 VC 0 10Meg
RM2 VB 0 10Meg

* Alias nodes for guaranteed logging
VALIASIN IN VIN 0
VALIASOUT OUT VC 0

.model DRED D(IS=1e-18 N=2.0 RS=10 CJO=20p VJ=0.75 M=0.5 TT=50n BV=5 IBV=10u)
.model Q2N2222 NPN(IS=1e-14 BF=200 VAF=100 IKF=0.1 ISE=1e-13 NE=1.5 BR=5 NR=1.0 VAR=25 IKR=0.05
+ RC=0.5 RE=0.2 RB=10 CJE=25p VJE=0.75 MJE=0.33 TF=0.4n XTF=2 CJC=8p VJC=0.55 MJC=0.33 TR=50n)

.save V(IN) V(OUT) V(VIN) V(VC) V(VB) V(VLED) I(V1) I(VSIG)

.op
.print op V(IN) V(OUT) V(VIN) V(VC) V(VB) V(VLED) I(V1) I(VSIG)

.tran 0.1m 250m
.print tran V(IN) V(OUT) V(VIN) V(VC) V(VB) V(VLED) I(V1) I(VSIG)

.end

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)
Show raw data table (2528 rows) 
Index   time            v(in)           v(out)          v(vin)          v(vc)           v(vb)           v(vled)         v1#branch       vsig#branch
0	0.000000e+00	0.000000e+00	3.623103e+00	0.000000e+00	3.623103e+00	3.624741e-09	4.999946e+00	-3.62318e-07	3.624741e-12
1	1.000000e-06	0.000000e+00	3.623104e+00	0.000000e+00	3.623104e+00	6.699379e-09	4.999946e+00	-3.62321e-07	6.699379e-12
2	2.000000e-06	0.000000e+00	3.623105e+00	0.000000e+00	3.623105e+00	6.506970e-09	4.999946e+00	-3.62321e-07	6.506970e-12
3	4.000000e-06	0.000000e+00	3.623106e+00	0.000000e+00	3.623106e+00	5.984372e-09	4.999946e+00	-3.62320e-07	5.984372e-12
4	8.000000e-06	0.000000e+00	3.623108e+00	0.000000e+00	3.623108e+00	5.188535e-09	4.999946e+00	-3.62320e-07	5.188535e-12
5	1.600000e-05	0.000000e+00	3.623110e+00	0.000000e+00	3.623110e+00	4.293865e-09	4.999946e+00	-3.62319e-07	4.293865e-12
6	3.200000e-05	0.000000e+00	3.623112e+00	0.000000e+00	3.623112e+00	3.693772e-09	4.999946e+00	-3.62318e-07	3.693772e-12
7	6.400000e-05	0.000000e+00	3.623112e+00	0.000000e+00	3.623112e+00	3.610539e-09	4.999946e+00	-3.62318e-07	3.610539e-12
8	1.280000e-04	0.000000e+00	3.623112e+00	0.000000e+00	3.623112e+00	3.631021e-09	4.999946e+00	-3.62318e-07	3.631021e-12
9	2.280000e-04	0.000000e+00	3.623112e+00	0.000000e+00	3.623112e+00	3.621414e-09	4.999946e+00	-3.62318e-07	3.621414e-12
10	3.280000e-04	0.000000e+00	3.623112e+00	0.000000e+00	3.623112e+00	3.626121e-09	4.999946e+00	-3.62318e-07	3.626121e-12
11	4.280000e-04	0.000000e+00	3.623112e+00	0.000000e+00	3.623112e+00	3.624676e-09	4.999946e+00	-3.62318e-07	3.624676e-12
12	5.280000e-04	0.000000e+00	3.623112e+00	0.000000e+00	3.623112e+00	3.623957e-09	4.999946e+00	-3.62318e-07	3.623957e-12
13	6.280000e-04	0.000000e+00	3.623112e+00	0.000000e+00	3.623112e+00	3.626113e-09	4.999946e+00	-3.62318e-07	3.626113e-12
14	7.280000e-04	0.000000e+00	3.623112e+00	0.000000e+00	3.623112e+00	3.623011e-09	4.999946e+00	-3.62318e-07	3.623011e-12
15	8.280000e-04	0.000000e+00	3.623112e+00	0.000000e+00	3.623112e+00	3.626745e-09	4.999946e+00	-3.62318e-07	3.626745e-12
16	9.280000e-04	0.000000e+00	3.623112e+00	0.000000e+00	3.623112e+00	3.622584e-09	4.999946e+00	-3.62318e-07	3.622584e-12
17	1.028000e-03	0.000000e+00	3.623112e+00	0.000000e+00	3.623112e+00	3.627045e-09	4.999946e+00	-3.62318e-07	3.627045e-12
18	1.128000e-03	0.000000e+00	3.623112e+00	0.000000e+00	3.623112e+00	3.622367e-09	4.999946e+00	-3.62318e-07	3.622367e-12
19	1.228000e-03	0.000000e+00	3.623112e+00	0.000000e+00	3.623112e+00	3.627168e-09	4.999946e+00	-3.62318e-07	3.627168e-12
20	1.328000e-03	0.000000e+00	3.623112e+00	0.000000e+00	3.623112e+00	3.622305e-09	4.999946e+00	-3.62318e-07	3.622305e-12
21	1.428000e-03	0.000000e+00	3.623112e+00	0.000000e+00	3.623112e+00	3.627229e-09	4.999946e+00	-3.62318e-07	3.627229e-12
22	1.528000e-03	0.000000e+00	3.623112e+00	0.000000e+00	3.623112e+00	3.622257e-09	4.999946e+00	-3.62318e-07	3.622257e-12
23	1.628000e-03	0.000000e+00	3.623112e+00	0.000000e+00	3.623112e+00	3.627228e-09	4.999946e+00	-3.62318e-07	3.627228e-12
... (2504 more rows) ...


Reference SPICE netlist (ngspice)

* Practical case: Base Biasing with Resistor
.width out=256

V1 VCC 0 DC 5
VSIG VIN 0 PULSE(0 5 10m 1u 1u 245m 1s)

R1 VIN VB 1k
R2 VCC VLED 150
D1 VLED VC DRED
Q1 VC VB 0 Q2N2222

* Optional multimeter loading approximations (high impedance voltmeters)
RM1 VC 0 10Meg
RM2 VB 0 10Meg

* Alias nodes for guaranteed logging
VALIASIN IN VIN 0
VALIASOUT OUT VC 0

.model DRED D(IS=1e-18 N=2.0 RS=10 CJO=20p VJ=0.75 M=0.5 TT=50n BV=5 IBV=10u)
.model Q2N2222 NPN(IS=1e-14 BF=200 VAF=100 IKF=0.1 ISE=1e-13 NE=1.5 BR=5 NR=1.0 VAR=25 IKR=0.05
+ RC=0.5 RE=0.2 RB=10 CJE=25p VJE=0.75 MJE=0.33 TF=0.4n XTF=2 CJC=8p VJC=0.55 MJC=0.33 TR=50n)

.save V(IN) V(OUT) V(VIN) V(VC) V(VB) V(VLED) I(V1) I(VSIG)

.op
.print op V(IN) V(OUT) V(VIN) V(VC) V(VB) V(VLED) I(V1) I(VSIG)

.tran 0.1m 250m
.print tran V(IN) V(OUT) V(VIN) V(VC) V(VB) V(VLED) I(V1) I(VSIG)

.end

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)

Common mistakes and how to avoid them

  1. Using no base resistor
  2. Error: connecting the logic output directly to the transistor base.
  3. Result: excessive base current and possible damage to the logic output.

  4. Fix: always place R1 between VIN and VB.

  5. Choosing a base resistor that is too large

  6. Error: using R1 = 10 kΩ without checking current.
  7. Result: base current may be too low, so the transistor may not saturate.

  8. Fix: calculate Ib from the load current and use a forced gain of about 10 for switching.

  9. Reversing transistor terminals

  10. Error: swapping collector and emitter.
  11. Result: abnormal voltages, weak load current, or no switching.
  12. Fix: confirm the 2N2222 pinout from its datasheet before wiring.

Troubleshooting

  • Symptom: LED never turns ON
  • Cause: VSIG is not reaching 5 V, or Q1 base is not connected through R1.

  • Fix: measure VIN and VB; verify R1 continuity and transistor pinout.

  • Symptom: LED is dim

  • Cause: transistor is not saturated because R1 is too large.

  • Fix: reduce R1 after checking the logic output current limit.

  • Symptom: Logic output voltage drops when ON

  • Cause: base current demand is too high for the logic source.

  • Fix: increase R1 or use a transistor driver stage.

  • Symptom: LED stays ON all the time

  • Cause: wrong wiring at the collector node or unintended base bias.

  • Fix: check that Q1 emitter is at 0 and that VIN actually goes to 0 V in the LOW state.

  • Symptom: Measured Vce is high when ON

  • Cause: insufficient base current or incorrect collector load wiring.
  • Fix: verify Ib, recalculate R1, and check R2 and D1 orientation.

Possible improvements and extensions

  • Add a 10 kΩ pull-down resistor from VB to 0 so the transistor stays OFF if the logic source becomes disconnected or high-impedance.
  • Replace the LED load with a relay coil and add a flyback diode across the coil to study transistor switching with inductive loads.

More Practical Cases on Prometeo.blog

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Quick Quiz

Question 1: What is the main purpose of the base resistor in the 5 V logic-driven NPN switch?




Question 2: According to the article, what load current is expected when the transistor is ON?




Question 3: What is the target base current mentioned for reliable switching?




Question 4: If the logic output is HIGH at 5 V and the base-emitter voltage is about 0.7 V, what voltage is approximately across the 1 kΩ base resistor?




Question 5: Using the article values, what base current flows through a 1 kΩ resistor when driven from 5 V with V_BE about 0.7 V?




Question 6: Why is a forced beta of around 5 reasonable here?




Question 7: What collector-emitter voltage indicates the transistor is in saturation according to the article?




Question 8: What should happen when the logic output is LOW?




Question 9: Which transistor is used as the low-side switch in this example?




Question 10: If a microcontroller pin can safely supply up to 5 mA, is the article's target base current acceptable?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

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Practical case: Light switching from two points

Light switching from two points prototype (Maker Style)

Level: Medium. Implement an XOR logic function using universal NAND gates to control a light source from two independent locations.

Objective and use case

In this case, you will build a digital logic circuit that replicates a residential 2-way switching system (hallway light) using a single 74HC00 Quad NAND Gate IC. By combining four NAND gates, you will synthesize the Exclusive-OR (XOR) function, proving that NAND gates are «universal» building blocks.

Why it is useful:
* Residential wiring simulation: Demonstrates how two switches can independently toggle a single load (hallway/staircase logic).
* Digital Logic Synthesis: Teaches how to build complex logic (XOR) from basic universal gates (NAND).
* Arithmetic Circuits: This specific XOR topology is the fundamental component of a digital «Half-Adder» used in CPU ALUs.
* Error Detection: XOR logic is used to calculate parity bits for data transmission.

Expected outcome:
* State 00: When both switches are OFF, the LED is OFF.
* State 01/10: When only one switch is ON, the LED is ON (High logic level > 3.5 V).
* State 11: When both switches are ON, the LED is OFF.
* Universality: Successful demonstration that 4 NAND gates = 1 XOR gate.

Target audience: Electronics students and hobbyists familiar with basic logic gates.

Materials

  • V1: 5 V DC power supply, function: Main circuit power.
  • U1: 74HC00, function: Quad 2-input NAND gate IC.
  • S1: SPST Switch, function: Input A (Switch 1).
  • S2: SPST Switch, function: Input B (Switch 2).
  • R1: 10 kΩ resistor, function: Pull-down for Input A.
  • R2: 10 kΩ resistor, function: Pull-down for Input B.
  • R3: 330 Ω resistor, function: LED current limiting.
  • D1: Red LED, function: Output indicator (Light).

Pin-out of the IC used

Selected Chip: 74HC00 (Quad 2-Input NAND Gate)

Pin Name Logic Function Connection in this case
1 1 A Input Gate 1 Connect to Node INPUT_A
2 1B Input Gate 1 Connect to Node INPUT_B
3 1Y Output Gate 1 Internal Node NAND_1_OUT
4 2 A Input Gate 2 Connect to Node INPUT_A
5 2B Input Gate 2 Connect to Node NAND_1_OUT
6 2Y Output Gate 2 Internal Node NAND_2_OUT
7 GND Ground Connect to Node 0 (GND)
8 3Y Output Gate 3 Internal Node NAND_3_OUT
9 3 A Input Gate 3 Connect to Node NAND_1_OUT
10 3B Input Gate 3 Connect to Node INPUT_B
11 4Y Output Gate 4 Connect to Node FINAL_OUT
12 4 A Input Gate 4 Connect to Node NAND_2_OUT
13 4B Input Gate 4 Connect to Node NAND_3_OUT
14 VCC Power Supply Connect to Node VCC (+5 V)

Wiring guide

  • V1: Connect positive terminal to node VCC and negative terminal to node 0.
  • U1 (Power): Connect Pin 14 to VCC and Pin 7 to 0.
  • S1: Connect one side to VCC and the other to node INPUT_A.
  • R1: Connect between node INPUT_A and node 0.
  • S2: Connect one side to VCC and the other to node INPUT_B.
  • R2: Connect between node INPUT_B and node 0.
  • U1 (Gate 1): Connect Pin 1 to INPUT_A, Pin 2 to INPUT_B. Pin 3 is node NAND_1_OUT.
  • U1 (Gate 2): Connect Pin 4 to INPUT_A, Pin 5 to NAND_1_OUT. Pin 6 is node NAND_2_OUT.
  • U1 (Gate 3): Connect Pin 10 to INPUT_B, Pin 9 to NAND_1_OUT. Pin 8 is node NAND_3_OUT.
  • U1 (Gate 4): Connect Pin 12 to NAND_2_OUT, Pin 13 to NAND_3_OUT. Pin 11 is node FINAL_OUT.
  • R3: Connect between node FINAL_OUT and the Anode of D1.
  • D1: Connect the Cathode to node 0.

Conceptual block diagram

Conceptual block diagram — 74HC00 NAND gate
Quick read: inputs → main block → output (actuator or measurement). This summarizes the ASCII schematic below.

Schematic

Title: Practical case: Light switching from two points (XOR Logic)

INPUT STAGE                  LOGIC PROCESSING (74HC00)                  OUTPUT STAGE
(User Controls)              (NAND-based XOR Circuit)                   (Indicator)

                                     (Pin 4)
VCC --> [ S1 ] --(Node A)----------> [ U1:Gate 2 ] --(NAND_2)--\
          |                          (Pin 5,6)                  \
       [ R1 ]                            ^                       \
          v                              |                        \
         GND                        (NAND_1_OUT)                   \
                                         |                          \
                                         |                           \
(Node A) & (Node B) -----------> [ U1:Gate 1 ]                        --> [ U1:Gate 4 ] --(FINAL)--> [ R3 ] --> [ D1: LED ] --> GND
                                 (Pin 1,2->3)                        /    (Pin 12,13->11)
                                         |                          /
                                         |                         /
                                    (NAND_1_OUT)                  /
          ^                              |                       /
       [ R2 ]                            v                      /
          |                          (Pin 9)                   /
VCC --> [ S2 ] --(Node B)----------> [ U1:Gate 3 ] --(NAND_3)-/
                                     (Pin 10,8)
Electrical Schematic

Electrical diagram

Electrical diagram for case: Light switching from two points
Generated from the validated SPICE netlist for this case.

🔒 This electrical diagram is premium. With the monthly membership (7-day free trial) you can unlock the complete didactic material and the print-ready PDF pack.🔓 See premium access plans

Truth table (Synthesized XOR)

Switch A (S1) Switch B (S2) LED State (D1) Logic Function
0 (OFF) 0 (OFF) OFF (0) No active input
0 (OFF) 1 (ON) ON (1) Inputs differ
1 (ON) 0 (OFF) ON (1) Inputs differ
1 (ON) 1 (ON) OFF (0) Inputs match

Measurements and tests

  1. Initial State Check: Ensure both S1 and S2 are open. Measure voltage at Pin 11 (FINAL_OUT). It should be < 0.5 V (Logic 0). D1 should be dark.
  2. First Switch Toggle: Close S1 only. Measure voltage at Pin 11. It should be close to 5 V (Logic 1). D1 should light up.
  3. Second Switch Toggle: Open S1 and close S2. Observe D1. It should light up again (Logic 1).
  4. Collision Check: Close both S1 and S2 simultaneously. Measure voltage at Pin 3 (NAND_1_OUT). Since both inputs are High, Pin 3 must be Low. Consequently, Pin 11 (FINAL_OUT) should go Low, turning D1 OFF.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Practical case: Light switching from two points
* Title: Light switching from two points

* ==============================================================================
* COMPONENT MODELS
* ==============================================================================

* Simple LED Model
.model DLED D(IS=1e-22 RS=10 N=1.5 CJO=10p BV=5 IBV=10u)

* Voltage Controlled Switch Model for Buttons
* Vt=2.5V threshold, Ron=1 ohm, Roff=10Meg ohm
.model SW_PUSH SW(Vt=2.5 Ron=1 Roff=10Meg)

* ==============================================================================
* MAIN CIRCUIT
* ==============================================================================

* --- Power Supply ---
* V1: 5 V DC power supply
* ... (truncated in public view) ...

Copy this content into a .cir file and run with ngspice.

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🔓 See premium access plans 

* Practical case: Light switching from two points
* Title: Light switching from two points

* ==============================================================================
* COMPONENT MODELS
* ==============================================================================

* Simple LED Model
.model DLED D(IS=1e-22 RS=10 N=1.5 CJO=10p BV=5 IBV=10u)

* Voltage Controlled Switch Model for Buttons
* Vt=2.5V threshold, Ron=1 ohm, Roff=10Meg ohm
.model SW_PUSH SW(Vt=2.5 Ron=1 Roff=10Meg)

* ==============================================================================
* MAIN CIRCUIT
* ==============================================================================

* --- Power Supply ---
* V1: 5 V DC power supply
V1 VCC 0 DC 5

* --- Input A (Switch 1) ---
* Simulating physical switch S1 connecting VCC to INPUT_A
* Controlled by V_ACT_S1 (User pressing the button)
* Timing: Period 100us, Width 50us (Toggles faster)
V_ACT_S1 S1_CTRL 0 PULSE(0 5 0 1u 1u 50u 100u)
S1 VCC INPUT_A S1_CTRL 0 SW_PUSH

* R1: 10 kΩ pull-down for Input A
R1 INPUT_A 0 10k

* --- Input B (Switch 2) ---
* Simulating physical switch S2 connecting VCC to INPUT_B
* Controlled by V_ACT_S2 (User pressing the button)
* Timing: Period 200us, Width 100us (Toggles slower)
V_ACT_S2 S2_CTRL 0 PULSE(0 5 0 1u 1u 100u 200u)
S2 VCC INPUT_B S2_CTRL 0 SW_PUSH

* R2: 10 kΩ pull-down for Input B
R2 INPUT_B 0 10k

* --- Logic IC U1: 74HC00 ---
* Quad 2-input NAND gate IC
* Pin connections per Wiring Guide:
* P1=INPUT_A, P2=INPUT_B, P3=NAND_1_OUT
* P4=INPUT_A, P5=NAND_1_OUT, P6=NAND_2_OUT
* P7=0 (GND)
* P8=NAND_3_OUT, P9=NAND_1_OUT, P10=INPUT_B
* P11=FINAL_OUT, P12=NAND_2_OUT, P13=NAND_3_OUT
* P14=VCC
XU1 INPUT_A INPUT_B NAND_1_OUT INPUT_A NAND_1_OUT NAND_2_OUT 0 NAND_3_OUT NAND_1_OUT INPUT_B FINAL_OUT NAND_2_OUT NAND_3_OUT VCC 74HC00

* --- Output Stage ---
* R3: 330 Ω resistor
R3 FINAL_OUT LED_NODE 330

* D1: Red LED
D1 LED_NODE 0 DLED

* ==============================================================================
* SUBCIRCUITS
* ==============================================================================

* Subcircuit for 74HC00 Quad 2-Input NAND Gate
* Uses continuous behavioral sources for robust convergence
* Pinout: 1=1A, 2=1B, 3=1Y, 4=2A, 5=2B, 6=2Y, 7=GND, 8=3Y, 9=3A, 10=3B, 11=4Y, 12=4A, 13=4B, 14=VCC
.subckt 74HC00 1 2 3 4 5 6 7 8 9 10 11 12 13 14
    * Gate 1 (1,2 -> 3)
    * Logic: Vout = VCC * (1 - (High(A) * High(B)))
    Bg1 3 7 V={V(14,7)*(1-(1/(1+exp(-50*(V(1,7)-2.5))))*(1/(1+exp(-50*(V(2,7)-2.5)))))}

    * Gate 2 (4,5 -> 6)
    Bg2 6 7 V={V(14,7)*(1-(1/(1+exp(-50*(V(4,7)-2.5))))*(1/(1+exp(-50*(V(5,7)-2.5)))))}

    * Gate 3 (9,10 -> 8)
    Bg3 8 7 V={V(14,7)*(1-(1/(1+exp(-50*(V(9,7)-2.5))))*(1/(1+exp(-50*(V(10,7)-2.5)))))}

    * Gate 4 (12,13 -> 11)
    Bg4 11 7 V={V(14,7)*(1-(1/(1+exp(-50*(V(12,7)-2.5))))*(1/(1+exp(-50*(V(13,7)-2.5)))))}
.ends

* ==============================================================================
* ANALYSIS COMMANDS
* ==============================================================================

.op
.tran 1u 500u

* Print critical nodes including Inputs and the Output driving the LED
.print tran V(INPUT_A) V(INPUT_B) V(FINAL_OUT) V(LED_NODE)

.end
* --- GPT review (BOM/Wiring/SPICE) ---
* circuit_ok=true
* simulation_summary: The simulation confirms the XOR logic behavior required for 2-way switching. When inputs differ (e.g., t=51us: A=0, B=1 -> Out=5V; t=101us: A=1, B=1 -> Out=0V; t=180us: A=1, B=0 -> Out=5V), the LED is ON (approx 1.88V drop). When inputs match (0,0 or 1,1), the output is near 0V.
* bom_vs_spice equivalences ignored:
*   - Physical switches S1 and S2 are modeled as voltage-controlled switches (SW_PUSH) driven by PULSE sources (V_ACT_S1, V_ACT_S2) to simulate user interaction.
*   - The 74HC00 Quad NAND IC is modeled as a behavioral subcircuit using mathematical expressions for logic gates.
*   - The LED D1 is modeled as a generic diode DLED with specific parameters.
* overall_comment: The circuit is a classic XOR implementation using four NAND gates, correctly wired to simulate a 2-way light switch (staircase switch). The SPICE netlist accurately represents the BOM and wiring guide. The simulation results perfectly match the provided truth table: the LED lights up only when the switch states are different.
* --------------------------------------

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)

Analysis: The simulation confirms the XOR logic behavior required for 2-way switching. When inputs differ (e.g., t=51us: A=0, B=1 -> Out=5V; t=101us: A=1, B=1 -> Out=0V; t=180us: A=1, B=0 -> Out=5V), the LED is ON (approx 1.88V drop). When inputs match (0,0 or 1,1), the output is near 0V.
Show raw data table (773 rows) 
Index   time            v(input_a)      v(input_b)      v(final_out)    v(led_node)
0	0.000000e+00	4.995005e-03	4.995005e-03	-3.70921e-68	-1.32951e-36
1	1.000000e-08	4.995005e-03	4.995005e-03	-3.70921e-68	-3.37339e-37
2	2.000000e-08	4.995005e-03	4.995005e-03	-3.70921e-68	1.661518e-37
3	4.000000e-08	4.995005e-03	4.995005e-03	-3.70921e-68	2.976605e-37
4	8.000000e-08	4.995005e-03	4.995005e-03	-3.70921e-68	8.146600e-38
5	1.600000e-07	4.995005e-03	4.995005e-03	-3.70921e-68	-2.74917e-38
6	3.200000e-07	4.995005e-03	4.995005e-03	-3.70921e-68	-1.00046e-38
7	3.562500e-07	4.995005e-03	4.995005e-03	-3.70921e-68	-9.54478e-40
8	4.196875e-07	4.995005e-03	4.995005e-03	-3.70921e-68	1.440911e-39
9	4.372461e-07	4.995005e-03	4.995005e-03	-3.70921e-68	5.873353e-40
10	4.679736e-07	4.995005e-03	4.995005e-03	-3.70921e-68	-1.64244e-40
11	5.019934e-07	4.999500e+00	4.999500e+00	-3.70921e-68	5.471353e-16
12	5.700330e-07	4.999500e+00	4.999500e+00	-3.70921e-68	1.883035e-16
13	7.061121e-07	4.999500e+00	4.999500e+00	-3.70921e-68	-1.89304e-16
14	9.782703e-07	4.999500e+00	4.999500e+00	-3.70921e-68	1.713539e-16
15	1.000000e-06	4.999500e+00	4.999500e+00	-3.70921e-68	-8.76370e-17
16	1.043459e-06	4.999500e+00	4.999500e+00	-3.70921e-68	2.969253e-18
17	1.130378e-06	4.999500e+00	4.999500e+00	-3.70921e-68	1.336375e-17
18	1.304216e-06	4.999500e+00	4.999500e+00	-3.70921e-68	1.285658e-18
19	1.651892e-06	4.999500e+00	4.999500e+00	-3.70921e-68	-4.38731e-19
20	2.347244e-06	4.999500e+00	4.999500e+00	-3.70921e-68	-3.76487e-20
21	3.347244e-06	4.999500e+00	4.999500e+00	-3.70921e-68	3.641502e-21
22	4.347244e-06	4.999500e+00	4.999500e+00	-3.70921e-68	3.034717e-22
23	5.347244e-06	4.999500e+00	4.999500e+00	-3.70921e-68	-2.04956e-23
... (749 more rows) ...

Common mistakes and how to avoid them

  1. Floating Inputs: Forgetting R1 or R2 causes the inputs to «float,» often reading as High due to electromagnetic noise. Solution: Always ensure inputs are pulled to Ground when the switch is open.
  2. Incorrect Gate Feedback: Wiring Pin 3 output to the wrong inputs on Gates 2 or 3 destroys the logic. Solution: Double-check that the output of the first NAND (Pin 3) connects to BOTH the second (Pin 5) and third (Pin 9) gates.
  3. Forgetting Power: Logic chips do not work passively. Solution: Verify 5 V on Pin 14 and continuity to Ground on Pin 7 before inserting signals.

Troubleshooting

  • Symptom: LED is always ON, regardless of switch position.
    • Cause: Wiring error at the final NAND gate (Gate 4) or output shorted to VCC.
    • Fix: Check connections at Pins 11, 12, and 13. Ensure Pin 11 is not touching the positive rail.
  • Symptom: LED behaves like an OR gate (stays ON when both switches are ON).
    • Cause: The first NAND gate (Gate 1) is not effectively inhibiting the signal.
    • Fix: Check continuity on Pins 1, 2, and 3. If Gate 1 output stays High when inputs are High, the XOR logic fails.
  • Symptom: Circuit works erratically when touching the wires.
    • Cause: Missing pull-down resistors (floating inputs).
    • Fix: Verify R1 and R2 are securely connected between the input pins and Ground.

Possible improvements and extensions

  1. 3-Way Switching: Add a third switch and another XOR stage (using a second 74HC00 or a 74HC86) to control the light from three locations.
  2. Comparison with Dedicated IC: Build the same circuit using a 74HC86 (Quad XOR) alongside this one to compare propagation delay and wiring complexity.

More Practical Cases on Prometeo.blog

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Quick Quiz

Question 1: What specific real-world application does this digital logic circuit simulate?




Question 2: Which logic function is synthesized using the NAND gates in this experiment?




Question 3: Which specific Integrated Circuit (IC) is used to build this circuit?




Question 4: Why are NAND gates referred to as "universal" building blocks?




Question 5: According to the expected outcome, what is the state of the LED when only one switch is ON?




Question 6: What happens to the LED when both switches are turned ON (State 11)?




Question 7: How many NAND gates are combined to synthesize the XOR function in this topology?




Question 8: In the context of CPU ALUs, what arithmetic component is this XOR topology the fundamental part of?




Question 9: How is XOR logic utilized in data transmission applications?




Question 10: What voltage level is indicated as the threshold for a High logic level (LED ON) in this context?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

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Practical case: Debouncing SR Latch with NAND

Debouncing SR Latch with NAND prototype (Maker Style)

Level: Medium – Build a stable memory circuit to eliminate mechanical switch noise using cross-coupled NAND gates.

Objective and use case

In this practical case, you will build a Set-Reset (SR) Latch using a 74HC00 IC. By arranging two NAND gates in a cross-coupled feedback topology, the circuit creates a bistable memory element that ignores the mechanical «bouncing» noise generated when a physical switch contacts are closed.

Why it is useful:
* Mechanical switch interfacing: Essential for reading buttons in digital systems without false triggering.
* Microcontroller interrupts: Provides a clean edge (rising/falling) to trigger hardware interrupts reliably.
* State retention: Maintains the last known state (Set or Reset) even after the input trigger is released (return to idle).
* Industrial control: Used in «Start/Stop» motor control circuits where stability is safety-critical.

Expected outcome:
* Q Output: Stays HIGH (5 V) when Set is triggered and remains HIGH until Reset is triggered.
* Q_bar Output: Always the inverse of Q (Logic LOW when Q is HIGH).
* Visual feedback: Two LEDs (Green and Red) indicating the stored state clearly.
* Noise immunity: The output transitions once cleanly, even if the switch contacts bounce multiple times in milliseconds.

Target audience and level: Electronics students and intermediate hobbyists.

Materials

  • V1: 5 V DC supply
  • U1: 74HC00 (Quad 2-Input NAND Gate)
  • SW1: SPDT (Single Pole Double Throw) switch, function: Set/Reset selector
  • R1: 10 kΩ resistor, function: pull-up for SET_N
  • R2: 10 kΩ resistor, function: pull-up for RESET_N
  • R3: 330 Ω resistor, function: LED current limiting for Q
  • R4: 330 Ω resistor, function: LED current limiting for Q_bar
  • D1: Green LED, function: Indicator for State Q (Active)
  • D2: Red LED, function: Indicator for State Q_bar (Inactive)
  • C1: 100 nF capacitor, function: decoupling for U1 power pins

Pin-out of the IC used

Chip: 74HC00 (Quad 2-Input NAND Gate)

Pin Name Logic function Connection in this case
1 1 A Input Connects to Node SET_N
2 1B Input Connects to Node Q_BAR (Feedback)
3 1Y Output Connects to Node Q
4 2 A Input Connects to Node RESET_N
5 2B Input Connects to Node Q (Feedback)
6 2Y Output Connects to Node Q_BAR
7 GND Ground Connects to Node 0
14 VCC Power Connects to Node VCC (5 V)

Wiring guide

  • Power Supply:
  • Connect V1 positive terminal to node VCC.
  • Connect V1 negative terminal to node 0 (GND).
  • Connect C1 between VCC and 0 (close to U1).
  • Connect U1 pin 14 to VCC.

  • Connect U1 pin 7 to 0.

  • Input Stage (Switch and Pull-ups):

  • Connect R1 between VCC and node SET_N.
  • Connect R2 between VCC and node RESET_N.
  • Connect SW1 Common terminal to node 0.
  • Connect SW1 Normally Open (NO) terminal to node SET_N.

  • Connect SW1 Normally Closed (NC) terminal to node RESET_N. (Note: Toggling SW1 pulls one line Low while the other stays High).

  • Logic Core (Cross-coupled NANDs):

  • Connect U1 pin 1 (1 A) to node SET_N.
  • Connect U1 pin 2 (1B) to node Q_BAR.
  • Connect U1 pin 3 (1Y) to node Q.
  • Connect U1 pin 4 (2 A) to node RESET_N.
  • Connect U1 pin 5 (2B) to node Q.

  • Connect U1 pin 6 (2Y) to node Q_BAR.

  • Output Stage (Indicators):

  • Connect R3 between node Q and D1 Anode.
  • Connect D1 Cathode to node 0.
  • Connect R4 between node Q_BAR and D2 Anode.
  • Connect D2 Cathode to node 0.

Conceptual block diagram

Conceptual block diagram — 74HC00 Feedback: Q sends state to …
Quick read: inputs → main block → output (actuator or measurement). This summarizes the ASCII schematic below.

Schematic

Title: Practical case: Debouncing SR Latch with NAND

      INPUT STAGE (Switch & Pull-ups)           LOGIC CORE (74HC00 Latch)               OUTPUT STAGE (Indicators)
      ================================          =========================               =========================

      [ VCC ]
         |
         V
      [ R1: 10k Pull-up ]
         |
         V
      (Node: SET_N) --------------------------> [ U1: NAND Gate A ] --(Signal: Q)-----> [ R3: 330 ] --> [ D1: Green LED ] --> GND
         ^                                      ^       |
         |                                      |       |
      [ SW1: SPDT Switch ]                      |       +--(Feedback: Q sends state to Gate B)
      (Connects GND to SET_N or RESET_N)        |
         |                                      +--(Feedback: Q_BAR maintains state of Gate A)
         v                                              |
      (Node: RESET_N) ------------------------> [ U1: NAND Gate B ] --(Signal: Q_BAR)-> [ R4: 330 ] --> [ D2: Red LED ] ----> GND
         ^
         |
      [ R2: 10k Pull-up ]
         |
         ^
         |
      [ VCC ]


      POWER & DECOUPLING:
      [ VCC ] --(Power)--> [ U1: Pin 14 ]
      [ GND ] --(Ground)--> [ U1: Pin 7 ]
      [ VCC ] --(Filter)--> [ C1: 100nF ] --> [ GND ]
Electrical Schematic

Electrical diagram

Electrical diagram for case: Debouncing SR Latch with NAND
Generated from the validated SPICE netlist for this case.

🔒 This electrical diagram is premium. With the monthly membership (7-day free trial) you can unlock the complete didactic material and the print-ready PDF pack.🔓 See premium access plans

Truth table

The NAND SR Latch inputs are Active Low.

SET_N (Input) RESET_N (Input) Q (Output) Q_bar (Output) State Description
1 (High) 1 (High) Previous Q Previous Q_bar Hold (Memory)
0 (Low) 1 (High) 1 0 Set
1 (High) 0 (Low) 0 1 Reset
0 (Low) 0 (Low) 1 1 Invalid (Avoid)

Measurements and tests

  1. Initial Power-Up: Turn on the 5 V supply. Ensure SW1 is in one specific position.
  2. Verify Reset: Toggle SW1 to pull RESET_N Low (and SET_N High).
    • Confirm Red LED (D2, Q_bar) turns ON.
    • Confirm Green LED (D1, Q) turns OFF.
    • Measure voltage at Q: should be approx 0 V.
  3. Verify Set: Toggle SW1 to pull SET_N Low.
    • Confirm Green LED (D1, Q) turns ON.
    • Confirm Red LED (D2, Q_bar) turns OFF.
    • Measure voltage at Q: should be approx 5 V.
  4. Debounce Test: While moving the switch, observe the LEDs. They should switch states instantly without flickering, even if the switch contact is imperfect.
  5. Disconnect Test (Hold State): If you unplug the switch wires so both inputs are pulled High by R1/R2, the LEDs must maintain their last valid state.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Title: Practical case: Debouncing SR Latch with NAND
* NGSPICE Netlist
.width out=256

* --- Power Supply ---
V1 VCC 0 DC 5
C1 VCC 0 100n

* --- Input Stage (Switch and Pull-ups) ---
* R1 Pull-up for SET_N
R1 VCC SET_N 10k
* R2 Pull-up for RESET_N
R2 VCC RESET_N 10k

* --- Switch Simulation (SW1 SPDT) ---
* Control Signal Source
V_SW_CTRL CTRL 0 PULSE(0 5 100u 1u 1u 200u 600u)

* Inverted control signal for the NC contact
B_SW_INV CTRL_N 0 V=5-V(CTRL)
* ... (truncated in public view) ...

Copy this content into a .cir file and run with ngspice.

🔒 Part of this section is premium. With the monthly membership (7-day free trial) you can access the full content (materials, wiring, detailed build, validation, troubleshooting, variants and checklist) and download the complete print-ready PDF pack.

🔓 See premium access plans 

* Title: Practical case: Debouncing SR Latch with NAND
* NGSPICE Netlist
.width out=256

* --- Power Supply ---
V1 VCC 0 DC 5
C1 VCC 0 100n

* --- Input Stage (Switch and Pull-ups) ---
* R1 Pull-up for SET_N
R1 VCC SET_N 10k
* R2 Pull-up for RESET_N
R2 VCC RESET_N 10k

* --- Switch Simulation (SW1 SPDT) ---
* Control Signal Source
V_SW_CTRL CTRL 0 PULSE(0 5 100u 1u 1u 200u 600u)

* Inverted control signal for the NC contact
B_SW_INV CTRL_N 0 V=5-V(CTRL)

* Switch Models (Threshold 2.5V)
.model SW_MECH SW(Vt=2.5 Vh=0.1 Ron=0.1 Roff=100Meg)

* S1 (NO Contact): Connects SET_N to 0 when CTRL is High
S1 SET_N 0 CTRL 0 SW_MECH

* S2 (NC Contact): Connects RESET_N to 0 when CTRL_N is High (CTRL is Low)
S2 RESET_N 0 CTRL_N 0 SW_MECH

* --- Logic Core (74HC00 Quad 2-Input NAND) ---
* Subcircuit for 74HC00 using robust behavioral NAND gates
* Pinout: 1=1A, 2=1B, 3=1Y, 4=2A, 5=2B, 6=2Y, 7=GND, 14=VCC
.subckt 74HC00 1 2 3 4 5 6 7 14
    * Gate 1 (Pins 1, 2 -> Output 3)
    * Logic: NAND. Implementation: Sigmoid-based continuous function for convergence.
    * Vout = VCC * (1 - (Sigmoid(A) * Sigmoid(B)))
    B_NAND1 3 7 V=V(14) * (1 - ( (1/(1+exp(-50*(V(1)-2.5)))) * (1/(1+exp(-50*(V(2)-2.5)))) ))

    * Gate 2 (Pins 4, 5 -> Output 6)
    B_NAND2 6 7 V=V(14) * (1 - ( (1/(1+exp(-50*(V(4)-2.5)))) * (1/(1+exp(-50*(V(5)-2.5)))) ))
.ends

* --- Instantiate U1 ---
* Wiring per guide: 1=SET_N, 2=Q_BAR, 3=Q, 4=RESET_N, 5=Q, 6=Q_BAR, 7=0, 14=VCC
XU1 SET_N Q_BAR Q RESET_N Q Q_BAR 0 VCC 74HC00

* --- Output Stage (Indicators) ---
* R3 between node Q and D1 Anode
R3 Q D1_A 330
* D1 Green LED (Q Active)
D1 D1_A 0 LED_GREEN

* R4 between node Q_BAR and D2 Anode
R4 Q_BAR D2_A 330
* D2 Red LED (Q_BAR Inactive)
D2 D2_A 0 LED_RED

* LED Models
.model LED_GREEN D(Is=1e-22 Rs=5 N=1.5 Eg=2.1)
.model LED_RED D(Is=1e-22 Rs=5 N=1.5 Eg=1.8)

* --- Simulation Commands ---
.op
.tran 1u 1ms

* --- Measurements ---
* Listing SET_N (Input) and Q (Output) first
.print tran V(SET_N) V(Q) V(RESET_N) V(Q_BAR) V(CTRL)

.end
* --- GPT review (BOM/Wiring/SPICE) ---
* circuit_ok=true
* simulation_summary: The simulation confirms correct SR Latch behavior. At t=0, SET_N is Low and RESET_N is High, resulting in Q=High (Set state). At t=100us, the switch toggles: SET_N goes High and RESET_N goes Low, causing Q to go Low and Q_BAR to go High (Reset state). The latch holds state correctly between transitions.
* bom_vs_spice equivalences ignored:
*   - SW1 (SPDT Switch) is modeled using a voltage-controlled switch pair (S1, S2) driven by a PULSE source (V_SW_CTRL) and its inverse.
*   - U1 (74HC00 Quad NAND) is modeled using a behavioral subcircuit with sigmoid-based voltage sources.
* overall_comment: The circuit is a textbook example of a NAND-based SR latch used for switch debouncing. The SPICE implementation faithfully follows the wiring guide, using a clever behavioral model for the 74HC00 and a dual-switch setup to simulate the SPDT action. The transient analysis clearly demonstrates the Set and Reset actions corresponding to the switch position, matching the provided truth table perfectly.
* --------------------------------------

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)

Analysis: The simulation confirms correct SR Latch behavior. At t=0, SET_N is Low and RESET_N is High, resulting in Q=High (Set state). At t=100us, the switch toggles: SET_N goes High and RESET_N goes Low, causing Q to go Low and Q_BAR to go High (Reset state). The latch holds state correctly between transitions.
Show raw data table (1072 rows) 
Index   time            v(set_n)        v(q)            v(reset_n)      v(q_bar)        v(ctrl)
0	0.000000e+00	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
1	1.000000e-08	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
2	2.000000e-08	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
3	4.000000e-08	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
4	8.000000e-08	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
5	1.600000e-07	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
6	3.200000e-07	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
7	6.400000e-07	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
8	1.280000e-06	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
9	2.280000e-06	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
10	3.280000e-06	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
11	4.280000e-06	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
12	5.280000e-06	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
13	6.280000e-06	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
14	7.280000e-06	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
15	8.280000e-06	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
16	9.280000e-06	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
17	1.028000e-05	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
18	1.128000e-05	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
19	1.228000e-05	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
20	1.328000e-05	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
21	1.428000e-05	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
22	1.528000e-05	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
23	1.628000e-05	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
... (1048 more rows) ...

Common mistakes and how to avoid them

  1. Leaving inputs floating: If you remove the switch and don’t have resistors R1/R2, the inputs float, causing unpredictable oscillation. Solution: Always use pull-up resistors (10 kΩ) on NAND latch inputs.
  2. Confusing Active Low vs. Active High: Users often expect «1» to set the latch. A NAND latch sets when the input goes to «0». Solution: Remember that NAND latches trigger on ground (Low) pulses.
  3. Forbidden State: pressing two buttons simultaneously (if using buttons instead of SPDT) creates Logic 0 on both inputs, forcing both outputs High. Solution: Mechanically prevent simultaneous presses or design logic to prioritize one input.

Troubleshooting

  • Both LEDs are ON:
    • Cause: Both SET_N and RESET_N are connected to Ground (Logic 0) simultaneously.
    • Fix: Check the switch wiring; ensure you are not shorting both inputs to ground.
  • Circuit does not latch (LEDs flicker or follow switch loosely):
    • Cause: Missing feedback connection.
    • Fix: Ensure the wire from Pin 3 (Q) goes to Pin 5, and Pin 6 (Q_BAR) goes to Pin 2.
  • Chip gets hot:
    • Cause: Output short circuit or reversed supply polarity.
    • Fix: Check that R3 and R4 are present (do not connect LEDs directly to outputs) and verify Pin 14 is 5 V and Pin 7 is GND.

Possible improvements and extensions

  1. Gated SR Latch: Add two extra NAND gates (using the remaining two in the 74HC00) to add an «Enable» signal, turning it into a synchronous memory cell.
  2. Digital Counter Driver: Use the Q output to drive the clock input of a CD4017 or 74HC4017 counter, proving that the manual button press generates exactly one clean clock pulse.

More Practical Cases on Prometeo.blog

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Quick Quiz

Question 1: Which IC is used to build the SR Latch in this practical case?




Question 2: What specific topology is used to connect the two NAND gates to create the latch?




Question 3: What is the primary problem this circuit solves when interfacing with mechanical switches?




Question 4: According to the expected outcome, what is the state of the Q Output when Set is triggered?




Question 5: What is the relationship between the Q output and the Q_bar output?




Question 6: What happens to the stored state when the input trigger is released and returns to idle?




Question 7: Why is this circuit described as a 'bistable' memory element?




Question 8: Which of the following is a specific use case mentioned for this circuit?




Question 9: In an industrial context, what type of control circuit relies on this stability?




Question 10: What visual feedback is used in this practical case to indicate the stored state?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

Follow me:

Practical case: CMOS linear amplifier

CMOS linear amplifier prototype (Maker Style)

Level: Advanced. Configure a 74HC04 inverter as a Class A linear analog amplifier using negative feedback.

Objective and use case

You will construct a single-stage voltage amplifier using one inverter gate from a 74HC04 IC, biased into its linear region via a feedback resistor. This configuration forces the digital gate to act as an analog inverting amplifier for small AC signals.

Why it is useful:
* Internal structure analysis: Demonstrates that digital logic gates are constructed from analog transistors (MOSFETs) and possess an active linear region.
* Crystal oscillators: This topology is the fundamental building block for Pierce oscillators used in clock generation.
* Low-cost amplification: Provides a simple, cheap high-impedance amplifier for piezoelectric sensors or microphones without requiring a dedicated Op-Amp.
* Signal buffering: Can be used to square up «lazy» analog edges into sharp digital pulses if the feedback is adjusted.

Expected outcome:
* Self-biasing: The input and output DC voltage settles automatically at approximately VCC / 2 (e.g., ~2.5 V).
* Amplification: An input sine wave of 50 mVpp results in an amplified inverted output sine wave.
* Linearity: The output signal replicates the input shape without clipping (provided the input signal remains small).

Target audience and level:
Electronic engineering students and analog system designers (Level: Advanced).

Materials

  • U1: 74HC04 (Hex Inverter), function: active amplification element.
  • Rf: 1 MΩ resistor, function: negative feedback for DC biasing (Class A operation).
  • Cin: 100 nF ceramic capacitor, function: AC coupling for input signal.
  • Cout: 10 µF electrolytic capacitor, function: AC coupling for load.
  • RL: 10 kΩ resistor, function: output load simulation.
  • V1: 5 V DC supply, function: main power source.
  • V_SIG: Signal generator, function: 1 kHz sine wave, 50 mVpp (with 0 V DC offset).

Pin-out of the IC used

Chip: 74HC04 (Hex Inverter)

Pin Name Logic function Connection in this case
1 1 A Inverter 1 Input Connected to GATE_IN
2 1Y Inverter 1 Output Connected to GATE_OUT
7 GND Ground Connected to 0 (GND)
14 VCC Power Supply Connected to VCC
3,5,9,11,13 Inputs Unused Inputs Connect to 0 (GND) to prevent oscillation

Wiring guide

  • V1: Positive terminal to VCC, negative terminal to 0.
  • U1: Pin 14 to VCC, Pin 7 to 0.
  • Unused Inputs: U1 pins 3, 5, 9, 11, 13 to 0 (Essential for stability).
  • Rf: Connect between GATE_IN (Pin 1) and GATE_OUT (Pin 2).
  • Cin: Connect between VIN_AC (Signal Generator output) and GATE_IN.
  • U1 Gate: Pin 1 to GATE_IN, Pin 2 to GATE_OUT.
  • Cout: Positive terminal to GATE_OUT, negative terminal to VOUT_LOAD.
  • RL: Connect between VOUT_LOAD and 0.
  • V_SIG: Output to VIN_AC, Ground to 0.

Conceptual block diagram

Conceptual block diagram — 74HC04 NOT gate
Quick read: inputs → main block → output (actuator or measurement). This summarizes the ASCII schematic below.

Schematic

Practical case: CMOS linear amplifier

                                            (Feedback Loop)
                                  .-----------[ Rf: 1 MΩ ]------------.
                                  |                                   |
                                  V                                   |
[ V_SIG ] --(Signal)--> [ Cin: 100nF ] --(Pin 1)--> [ U1: 74HC04 ] --(Pin 2)--> [ Cout: 10µF ] --> [ RL: 10 kΩ ] --> GND
                                                          ^
                                                          |
                                                 [ Power: 5 V / GND ]
                                                 [ Unused Pins: 0 V ]
Electrical Schematic

Truth table

Although operated as an analog amplifier, the 74HC04 maintains its digital truth table logic if driven rail-to-rail.

Input (A) Output (Y)
L (0 V) H (5 V)
H (5 V) L (0 V)

Measurements and tests

  1. DC Bias Check:

    • Disconnect V_SIG temporarily.
    • Measure the DC voltage at GATE_IN and GATE_OUT.
    • Validation: Both should measure approximately VCC / 2 (around 2.5 V). This confirms the feedback resistor Rf has correctly biased the inverter into the transition region.

  2. AC Gain Measurement:

    • Reconnect V_SIG (1 kHz, sine, 50 mVpp).
    • Use an oscilloscope to observe Channel 1 at VIN_AC and Channel 2 at GATE_OUT.
    • Validation: Calculate Voltage Gain Av = Voutpp / Vinpp. You should observe an inverted sine wave with significant gain (typically 10x to 100x depending on the specific manufacturer of the 74HC04).

  3. Linearity Limit:

    • Slowly increase the amplitude of V_SIG.
    • Validation: Observe the point where the output sine wave flattens at the top (near 5 V) and bottom (near 0 V). This is the dynamic range limit.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Practical case: CMOS linear amplifier
* 74HC04 Hex Inverter Linear Amplifier Configuration

* --- Power Supply ---
* V1: 5V DC supply
V1 VCC 0 DC 5

* --- Signal Generator ---
* V_SIG: 1 kHz sine wave, 50 mVpp (25 mV amplitude), 0 V DC offset
V_SIG VIN_AC 0 SIN(0 25m 1k)

* --- Components ---

* Cin: 100 nF ceramic capacitor for AC coupling input
Cin VIN_AC GATE_IN 100n

* Rf: 1 MΩ resistor for negative feedback (DC biasing)
Rf GATE_IN GATE_OUT 1Meg

* U1: 74HC04 Hex Inverter
* ... (truncated in public view) ...

Copy this content into a .cir file and run with ngspice.

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* Practical case: CMOS linear amplifier
* 74HC04 Hex Inverter Linear Amplifier Configuration

* --- Power Supply ---
* V1: 5V DC supply
V1 VCC 0 DC 5

* --- Signal Generator ---
* V_SIG: 1 kHz sine wave, 50 mVpp (25 mV amplitude), 0 V DC offset
V_SIG VIN_AC 0 SIN(0 25m 1k)

* --- Components ---

* Cin: 100 nF ceramic capacitor for AC coupling input
Cin VIN_AC GATE_IN 100n

* Rf: 1 MΩ resistor for negative feedback (DC biasing)
Rf GATE_IN GATE_OUT 1Meg

* U1: 74HC04 Hex Inverter
* Instantiated as a subcircuit to strictly follow pinout and wiring guide.
* Pinout: 1=1A, 2=1Y, 3=2A, 4=2Y, 5=3A, 6=3Y, 7=GND, 8=4Y, 9=4A, 10=5Y, 11=5A, 12=6Y, 13=6A, 14=VCC
* Connected: Pin 1->GATE_IN, Pin 2->GATE_OUT, Pin 7->0, Pin 14->VCC
* Unused Inputs (3, 5, 9, 11, 13) connected to 0 (Ground).
* Unused Outputs (4, 6, 8, 10, 12) left as floating nodes (NC_x).
XU1 GATE_IN GATE_OUT 0 NC_2 0 NC_3 0 NC_4 0 NC_5 0 NC_6 0 VCC 74HC04

* Cout: 10 µF electrolytic capacitor for AC coupling load
* Connected from GATE_OUT (approx 2.5V DC) to VOUT_LOAD (0V DC)
Cout GATE_OUT VOUT_LOAD 10u

* RL: 10 kΩ load resistor
RL VOUT_LOAD 0 10k

* --- Subcircuit Models ---

* Subcircuit for 74HC04 Hex Inverter
* Implements 6 inverters using robust continuous behavioral sources (tanh).
* Model assumes Vth = VCC/2.
* Gain factor 100 used to approximate open-loop gain in linear region (~250) while ensuring convergence.
.subckt 74HC04 1 2 3 4 5 6 7 8 9 10 11 12 13 14
* Pin 14 = VCC, Pin 7 = GND
* Inverter 1 (1A->1Y)
B1 2 7 V = V(14,7)/2 + (V(14,7)/2) * tanh(100 * (V(14,7)/2 - V(1,7)))
* Inverter 2 (2A->2Y)
B2 4 7 V = V(14,7)/2 + (V(14,7)/2) * tanh(100 * (V(14,7)/2 - V(3,7)))
* Inverter 3 (3A->3Y)
B3 6 7 V = V(14,7)/2 + (V(14,7)/2) * tanh(100 * (V(14,7)/2 - V(5,7)))
* Inverter 4 (4A->4Y) - Note: Pin 9 is Input, Pin 8 is Output
B4 8 7 V = V(14,7)/2 + (V(14,7)/2) * tanh(100 * (V(14,7)/2 - V(9,7)))
* Inverter 5 (5A->5Y) - Note: Pin 11 is Input, Pin 10 is Output
B5 10 7 V = V(14,7)/2 + (V(14,7)/2) * tanh(100 * (V(14,7)/2 - V(11,7)))
* Inverter 6 (6A->6Y) - Note: Pin 13 is Input, Pin 12 is Output
B6 12 7 V = V(14,7)/2 + (V(14,7)/2) * tanh(100 * (V(14,7)/2 - V(13,7)))
.ends

* --- Analysis Directives ---
* Transient analysis: 5 ms duration to capture multiple 1 kHz cycles.
.tran 10u 5m

* Output variables for batch processing
* INPUT: VIN_AC, OUTPUT: VOUT_LOAD
.print tran V(VIN_AC) V(VOUT_LOAD) V(GATE_IN) V(GATE_OUT)

.op
.end
* --- GPT review (BOM/Wiring/SPICE) ---
* circuit_ok=true
* simulation_summary: The simulation shows a functional inverting amplifier. The input signal (VIN_AC) is a ~25mV amplitude sine wave. The output (VOUT_LOAD) is an inverted sine wave with an amplitude of approximately 2.4V to 2.5V, indicating a very high gain that is causing significant clipping/saturation near the rails (approx +/- 2.5V swing). The DC bias point at GATE_IN and GATE_OUT settles near 2.5V (VCC/2) as expected for this self-biasing topology.
* bom_vs_spice equivalences ignored:
*   - U1 (74HC04) is modeled as a subcircuit using continuous behavioral voltage sources (tanh functions) to approximate the analog transfer curve of CMOS inverters.
* overall_comment: The circuit is a classic example of using a digital CMOS inverter as a linear class A amplifier. The netlist correctly implements the self-biasing scheme (Rf feedback) and AC coupling. The simulation results confirm the high open-loop gain of the HC series inverter, resulting in a heavily clipped output for a 50mVpp input. As a didactic example, it effectively demonstrates the concept, though a teacher might want to reduce the input amplitude or add an input series resistor to reduce the gain if a cleaner sine wave is desired.
* --------------------------------------

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)

Analysis: The simulation shows a functional inverting amplifier. The input signal (VIN_AC) is a ~25mV amplitude sine wave. The output (VOUT_LOAD) is an inverted sine wave with an amplitude of approximately 2.4V to 2.5V, indicating a very high gain that is causing significant clipping/saturation near the rails (approx +/- 2.5V swing). The DC bias point at GATE_IN and GATE_OUT settles near 2.5V (VCC/2) as expected for this self-biasing topology.
Show raw data table (508 rows) 
Index   time            v(vin_ac)       v(vout_load)    v(gate_in)      v(gate_out)
0	0.000000e+00	0.000000e+00	0.000000e+00	2.500000e+00	2.500000e+00
1	1.000000e-07	1.570796e-05	-3.92600e-03	2.500016e+00	2.496074e+00
2	2.000000e-07	3.141592e-05	-7.85100e-03	2.500031e+00	2.492149e+00
3	4.000000e-07	6.283179e-05	-1.56989e-02	2.500063e+00	2.484301e+00
4	8.000000e-07	1.256632e-04	-3.13823e-02	2.500126e+00	2.468618e+00
5	1.600000e-06	2.513232e-04	-6.26967e-02	2.500251e+00	2.437303e+00
6	3.200000e-06	5.026210e-04	-1.25097e-01	2.500501e+00	2.374901e+00
7	6.400000e-06	1.005039e-03	-2.48425e-01	2.500997e+00	2.251567e+00
8	1.280000e-05	2.008453e-03	-4.87825e-01	2.501977e+00	2.012143e+00
9	2.280000e-05	3.569178e-03	-8.34430e-01	2.503471e+00	1.665472e+00
10	3.280000e-05	5.115818e-03	-1.13904e+00	2.504919e+00	1.360762e+00
11	4.280000e-05	6.642268e-03	-1.39785e+00	2.506318e+00	1.101832e+00
12	5.280000e-05	8.142504e-03	-1.61199e+00	2.507667e+00	8.875322e-01
13	6.280000e-05	9.610606e-03	-1.78571e+00	2.508964e+00	7.136492e-01
14	7.280000e-05	1.104078e-02	-1.92461e+00	2.510208e+00	5.745580e-01
15	8.280000e-05	1.242738e-02	-2.03459e+00	2.511395e+00	4.643784e-01
16	9.280000e-05	1.376493e-02	-2.12112e+00	2.512524e+00	3.776434e-01
17	1.028000e-04	1.504816e-02	-2.18894e+00	2.513590e+00	3.096072e-01
18	1.128000e-04	1.627201e-02	-2.24200e+00	2.514591e+00	2.563270e-01
19	1.228000e-04	1.743163e-02	-2.28348e+00	2.515522e+00	2.146211e-01
20	1.328000e-04	1.852246e-02	-2.31590e+00	2.516381e+00	1.819734e-01
21	1.428000e-04	1.954019e-02	-2.34122e+00	2.517164e+00	1.564217e-01
22	1.528000e-04	2.048080e-02	-2.36095e+00	2.517868e+00	1.364514e-01
23	1.628000e-04	2.134059e-02	-2.37626e+00	2.518489e+00	1.209036e-01
... (484 more rows) ...

Common mistakes and how to avoid them

  1. Using the wrong logic family: Students often use 74LS04 or 74HCT04. These have internal pull-ups or different input thresholds that prevent symmetrical linear biasing. Solution: Ensure you use the 74HC04 (CMOS) or CD4069UB.
  2. Input signal too large: Applying a standard TTL/CMOS logic signal (0-5 V) will result in a square wave output, not amplification. Solution: Keep the input signal small (under 100 mVpp) to stay within the linear region.
  3. Floating unused inputs: Leaving pins 3, 5, 9, etc., disconnected causes internal noise and excessive power consumption. Solution: Always tie unused inputs of CMOS chips to Ground (0).

Troubleshooting

  • Symptom: Output is stuck at 0 V or 5 V.
    • Cause: Feedback resistor Rf is missing or open circuit.
    • Fix: Check continuity of Rf (1 MΩ). It is required to pull the input voltage to the tipping point.
  • Symptom: High frequency noise superimposed on the signal.
    • Cause: Parasitic oscillation due to high gain and stray capacitance.
    • Fix: Shorten wires or add a small capacitor (e.g., 10 pF) in parallel with Rf to reduce bandwidth.
  • Symptom: Gain is very low ($< 2$).
    • Cause: Load resistance RL is too small.
    • Fix: The output impedance of a 74HC04 in linear mode is relatively high. Increase RL to 100 kΩ or remove it for testing.

Possible improvements and extensions

  1. Crystal Oscillator: Replace the signal generator with a quartz crystal and two load capacitors (to ground) at the input and output pins to create a stable clock source.
  2. Cascaded Amplifier: Connect the output of the first stage (via a capacitor) to a second identically configured 74HC04 stage to achieve much higher total voltage gain.

More Practical Cases on Prometeo.blog

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Go to Amazon

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Quick Quiz

Question 1: What is the primary function of the feedback resistor in this circuit configuration?




Question 2: In this self-biasing configuration, what is the expected DC voltage at the input and output (assuming a 5 V supply)?




Question 3: Which internal components of the 74HC04 allow it to function as an analog amplifier?




Question 4: This linear inverter topology is the fundamental building block for which common circuit?




Question 5: When configured with negative feedback, the inverter operates as which class of amplifier?




Question 6: What is the phase relationship between the AC input signal and the amplified output signal?




Question 7: What is a key advantage of this configuration regarding the input impedance?




Question 8: To maintain linear operation without clipping, how should the input signal be characterized?




Question 9: Besides amplification, what other use case is mentioned for this circuit topology?




Question 10: Why is the 74HC04 specifically capable of this operation?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

Follow me:

Practical case: Ring Oscillator and Delay

Ring Oscillator and Delay prototype (Maker Style)

Level: Advanced — Build and analyze a 5-stage ring oscillator to calculate component propagation delay.

Objective and use case

In this case, you will construct a ring oscillator by cascading an odd number (5) of NOT gates (inverters) in a closed feedback loop using a 74HC04 IC. You will measure the resulting oscillation frequency to calculate the intrinsic propagation delay of the logic gates.

Why it is useful:
* Process characterization: Used in semiconductor manufacturing to test the speed and quality of silicon wafers.
* Clock generation: Fundamental topology for generating internal clocks in ASICs and FPGAs.
* Random Number Generation: The inherent jitter in ring oscillators is a source of entropy for True Random Number Generators (TRNG).
* Time-to-Digital Converters (TDC): Used to measure time intervals with high precision.

Expected outcome:
* A stable square wave output oscillating in the MHz range (typically 20 MHz–50 MHz for 74HC logic on a breadboard).
* Measurement of oscillation frequency (fosc).
* Calculation of the average propagation delay (tpd) per gate.
* Visual observation of rise (tr) and fall (tf) times due to capacitive loading.

Target audience and level:
Advanced Electronics Students; Engineering Undergraduates.

Materials

  • U1: 74HC04 Hex Inverter IC, function: logic gates for the ring
  • C1: 100 nF ceramic capacitor, function: power supply decoupling (critical for stability)
  • C2: 10 pF capacitor, function: simulated load (optional, represents probe capacitance)
  • V1: 5 V DC supply
  • W1-W5: Jumper wires, function: inter-stage connections

Pin-out of the IC used

Selected Chip: 74HC04 (Hex Inverter)

Pin Name Logic function Connection in this case
1 1 A Input 1 From Output 5 (Node N5)
2 1Y Output 1 To Input 2 (Node N1)
3 2 A Input 2 From Output 1 (Node N1)
4 2Y Output 2 To Input 3 (Node N2)
5 3 A Input 3 From Output 2 (Node N2)
6 3Y Output 3 To Input 4 (Node N3)
7 GND Ground Connect to Node 0
8 4Y Output 4 To Input 5 (Node N4)
9 4 A Input 4 From Output 3 (Node N3)
10 5Y Output 5 To Input 1 (Node N5 – Feedback)
11 5 A Input 5 From Output 4 (Node N4)
14 VCC Power Supply Connect to Node VCC (+5 V)

Note: Pins 12 (6Y) and 13 (6 A) are unused and should be left open or tied to GND/VCC depending on specific noise requirements, though for this test leaving them open is acceptable.

Wiring guide

This circuit relies on minimal trace length to sustain high-frequency oscillation.

  • V1 connects between node VCC and node 0 (GND).
  • C1 connects between node VCC and node 0 (place physically close to U1).
  • U1 (Pin 14) connects to node VCC.
  • U1 (Pin 7) connects to node 0.
  • U1 (Pin 1 – Input 1) connects to node N5 (Feedback loop closure).
  • U1 (Pin 2 – Output 1) connects to node N1.
  • U1 (Pin 3 – Input 2) connects to node N1.
  • U1 (Pin 4 – Output 2) connects to node N2.
  • U1 (Pin 5 – Input 3) connects to node N2.
  • U1 (Pin 6 – Output 3) connects to node N3.
  • U1 (Pin 9 – Input 4) connects to node N3.
  • U1 (Pin 8 – Output 4) connects to node N4.
  • U1 (Pin 11 – Input 5) connects to node N4.
  • U1 (Pin 10 – Output 5) connects to node N5.
  • C2 (Optional Load) connects between node N5 and node 0 to simulate probe capacitance.

Conceptual block diagram

Conceptual block diagram — 74HC04 NOT gate
Quick read: inputs → main block → output (actuator or measurement). This summarizes the ASCII schematic below.

Schematic

POWER SUPPLY & DECOUPLING:
      VCC (5 V) --> [ Node VCC ] --(Pin 14)--> [ U1: 74HC04 Power ]
                      |
                    [ C1: 100nF ]
                      |
      GND (0 V) --> [ Node 0 ] --(Pin 7)---> [ U1: 74HC04 GND ]


SIGNAL FLOW (RING OSCILLATOR):
(Logic flows Left to Right, wrapping around at the end)

      [ Feedback N5 ] --> [ U1: Gate 1 ] --(Node N1)--> [ U1: Gate 2 ] --(Node N2)--> [ U1: Gate 3 ] --(Node N3)--> \
      (Input Pin 1)       (In:1 / Out:2)                (In:3 / Out:4)                (In:5 / Out:6)                |
                                                                                                                    |
      /-------------------------------------------------------------------------------------------------------------/
      |
      \--> [ U1: Gate 4 ] --(Node N4)--> [ U1: Gate 5 ] --(Node N5)--> [ C2: 10pF ] --> GND
           (In:9 / Out:8)                (In:11 / Out:10)      |
                                                               |
                                                      (Loop back to Start)
Electrical Schematic

Truth table (Single NOT Gate)

Input (A) Output (Y)
L H
H L

In a ring configuration with an odd number of stages, the logic never settles, causing perpetual oscillation.

Measurements and tests

  1. Setup: Ensure wiring is short and neat. Long wires add parasitic inductance and capacitance which will significantly lower the frequency.
  2. Visualization: Connect an oscilloscope probe (x10 attenuation recommended to reduce loading) to Node N5 (or any node N1 through N4).
  3. Frequency Measurement: Measure the frequency of the oscillation (fosc). For a 74HC series at 5 V, expect approx 20MHz – 40MHz depending on stray capacitance.
  4. Propagation Delay Calculation: Calculate the average propagation delay per gate (tpd) using the formula:
    $tpd = (1 / (2 × N × fosc))$
    Where $N = 5$ (number of stages).
    Example: If $f_{osc} = 25 MHz$, then $T = 40 ns$. $t_{pd} = 40 ns / 10 = 4 ns$.
  5. Waveform Analysis: Zoom in on the edges. Notice that the wave is not a perfect square; the rise time ($t_{r}$) and fall time ($t_{f}$) are visible due to the capacitive charging of the next gate’s input.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Practical case: Ring Oscillator and Delay
.width out=256
* Ngspice Netlist

* --- Power Supply ---
* V1: 5 V DC supply connecting VCC to GND (0)
V1 VCC 0 DC 5

* --- Decoupling Capacitor ---
* C1: 100 nF ceramic capacitor, power supply decoupling
C1 VCC 0 100n

* --- Integrated Circuit U1: 74HC04 Hex Inverter ---
* Modeled as a subcircuit to strictly follow physical pinout and wiring guide.
* Pin Mapping (Standard DIP-14):
* 1:1A  2:1Y  3:2A  4:2Y  5:3A  6:3Y  7:GND
* 8:4Y  9:4A 10:5Y 11:5A 12:6Y 13:6A 14:VCC
*
* Wiring Connections based on Guide:
* Pin 1 (In1)  -> N5
* ... (truncated in public view) ...

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* Practical case: Ring Oscillator and Delay
.width out=256
* Ngspice Netlist

* --- Power Supply ---
* V1: 5 V DC supply connecting VCC to GND (0)
V1 VCC 0 DC 5

* --- Decoupling Capacitor ---
* C1: 100 nF ceramic capacitor, power supply decoupling
C1 VCC 0 100n

* --- Integrated Circuit U1: 74HC04 Hex Inverter ---
* Modeled as a subcircuit to strictly follow physical pinout and wiring guide.
* Pin Mapping (Standard DIP-14):
* 1:1A  2:1Y  3:2A  4:2Y  5:3A  6:3Y  7:GND
* 8:4Y  9:4A 10:5Y 11:5A 12:6Y 13:6A 14:VCC
*
* Wiring Connections based on Guide:
* Pin 1 (In1)  -> N5
* Pin 2 (Out1) -> N1
* Pin 3 (In2)  -> N1
* Pin 4 (Out2) -> N2
* Pin 5 (In3)  -> N2
* Pin 6 (Out3) -> N3
* Pin 7 (GND)  -> 0
* Pin 8 (Out4) -> N4
* Pin 9 (In4)  -> N3
* Pin 10 (Out5)-> N5
* Pin 11 (In5) -> N4
* Pin 12 (Out6)-> NC_OUT (Unused)
* Pin 13 (In6) -> NC_IN  (Unused)
* Pin 14 (VCC) -> VCC

XU1 N5 N1 N1 N2 N2 N3 0 N4 N3 N5 N4 NC_OUT NC_IN VCC 74HC04

* --- Load Capacitor ---
* C2: 10 pF capacitor, simulated load on Output Node N5
C2 N5 0 10p

* --- Unused Pin Termination ---
* High resistance to ground to prevent floating node errors
R_NC1 NC_OUT 0 1G
R_NC2 NC_IN 0 1G

* --- Subcircuit Definitions ---

.subckt 74HC04 1 2 3 4 5 6 7 8 9 10 11 12 13 14
    * Local Power: 14=VCC, 7=GND
    * Gate 1
    X1 1 2 14 7 inv_gate
    * Gate 2
    X2 3 4 14 7 inv_gate
    * Gate 3
    X3 5 6 14 7 inv_gate
    * Gate 4 (Note: Pin 9 is Input, Pin 8 is Output)
    X4 9 8 14 7 inv_gate
    * Gate 5 (Note: Pin 11 is Input, Pin 10 is Output)
    X5 11 10 14 7 inv_gate
    * Gate 6 (Unused)
    X6 13 12 14 7 inv_gate
.ends

.subckt inv_gate in out vcc gnd
    * Internal RC Delay Stage
    * Essential for Ring Oscillator simulation to function (creates phase shift)
    * R=1k, C=10p yields approx 10ns delay per stage
    R_delay in int 1k
    C_delay int gnd 10p
    
    * Behavioral Voltage Source (Sigmoid Inverter)
    * Uses continuous function for convergence: Vout = Vcc / (1 + exp(k*(Vin - Vcc/2)))
    * Gain factor k=10
    B_logic out gnd V=V(vcc) / (1 + exp(10 * (V(int) - V(vcc)/2)))
.ends

* --- Simulation Directives ---

* Initial Condition:
* Force node N1 to 0V at t=0 to prevent metastable state (kickstart oscillation)
.ic V(N1)=0

* Transient Analysis:
* Run for 2 microseconds, step size 1 nanosecond
* 'uic' (Use Initial Conditions) is required for .ic to take effect
.tran 1n 2u uic

* Operating Point (for check)
.op

* Output Printing:
* V(N5) is the primary OSC_OUT node
.print tran V(N5) V(N1) V(N2) V(N3) V(N4)

.end
* --- GPT review (BOM/Wiring/SPICE) ---
* circuit_ok=true
* simulation_summary: The transient analysis shows sustained oscillation on nodes N1 through N5. The voltages swing between ~0V and ~5V. The frequency can be inferred from the timestamps (e.g., N5 rising edges around 1.43us and subsequent cycles), confirming the ring oscillator behavior.
* bom_vs_spice equivalences ignored:
*   - U1 (74HC04 Hex Inverter) is modeled as a subcircuit using behavioral voltage sources and RC delay stages to simulate propagation delay and logic inversion.
* overall_comment: The circuit is well-modeled for a didactic example. The inclusion of internal RC delay stages inside the inverter subcircuit is crucial for a ring oscillator simulation, as ideal SPICE inverters often fail to oscillate or converge without explicit time-dependent behavior. The initial condition (.ic V(N1)=0) correctly kickstarts the oscillation. The connectivity matches the wiring guide perfectly.
* --------------------------------------

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)

Analysis: The transient analysis shows sustained oscillation on nodes N1 through N5. The voltages swing between ~0V and ~5V. The frequency can be inferred from the timestamps (e.g., N5 rising edges around 1.43us and subsequent cycles), confirming the ring oscillator behavior.
Show raw data table (2039 rows) 
Index   time            v(n5)           v(n1)           v(n2)           v(n3)           v(n4)
0	1.000000e-11	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00
1	1.028000e-11	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00
2	1.084000e-11	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00
3	1.196000e-11	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00
4	1.420000e-11	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00
5	1.868000e-11	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00
6	2.764000e-11	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00
7	4.556000e-11	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00
8	8.140000e-11	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00
9	1.530800e-10	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00
10	2.964400e-10	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00
11	5.831600e-10	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00
12	1.000000e-09	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00
13	1.057344e-09	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00
14	1.172032e-09	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00
15	1.401408e-09	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00
16	1.860160e-09	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00
17	2.777664e-09	4.999998e+00	4.999998e+00	4.999998e+00	4.999998e+00	4.999998e+00
18	3.777664e-09	4.999526e+00	4.999526e+00	4.999526e+00	4.999526e+00	4.999526e+00
19	4.777664e-09	4.987728e+00	4.987728e+00	4.987728e+00	4.987728e+00	4.987728e+00
20	5.777664e-09	4.795985e+00	4.795985e+00	4.795985e+00	4.795985e+00	4.795985e+00
21	6.777664e-09	3.794650e+00	3.794650e+00	3.794650e+00	3.794650e+00	3.794650e+00
22	7.777664e-09	2.828762e+00	2.828762e+00	2.828762e+00	2.828762e+00	2.828762e+00
23	8.777664e-09	2.564867e+00	2.564867e+00	2.564867e+00	2.564867e+00	2.564867e+00
... (2015 more rows) ...

Common mistakes and how to avoid them

  1. Using an even number of gates: If you use 4 or 6 gates, the logic will settle into a stable state (latch up) rather than oscillate. Always use an odd number (3, 5, 7…).
  2. Breadboard capacitance: Standard breadboards have high parasitic capacitance between rows (approx 2-5pF). This will make the oscillator run slower than the datasheet specs imply. Avoid long jumper loops.
  3. Missing decoupling capacitor: Without C1 close to the chip, the high-frequency switching current will cause VCC sag, resulting in erratic frequency or no oscillation.

Troubleshooting

  • Output is stuck High or Low: Check that you have an odd number of inverters in the loop. Verify the feedback wire connects the last output to the first input.
  • Frequency is unstable (jitter): Likely power supply noise. Ensure C1 (100nF) is installed extremely close to pins 14 and 7.
  • Scope shows a sine wave instead of square: At very high frequencies (approaching the bandwidth limit of the scope or probe), square waves look like sine waves due to the attenuation of higher harmonics. Ensure your scope bandwidth is at least 100 MHz.
  • Circuit gets hot: Check for short circuits between outputs. Never connect two outputs together.

Possible improvements and extensions

  1. Enable Control: Replace the first inverter with a NAND gate (e.g., using 74HC00). Use one input for the feedback loop and the other as an Enable/Disable control signal.
  2. Buffered Output: Use the 6th unused inverter in the 74HC04 package as a buffer connected to one of the ring nodes. Connect your probe/load to this buffer output. This isolates the ring oscillator from the load capacitance, providing a more accurate frequency measurement.

More Practical Cases on Prometeo.blog

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Quick Quiz

Question 1: What specific arrangement of NOT gates is required to construct a ring oscillator?




Question 2: What is a strict requirement regarding the number of stages in a ring oscillator to ensure oscillation?




Question 3: What is the primary physical parameter calculated by measuring the oscillation frequency in this experiment?




Question 4: Which characteristic of ring oscillators allows them to be used for True Random Number Generators (TRNG)?




Question 5: What is the typical expected frequency range for a 5-stage ring oscillator using 74HC logic on a breadboard?




Question 6: What is the function of the 100 nF ceramic capacitor (C1) typically placed near the IC in this circuit?




Question 7: In the context of semiconductor manufacturing, why are ring oscillators useful?




Question 8: What does the optional 10 pF capacitor (C2) simulate in this experiment?




Question 9: Which IC is specifically selected to provide the logic gates for this ring oscillator?




Question 10: What type of waveform is expected at the output of the ring oscillator?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

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Practical case: Frequency divider by 2, 4 and 8

Frequency divider by 2, 4 and 8 prototype (Maker Style)

Level: Basic – Verify the frequency division relationship on the Q outputs of a binary counter relative to the clock.

Objective and use case

In this practical case, you will build a digital circuit using a 4-bit binary counter (74HC393) to divide an input clock signal frequency by factors of 2 (2^1), 4 (2^2), and 8 (2^3).

  • Digital Clocks: Used to divide high-frequency crystal oscillator signals down to 1 Hz for keeping time (seconds).
  • Audio Synthesis: Used to generate lower octaves from a base tone (frequency halving results in a tone one octave lower).
  • Baud Rate Generation: Used in UART communication to derive specific data transmission speeds from a master system clock.
  • Address Counters: Used to sequence through memory addresses in microcontrollers.

Expected outcome:
* Q0 Output: A square wave with a frequency exactly half of the input clock (f/2).
* Q1 Output: A square wave with a frequency one-quarter of the input clock (f/4).
* Q2 Output: A square wave with a frequency one-eighth of the input clock (f/8).
* Target Audience: Basic level students and hobbyists.

Materials

  • V1: 5 V DC supply, function: Main power source.
  • V_CLK: Pulse generator (0 V to 5 V, 1 kHz, 50% duty cycle), function: Input Clock signal.
  • U1: 74HC393, function: Dual 4-bit Binary Counter.
  • R1: 330 Ω resistor, function: Current limiting for LED D1.
  • R2: 330 Ω resistor, function: Current limiting for LED D2.
  • R3: 330 Ω resistor, function: Current limiting for LED D3.
  • D1: Red LED, function: Visual indicator for Q0 (f/2).
  • D2: Green LED, function: Visual indicator for Q1 (f/4).
  • D3: Yellow LED, function: Visual indicator for Q2 (f/8).
  • Scope: 4-Channel Oscilloscope, function: Waveform analysis.

Pin-out of the IC used

Selected Chip: 74HC393 (Dual 4-bit Binary Counter). We will use the first counter block (Side 1).

Pin Name Logic function Connection in this case
1 1CP (CLK) Clock Input (Falling edge trigger) Connected to CLK_IN
2 1MR Master Reset (Active High) Connected to 0 (GND)
3 1Q0 Output Bit 0 (Divide by 2) Connected to Q0
4 1Q1 Output Bit 1 (Divide by 4) Connected to Q1
5 1Q2 Output Bit 2 (Divide by 8) Connected to Q2
7 GND Ground Connected to 0
14 VCC Power Supply (+5 V) Connected to VCC

Wiring guide

  • V1 connects between node VCC and node 0 (GND).
  • U1 pin 14 connects to node VCC.
  • U1 pin 7 connects to node 0 (GND).
  • U1 pin 2 (Reset) connects to node 0 (GND) to enable counting.
  • V_CLK connects between node CLK_IN and node 0 (GND).
  • U1 pin 1 connects to node CLK_IN.
  • U1 pin 3 connects to node Q0.
  • U1 pin 4 connects to node Q1.
  • U1 pin 5 connects to node Q2.
  • R1 connects between node Q0 and node LED_Q0.
  • D1 anode connects to LED_Q0, cathode connects to 0 (GND).
  • R2 connects between node Q1 and node LED_Q1.
  • D2 anode connects to LED_Q1, cathode connects to 0 (GND).
  • R3 connects between node Q2 and node LED_Q2.
  • D3 anode connects to LED_Q2, cathode connects to 0 (GND).

Conceptual block diagram

Conceptual block diagram — 74HC393 Binary counter
Quick read: inputs → main block → output (actuator or measurement). This summarizes the ASCII schematic below.

Schematic

INPUTS                                   PROCESSING                                     OUTPUTS / LOADS
(Left)                                    (Center)                                          (Right)

                                   +-----------------------+
                                   |                       |
 [ V_CLK: 1kHz ] --(Pin 1: CP)---> |                       | --(Pin 3: Q0)--> [ R1: 330 ] --> [ D1: Red ] --> GND
                                   |                       |       |
                                   |      U1: 74HC393      |       '--------(Scope Ch1: f/2)
                                   |      Dual 4-bit       |
                                   |      Bin Counter      |
 [ GND ] ---------(Pin 2: MR)--->  |                       | --(Pin 4: Q1)--> [ R2: 330 ] --> [ D2: Grn ] --> GND
             (Reset Disabled)      |   (Power: VCC=Pin 14, |       |
                                   |           GND=Pin 7)  |       '--------(Scope Ch2: f/4)
                                   |                       |
                                   |                       |
                                   |                       | --(Pin 5: Q2)--> [ R3: 330 ] --> [ D3: Yel ] --> GND
                                   |                       |       |
                                   +-----------------------+       '--------(Scope Ch3: f/8)
Electrical Schematic

Electrical diagram

Electrical diagram for case: Practical case: Frequency divider by 2, 4 and 8
Generated from the validated SPICE netlist for this case.

🔒 This electrical diagram is premium. With the monthly membership (7-day free trial) you can unlock the complete didactic material and the print-ready PDF pack.🔓 See premium access plans

Measurements and tests

To validate the circuit, perform the following measurements using the 4-channel oscilloscope:

  1. Setup: Connect the Ground clip of all oscilloscope probes to node 0 (GND).
  2. Channel 1 (Input): Connect to CLK_IN. Verify the frequency is 1 kHz.
  3. Channel 2 (Q0): Connect to Q0. Measure the frequency. It must be 500 Hz ($1kHz / 2$).
  4. Channel 3 (Q1): Connect to Q1. Measure the frequency. It must be 250 Hz ($1kHz / 4$).
  5. Channel 4 (Q2): Connect to Q2. Measure the frequency. It must be 125 Hz ($1kHz / 8$).
  6. Visual Check: If you lower the input clock frequency to 10 Hz, you should see D1 blinking fastest, D2 slower, and D3 slowest.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Practical case: Frequency divider by 2, 4 and 8

.width out=256

* --- Models ---
* Generic LED Model
.model DLED D(IS=1e-14 N=2 RS=10 BV=5 IBV=10u CJO=10p)

* --- Power Supply ---
* V1: 5V Main Supply
V1 VCC 0 DC 5

* --- Input Signal ---
* V_CLK: 1kHz Pulse, 0V to 5V, 50% Duty Cycle
V_CLK CLK_IN 0 PULSE(0 5 0 1u 1u 0.5m 1m)

* --- Subcircuit: 74HC393 (Behavioral XSPICE) ---
* Dual 4-bit Binary Counter
* Implements Counter 1 logic using XSPICE primitives.
* Pinout (DIP-14): 1=1CP, 2=1MR, 3=1Q0, 4=1Q1, 5=1Q2, 6=1Q3, 7=GND
* ... (truncated in public view) ...

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🔓 See premium access plans 

* Practical case: Frequency divider by 2, 4 and 8

.width out=256

* --- Models ---
* Generic LED Model
.model DLED D(IS=1e-14 N=2 RS=10 BV=5 IBV=10u CJO=10p)

* --- Power Supply ---
* V1: 5V Main Supply
V1 VCC 0 DC 5

* --- Input Signal ---
* V_CLK: 1kHz Pulse, 0V to 5V, 50% Duty Cycle
V_CLK CLK_IN 0 PULSE(0 5 0 1u 1u 0.5m 1m)

* --- Subcircuit: 74HC393 (Behavioral XSPICE) ---
* Dual 4-bit Binary Counter
* Implements Counter 1 logic using XSPICE primitives.
* Pinout (DIP-14): 1=1CP, 2=1MR, 3=1Q0, 4=1Q1, 5=1Q2, 6=1Q3, 7=GND
*                  8=2Q3, 9=2Q2, 10=2Q1, 11=2Q0, 12=2MR, 13=2CP, 14=VCC
.subckt 74HC393 1CP 1MR 1Q0 1Q1 1Q2 1Q3 GND 2Q3 2Q2 2Q1 2Q0 2MR 2CP VCC

    * ADC Bridge to read analog inputs (Clock and Reset)
    .model adc_mod adc_bridge(in_low=1.5 in_high=3.5)
    A_IN [1CP 1MR] [d_1cp d_1mr] adc_mod
    
    * ADC Bridge to read GND for Logic Low (used for SET inputs)
    A_GND [GND] [d_low] adc_mod

    * Logic Models
    .model inv_mod d_inverter(rise_delay=10n fall_delay=10n)
    .model dff_mod d_dff(clk_delay=10n rise_delay=10n fall_delay=10n)
    .model dac_mod dac_bridge(out_low=0.0 out_high=5.0)

    * --- Counter Logic (Side 1) ---
    * 74HC393 triggers on High-to-Low transition of CP.
    * XSPICE DFF triggers on Rising Edge. So we invert CP.
    A_INV1 d_1cp d_1cp_inv inv_mod

    * Stage 1 (Q0): Divider by 2
    * T-FF behavior: D = ~Q. Clock = ~CP. Reset = MR.
    * Port order: din clk set reset out nout
    A_DFF1 d_1q0_bar d_1cp_inv d_low d_1mr d_1q0 d_1q0_bar dff_mod

    * Stage 2 (Q1): Divider by 4
    * Ripples from Q0 Falling Edge.
    * Q0 Falling = ~Q0 Rising. Use d_1q0_bar as clock.
    A_DFF2 d_1q1_bar d_1q0_bar d_low d_1mr d_1q1 d_1q1_bar dff_mod

    * Stage 3 (Q2): Divider by 8
    * Ripples from Q1 Falling Edge. Use d_1q1_bar as clock.
    A_DFF3 d_1q2_bar d_1q1_bar d_low d_1mr d_1q2 d_1q2_bar dff_mod

    * Stage 4 (Q3): Divider by 16 (Not used externally but part of logic)
    A_DFF4 d_1q3_bar d_1q2_bar d_low d_1mr d_1q3 d_1q3_bar dff_mod

    * Drive Outputs
    A_OUT [d_1q0 d_1q1 d_1q2 d_1q3] [1Q0 1Q1 1Q2 1Q3] dac_mod

    * Side 2 is unused, inputs grounded in main circuit, outputs open.
.ends 74HC393

* --- Main Circuit Instances ---
* U1: 74HC393 Counter
* Pin connections based on Wiring Guide:
* 1(CLK_IN), 2(0/Reset), 3(Q0), 4(Q1), 5(Q2), 7(0/GND), 14(VCC)
* Unused outputs mapped to NC nodes. Unused inputs to 0.
* Subcircuit Pin Order: 1CP 1MR 1Q0 1Q1 1Q2 1Q3 GND 2Q3 2Q2 2Q1 2Q0 2MR 2CP VCC
XU1 CLK_IN 0 Q0 Q1 Q2 NC_1Q3 0 NC_2Q3 NC_2Q2 NC_2Q1 NC_2Q0 0 0 VCC 74HC393

* --- Output Paths (LEDs and Resistors) ---
* Path 1: Q0 -> R1 -> D1 (Red)
R1 Q0 LED_Q0 330
D1 LED_Q0 0 DLED

* Path 2: Q1 -> R2 -> D2 (Green)
R2 Q1 LED_Q1 330
D2 LED_Q1 0 DLED

* Path 3: Q2 -> R3 -> D3 (Yellow)
R3 Q2 LED_Q2 330
D3 LED_Q2 0 DLED

* --- Simulation & Output ---
.op
.tran 10u 20m
.print tran V(CLK_IN) V(Q0) V(Q1) V(Q2)

.end
* --- GPT review (BOM/Wiring/SPICE) ---
* circuit_ok=true
* simulation_summary: The simulation shows a clear binary counting sequence. CLK_IN is a 1kHz clock (period 1ms). Q0 toggles every 1ms (f/2, period 2ms). Q1 toggles every 2ms (f/4, period 4ms). Q2 toggles every 4ms (f/8, period 8ms). The outputs transition cleanly between 0V and 5V.
* bom_vs_spice equivalences ignored:
*   - LEDs (D1, D2, D3) are modeled using a generic diode model (DLED) with specific parameters.
*   - U1 (74HC393) is modeled as a behavioral subcircuit using XSPICE primitives (ADC/DAC bridges, DFFs) instead of a transistor-level model.
* overall_comment: The circuit is perfectly functional and accurately represents a 3-bit binary ripple counter (frequency divider). The behavioral model for the 74HC393 is correctly implemented with the necessary ADC/DAC bridges for XSPICE. The wiring matches the guide exactly, and the simulation results confirm the expected frequency division ratios (f/2, f/4, f/8). It is an excellent didactic example.
* --------------------------------------

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)

Analysis: The simulation shows a clear binary counting sequence. CLK_IN is a 1kHz clock (period 1ms). Q0 toggles every 1ms (f/2, period 2ms). Q1 toggles every 2ms (f/4, period 4ms). Q2 toggles every 4ms (f/8, period 8ms). The outputs transition cleanly between 0V and 5V.
Show raw data table (3323 rows) 
Index   time            v(clk_in)       v(q0)           v(q1)           v(q2)
0	0.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
1	1.000000e-08	5.000000e-02	0.000000e+00	0.000000e+00	0.000000e+00
2	2.000000e-08	1.000000e-01	0.000000e+00	0.000000e+00	0.000000e+00
3	4.000000e-08	2.000000e-01	0.000000e+00	0.000000e+00	0.000000e+00
4	8.000000e-08	4.000000e-01	0.000000e+00	0.000000e+00	0.000000e+00
5	1.600000e-07	8.000000e-01	0.000000e+00	0.000000e+00	0.000000e+00
6	3.200000e-07	1.600000e+00	0.000000e+00	0.000000e+00	0.000000e+00
7	6.400000e-07	3.200000e+00	0.000000e+00	0.000000e+00	0.000000e+00
8	1.000000e-06	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
9	1.064000e-06	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
10	1.192000e-06	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
11	1.448000e-06	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
12	1.960000e-06	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
13	2.984000e-06	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
14	5.032000e-06	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
15	9.128000e-06	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
16	1.732000e-05	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
17	2.732000e-05	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
18	3.732000e-05	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
19	4.732000e-05	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
20	5.732000e-05	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
21	6.732000e-05	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
22	7.732000e-05	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
23	8.732000e-05	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
... (3299 more rows) ...

Common mistakes and how to avoid them

  1. Floating the Master Reset (MR) pin: Leaving pin 2 disconnected causes the counter to reset randomly due to noise. Solution: Always tie the MR pin to GND (Logic 0) for normal counting operation.
  2. Confusing Pin Numbers: The 74HC393 has two counters inside. Students often mix pins from Counter 1 and Counter 2. Solution: Strictly follow the datasheet and use pins 1, 2, 3, 4, 5, and 6 for the first counter only.
  3. Ignoring VCC/GND: Forgetting to power the chip leads to unpredictable output or no activity. Solution: Always connect Pin 14 to +5 V and Pin 7 to GND before testing.

Troubleshooting

  • Symptom: No LEDs light up, and outputs remain at 0 V.
    • Cause: Master Reset (Pin 2) might be connected to VCC instead of GND.
    • Fix: Move connection of Pin 2 to GND.
  • Symptom: LEDs are always on or flickering very dimly.
    • Cause: Frequency is too high for the eye to see blinking (e.g., 1 kHz).
    • Fix: Use the oscilloscope to verify the signal, or lower V_CLK frequency to < 10 Hz for visual confirmation.
  • Symptom: Output frequency is unstable or erratic.
    • Cause: Noisy power supply or lack of decoupling capacitor.
    • Fix: Add a 100 nF capacitor across VCC and GND near the IC.

Possible improvements and extensions

  1. Divide by 16 and 256: Cascade the first counter into the second counter of the U1 chip (connect 1Q3 to 2CP) to achieve higher division ratios up to 256.
  2. Variable Audio Generator: Connect the outputs to a simple speaker driver and use a variable potentiometer on a 555 timer (as the clock) to hear how the pitch drops by octaves as you switch between Q0, Q1, and Q2.

More Practical Cases on Prometeo.blog

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Quick Quiz

Question 1: What is the primary function of the 74HC393 IC used in this circuit?




Question 2: What is the frequency relationship of the Q0 output relative to the input clock (f)?




Question 3: If the input clock frequency is 1 kHz, what is the expected frequency at the Q1 output?




Question 4: What is the expected frequency relationship at the Q2 output?




Question 5: In audio synthesis, what is the result of halving a tone's frequency?




Question 6: What is the purpose of using this circuit in digital clocks?




Question 7: What DC supply voltage is specified for this circuit?




Question 8: How is this circuit applied in UART communication?




Question 9: Which power of 2 represents the division factor for the Q1 output?




Question 10: What is the role of address counters in microcontrollers?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

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