Level: Basic – Learn to use an NPN transistor as a switch to drive a DC motor, including the use of a flyback diode.

Objective and use case

In this practical case, you will build a low-side switch circuit using an NPN transistor to safely control a high-current DC motor from a low-power control signal.

This topology is highly useful in the real world for several reasons:
* Interfacing low-voltage microcontrollers (like an Arduino or Raspberry Pi) with higher power loads that require external power supplies.
* Automating small cooling fans in temperature-controlled systems.
* Building basic drive systems for small hobbyist robotics.
* Protecting delicate control logic from the damaging voltage spikes generated by inductive loads.

Expected outcome:
* Applying a 5 V control signal to the base circuit will saturate the transistor.
* The DC motor will spin as the transistor bridges its connection to ground.
* The flyback diode will safely dissipate the motor’s inductive kickback when the control signal is turned off.
* Measurable base voltage (VBE) around 0.7 V, near-zero collector-emitter voltage (VCE) indicating saturation, and clearly observable base current (IB) and collector current (IC).

Target audience and level: Beginners in electronics and hobbyists looking to control mechanical loads safely.

Materials

• V1: 9 V DC supply, function: main power source for the DC motor
• V2: 5 V DC supply, function: simulated control signal source
• SW1: SPST switch, function: manual control of the base signal
• Q1: 2N2222 NPN transistor, function: low-side switch to drive the motor
• M1: 9 V DC motor, function: inductive mechanical load
• D1: 1N4007 diode, function: flyback diode to suppress inductive spikes
• R1: 1 kΩ resistor, function: base current limiting resistor
• R2: 10 kΩ resistor, function: pull-down resistor for the control signal

Wiring guide

• V1: connects between nodes 9 V_PWR and 0
• V2: connects between nodes 5 V_CTRL and 0
• SW1: connects between nodes 5 V_CTRL and CTRL_IN
• R2: connects between nodes CTRL_IN and 0
• R1: connects between nodes CTRL_IN and BASE
• Q1: Collector connects to node COLLECTOR, Base connects to node BASE, Emitter connects to node 0
• M1: connects between nodes 9 V_PWR and COLLECTOR
• D1: Anode connects to node COLLECTOR, Cathode connects to node 9 V_PWR

Conceptual block diagram

Schematic

[ 5 V_CTRL ] --> [ SW1 ] --(CTRL_IN)--+--> [ R1: 1 kΩ ] --(BASE)--> [ Q1:Base ]
                                           |                                |
                                       [ R2: 10 kΩ ]                         |
                                           |                                |
                                          GND                               |
                                                                            |
      [ 9 V_PWR ] --+--> [ M1: 9 V Motor ] -----------------+--(COLLECTOR)--> [ Q1:Collector ] --( )-- [ Q1:Emitter ] --> GND
                   |                                      |
                   +--> [ D1: 1N4007 (Cath->Anode) ] -----+

Electrical diagram

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Measurements and tests

1. Verify Control Signal: Close SW1. Measure the voltage at node CTRL_IN with respect to node 0. It should read 5 V. When open, it should read 0 V due to the pull-down resistor R2.
2. Measure Base-Emitter Voltage (VBE): With SW1 closed, place your multimeter probes across node BASE and node 0. You should measure approximately 0.7 V, confirming the transistor’s base-emitter junction is forward-biased.
3. Measure Collector-Emitter Voltage (VCE): With the motor running (SW1 closed), measure the voltage between node COLLECTOR and node 0. A reading of around 0.2 V indicates the transistor is correctly operating in the saturation region. When SW1 is open, this voltage should rise to 9 V.
4. Measure Base Current (IB): Set your multimeter to measure current (mA range) and place it in series between R1 and node BASE. You should measure a small current (around 4.3 mA).
5. Measure Collector Current (IC): Place your ammeter in series between M1 and node COLLECTOR. You will measure the actual current drawn by the motor (which could range from tens to hundreds of mA depending on the specific motor).

SPICE netlist and simulation

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

* DC Motor Control with a Transistor
.width out=256

* Power Supplies
V1 9V_PWR 0 DC 9
V2 5V_CTRL 0 DC 5

* Switch SW1 modeled as a voltage-controlled switch to simulate user interaction
S1 5V_CTRL CTRL_IN SW_CTRL 0 mySW
.model mySW SW(Vt=2.5 Vh=0.5 Ron=0.1 Roff=100MEG)

* Control signal to simulate the user pressing the switch
V_SW_CTRL SW_CTRL 0 PULSE(0 5 10m 1u 1u 245m 1s)

* Resistors
R2 CTRL_IN 0 10k
R1 CTRL_IN BASE 1k

* Transistor Q1 (Low-side switch)
Q1 COLLECTOR BASE 0 2N2222MOD
* ... (truncated in public view) ...

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* DC Motor Control with a Transistor
.width out=256

* Power Supplies
V1 9V_PWR 0 DC 9
V2 5V_CTRL 0 DC 5

* Switch SW1 modeled as a voltage-controlled switch to simulate user interaction
S1 5V_CTRL CTRL_IN SW_CTRL 0 mySW
.model mySW SW(Vt=2.5 Vh=0.5 Ron=0.1 Roff=100MEG)

* Control signal to simulate the user pressing the switch
V_SW_CTRL SW_CTRL 0 PULSE(0 5 10m 1u 1u 245m 1s)

* Resistors
R2 CTRL_IN 0 10k
R1 CTRL_IN BASE 1k

* Transistor Q1 (Low-side switch)
Q1 COLLECTOR BASE 0 2N2222MOD

* Motor M1 modeled as a series inductor and resistor representing the inductive mechanical load
LM1 9V_PWR M1_INT 1mH
RM1 M1_INT COLLECTOR 20

* Flyback diode D1
D1 COLLECTOR 9V_PWR 1N4007MOD

* Component Models
.model 2N2222MOD NPN(IS=1E-14 VAF=100 BF=200 IKF=0.3 XTB=1.5 BR=3 CJC=8E-12 CJE=25E-12 TR=100E-9 TF=400E-12 ITF=1 VTF=2 XTF=3 RB=10 RC=0.3 RE=0.2)
.model 1N4007MOD D(IS=7.02767n RS=0.0341512 N=1.80803 EG=1.11 XTI=3.0 BV=1000 IBV=5e-08 CJO=1e-11 VJ=0.7 M=0.5 FC=0.5 TT=1e-07)

* Simulation Commands
.op
.tran 0.1m 250m
.print tran V(CTRL_IN) V(COLLECTOR) V(BASE) I(LM1)
.end

Simulation Results (Transient Analysis)

Analysis: The simulation shows that when the control signal (v(ctrl_in)) goes high to ~5V at t=10ms, the transistor turns on, pulling the collector voltage down from 9V to ~1.64V. The base voltage rises to ~0.94V, and the motor current (lm1#branch) ramps up to ~368mA, indicating successful motor activation.

Show raw data table (2541 rows)

Index   time            v(ctrl_in)      v(collector)    v(base)         lm1#branch
0	0.000000e+00	5.000400e-04	9.000000e+00	5.000490e-04	1.799750e-11
1	1.000000e-06	5.000400e-04	9.000000e+00	5.000490e-04	1.800624e-11
2	2.000000e-06	5.000400e-04	9.000000e+00	5.000490e-04	1.800815e-11
3	4.000000e-06	5.000400e-04	9.000000e+00	5.000490e-04	1.800528e-11
4	8.000000e-06	5.000400e-04	9.000000e+00	5.000490e-04	1.799050e-11
5	1.600000e-05	5.000400e-04	9.000000e+00	5.000490e-04	1.798412e-11
6	3.200000e-05	5.000400e-04	9.000000e+00	5.000490e-04	1.797999e-11
7	6.400000e-05	5.000400e-04	9.000000e+00	5.000490e-04	1.798801e-11
8	1.280000e-04	5.000400e-04	9.000000e+00	5.000490e-04	1.797977e-11
9	2.280000e-04	5.000400e-04	9.000000e+00	5.000490e-04	1.799637e-11
10	3.280000e-04	5.000400e-04	9.000000e+00	5.000490e-04	1.799685e-11
11	4.280000e-04	5.000400e-04	9.000000e+00	5.000490e-04	1.799640e-11
12	5.280000e-04	5.000400e-04	9.000000e+00	5.000490e-04	1.799689e-11
13	6.280000e-04	5.000400e-04	9.000000e+00	5.000490e-04	1.799636e-11
14	7.280000e-04	5.000400e-04	9.000000e+00	5.000490e-04	1.799685e-11
15	8.280000e-04	5.000400e-04	9.000000e+00	5.000490e-04	1.799639e-11
16	9.280000e-04	5.000400e-04	9.000000e+00	5.000490e-04	1.799690e-11
17	1.028000e-03	5.000400e-04	9.000000e+00	5.000490e-04	1.799645e-11
18	1.128000e-03	5.000400e-04	9.000000e+00	5.000490e-04	1.799690e-11
19	1.228000e-03	5.000400e-04	9.000000e+00	5.000490e-04	1.799640e-11
20	1.328000e-03	5.000400e-04	9.000000e+00	5.000490e-04	1.799689e-11
21	1.428000e-03	5.000400e-04	9.000000e+00	5.000490e-04	1.799641e-11
22	1.528000e-03	5.000400e-04	9.000000e+00	5.000490e-04	1.799690e-11
23	1.628000e-03	5.000400e-04	9.000000e+00	5.000490e-04	1.799640e-11
... (2517 more rows) ...

Common mistakes and how to avoid them

• Omitting the flyback diode (D1): A DC motor is an inductive load. When the transistor turns off, the collapsing magnetic field creates a massive voltage spike. Without the diode, this spike will instantly destroy the transistor. Always place a diode in parallel with the motor, reverse-biased relative to the normal current flow.
• Forgetting the base resistor (R1): Connecting a 5 V control signal directly to the transistor’s base will draw excessive current, immediately destroying the control source (e.g., your microcontroller) or the transistor. Always use a current-limiting resistor.
• Swapping the Collector and Emitter pins: Inserting the NPN transistor backward will result in very poor current gain (hFE). The motor may barely turn, and the transistor will heat up significantly because it cannot fully saturate. Double-check the datasheet for your specific transistor’s pinout.

Troubleshooting

• Symptom: The motor does not spin when the switch is closed.
  • Cause: The transistor is not turning on, or the motor lacks power.
  • Fix: Measure the voltage at node BASE. If it is 0 V, check your switch SW1 and resistor R1. Measure node 9 V_PWR to ensure the main power supply is active.
• Symptom: The transistor becomes extremely hot very quickly.
  • Cause: The transistor is operating in the active/linear region instead of fully saturating, usually because the base current (IB) is too low for the required collector current (IC).
  • Fix: Calculate the required base current (IC / hFE). If the current is too low, reduce the value of R1 (e.g., to 470 Ω or 330 Ω) to allow more base current, ensuring saturation.
• Symptom: The microcontroller resets or behaves erratically when the motor turns on/off.
  • Cause: Electrical noise from the motor brushes or voltage drops on the power line.
  • Fix: Ensure the motor power supply (V1) is completely separate from the control logic supply (V2), sharing only the ground (0) connection. Add a 100 nF ceramic capacitor across the motor terminals to suppress brush noise.

Possible improvements and extensions

• PWM Speed Control: Replace the manual switch (SW1) with a Pulse Width Modulation (PWM) signal from a microcontroller. By rapidly turning the transistor on and off, you can smoothly control the rotational speed of the motor rather than just having it on or off.
• Optoisolation for superior safety: Introduce an optocoupler between the control signal and the transistor base. This physically separates the low-voltage control circuit from the higher-voltage motor circuit using light, providing total electrical isolation and preventing catastrophic failures from reaching your logic board.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

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

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Who we are

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).

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

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)

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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* 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)

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

Carlos Núñez Zorrilla

Electronics & Computer Engineer

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

Follow me:

Who we are

Level: Basic. Verify the operation of a 4-bit binary counter by visualizing the counting sequence with LEDs.

Objective and use case

In this practical case, you will build a synchronous digital circuit using the 74HC161 integrated circuit to count clock pulses in binary from 0 (0000) to 15 (1111). You will visualize the output states using four LEDs representing the bits from LSB (Least Significant Bit) to MSB (Most Significant Bit).

Why it is useful:
* Digital Clocks: It forms the fundamental building block for tracking time (seconds, minutes, hours).
* Frequency Division: Counters are used to reduce high-frequency clock signals to lower, usable frequencies for other components.
* Memory Addressing: In computing systems, counters generate sequential addresses to access data in memory.
* Event Counting: Useful for industrial automation to count items on a conveyor belt or sensor triggers.
* State Machines: Provides the sequence of states required for controlling complex digital logic operations.

Expected outcome:
* Four LEDs (D1–D4) will light up in a binary pattern (0000, 0001, 0010… 1111).
* The sequence repeats every 16 clock pulses.
* Activating the reset switch forces all LEDs to turn OFF immediately.
* Logic High output voltage approx. 5 V; Logic Low approx. 0 V.

Target audience and level:
Students and hobbyists familiar with basic logic levels entering sequential logic design.

Materials

• U1: 74HC161, function: 4-bit synchronous binary counter IC
• V1: 5 V DC supply, function: main power source
• V2: Pulse voltage source (0 V to 5 V), function: Clock signal (1 Hz for visualization)
• R1: 330 Ω resistor, function: current limiting for LED Q0
• R2: 330 Ω resistor, function: current limiting for LED Q1
• R3: 330 Ω resistor, function: current limiting for LED Q2
• R4: 330 Ω resistor, function: current limiting for LED Q3
• R5: 10 kΩ resistor, function: pull-up for Master Reset
• D1: Red LED, function: Indicator for Q0 (LSB)
• D2: Red LED, function: Indicator for Q1
• D3: Red LED, function: Indicator for Q2
• D4: Red LED, function: Indicator for Q3 (MSB)
• S1: Momentary push button (normally open), function: Reset trigger

Pin-out of the IC used

Selected Chip: 74HC161 (4-bit Synchronous Binary Counter, Asynchronous Reset)

Note: Pins 3, 4, 5, 6 (Parallel Data Inputs) and 15 (Ripple Carry Output) are not used in this basic counting configuration and inputs can be tied to ground if preferred, though usually irrelevant when Load is disabled.

Wiring guide

Construct the circuit following these explicit node connections:

• Power Nodes:

  • Connect V1 positive terminal to node VCC.
  • Connect V1 negative terminal to node 0 (GND).
  • Connect U1 pin 16 to VCC.
  • Connect U1 pin 8 to 0.

• Control Inputs:

  • Connect V2 (Clock Source) positive to node CLK. Connect V2 negative to 0.
  • Connect U1 pin 2 to node CLK.
  • Connect U1 pins 7 (CEP), 10 (CET), and 9 (\overlinePE) directly to VCC to enable counting and disable parallel loading.
  • Reset Circuit: Connect R5 between VCC and node RESET_N. Connect S1 between node RESET_N and 0. Connect U1 pin 1 to RESET_N.

• Outputs (LED Indicators):

  • Bit 0 (LSB): Connect U1 pin 14 to node Q0. Connect R1 between Q0 and node LED_A1. Connect D1 anode to LED_A1 and cathode to 0.
  • Bit 1: Connect U1 pin 13 to node Q1. Connect R2 between Q1 and node LED_A2. Connect D2 anode to LED_A2 and cathode to 0.
  • Bit 2: Connect U1 pin 12 to node Q2. Connect R3 between Q2 and node LED_A3. Connect D3 anode to LED_A3 and cathode to 0.
  • Bit 3 (MSB): Connect U1 pin 11 to node Q3. Connect R4 between Q3 and node LED_A4. Connect D4 anode to LED_A4 and cathode to 0.

Conceptual block diagram

Schematic

+-------------------------------------------------------------------------------------------------------+
|                                  PRACTICAL CASE: 4-BIT UP COUNTER                                     |
+-------------------------------------------------------------------------------------------------------+

      INPUTS & CONTROL                     PROCESSING (U1)                     OUTPUTS & LOAD
   (Left-to-Right Flow)                   (74HC161 Counter)                 (LED Visualization)

                                     +-------------------------+
                                     |                         |
[ V2: Clock Source ] --(CLK 1Hz)---> | [Pin 2] CP              |
                                     |                         |
                                     |                         |          (Bit 0 - LSB)
[ Reset Logic ]                      |             [Pin 14] Q0 | --(Q0)--> [ R1: 330 ] --> [ D1: Red ] --> GND
(VCC->R5->Node->S1->GND) --(RST_N)-> | [Pin 1] ~MR             |
                                     |                         |
                                     |                         |          (Bit 1)
                                     |             [Pin 13] Q1 | --(Q1)--> [ R2: 330 ] --> [ D2: Red ] --> GND
[ VCC: 5 V Source ] --(Enable High)-> | [Pin 7]  CEP            |
                   --(Enable High)-> | [Pin 10] CET            |
                   --(Disable Load)> | [Pin 9]  ~PE            |          (Bit 2)
                                     |             [Pin 12] Q2 | --(Q2)--> [ R3: 330 ] --> [ D3: Red ] --> GND
                                     |                         |
                                     |                         |
                                     |                         |          (Bit 3 - MSB)
                                     |             [Pin 11] Q3 | --(Q3)--> [ R4: 330 ] --> [ D4: Red ] --> GND
                                     |                         |
                                     +-------------------------+
                                            |           |
                                         [Pin 16]    [Pin 8]
                                            |           |
                                           VCC         GND

Measurements and tests

1. Supply Check: Before connecting the IC, measure voltage between VCC and 0 to ensure it is stable at 5 V.
2. Clock Verification: Set V2 to a low frequency (e.g., 1 Hz). Verify the signal at node CLK oscillates between 0 V and 5 V.
3. Sequence Observation: Power on the circuit. Observe D1 through D4. They should toggle in the binary sequence:
   • 0: All OFF
   • 1: D1 ON
   • 2: D2 ON
   • 3: D1 & D2 ON
   • … up to 15: All ON.
4. Reset Test: While the counter is running, press S1. All LEDs must turn OFF immediately (Asynchronous Reset) or at the next clock edge (if using a synchronous reset variant, though standard 74HC161 Reset is usually asynchronous).
5. Logic Levels: Use a multimeter to measure node Q3 when D4 is lit. It should read close to 5 V (Logic High).

SPICE netlist and simulation

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

* Practical case: 4-bit up counter with LEDs (74HC161)
* NGSPICE Netlist
* Requires XSPICE extensions

.width out=256

* --- Power Supplies ---
V1 VCC 0 DC 5
* Clock Source: 1 Hz Pulse (0V to 5V), 50% duty cycle
* Corrected to 1 Hz per BOM (Period = 1s, Pulse Width = 0.5s)
V2 CLK 0 PULSE(0 5 0 1u 1u 0.5 1)

* --- Reset Circuit ---
* Pull-up resistor for Master Reset
R5 VCC RESET_N 10k
* S1: Momentary Push Button (Normally Open)
* Implemented as a Voltage Controlled Switch driven by V_BTN source
S1 RESET_N 0 CTRL_RST 0 SW_BTN
* Button Actuator (Simulates a press at 8s for 1s duration to test reset)
V_BTN CTRL_RST 0 PULSE(0 1 8 1m 1m 1 20)
* ... (truncated in public view) ...

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* Practical case: 4-bit up counter with LEDs (74HC161)
* NGSPICE Netlist
* Requires XSPICE extensions

.width out=256

* --- Power Supplies ---
V1 VCC 0 DC 5
* Clock Source: 1 Hz Pulse (0V to 5V), 50% duty cycle
* Corrected to 1 Hz per BOM (Period = 1s, Pulse Width = 0.5s)
V2 CLK 0 PULSE(0 5 0 1u 1u 0.5 1)

* --- Reset Circuit ---
* Pull-up resistor for Master Reset
R5 VCC RESET_N 10k
* S1: Momentary Push Button (Normally Open)
* Implemented as a Voltage Controlled Switch driven by V_BTN source
S1 RESET_N 0 CTRL_RST 0 SW_BTN
* Button Actuator (Simulates a press at 8s for 1s duration to test reset)
V_BTN CTRL_RST 0 PULSE(0 1 8 1m 1m 1 20)
.model SW_BTN sw(vt=0.5 ron=1 roff=10Meg)

* --- 74HC161 4-bit Binary Counter Subcircuit Instance ---
* Connections match Wiring Guide:
* Pin 1 (MR_N) -> RESET_N
* Pin 2 (CP) -> CLK
* Pin 3-6 (D0-D3) -> 0 (GND)
* Pin 7 (CEP) -> VCC
* Pin 8 (GND) -> 0
* Pin 9 (PE_N) -> VCC
* Pin 10 (CET) -> VCC
* Pin 11-14 (Q3-Q0) -> Output Nodes
* Pin 15 (TC) -> TC_NC (Floating)
* Pin 16 (VCC) -> VCC
XU1 RESET_N CLK 0 0 0 0 VCC 0 VCC VCC Q3 Q2 Q1 Q0 TC_NC VCC 74HC161

* --- LED Output Indicators ---
* Bit 0 (LSB)
R1 Q0 LED_A1 330
D1 LED_A1 0 LED_RED
* Bit 1
R2 Q1 LED_A2 330
D2 LED_A2 0 LED_RED
* Bit 2
R3 Q2 LED_A3 330
D3 LED_A3 0 LED_RED
* Bit 3 (MSB)
R4 Q3 LED_A4 330
D4 LED_A4 0 LED_RED

* --- Models ---
.model LED_RED D(Is=1e-14 Rs=5 N=2)

* --- Subcircuit Definition: 74HC161 ---
* Behavioral XSPICE implementation of a 4-bit Counter with Async Reset
.subckt 74HC161 MR_N CP D0 D1 D2 D3 CEP GND PE_N CET Q3 Q2 Q1 Q0 TC VCC
    * XSPICE Models
    .model adc_in adc_bridge(in_low=2.0 in_high=3.0)
    .model dac_out dac_bridge(out_low=0.0 out_high=5.0)
    .model dff_mod d_dff(rise_delay=10n fall_delay=10n)
    .model inv_mod d_inverter(rise_delay=5n fall_delay=5n)

    * Input Bridges (Analog to Digital)
    A_IN [MR_N CP] [mr_dig cp_dig] adc_in

    * Reset Logic (MR_N is active low, d_dff reset is active high)
    A_RST_INV mr_dig rst_high inv_mod

    * Counter Chain (Ripple Up Counter)
    * Bit 0: Toggles on CP rising edge
    A_D0 q0_inv cp_dig NULL rst_high q0_dig q0_inv dff_mod

    * Bit 1: Toggles on Q0 falling edge (Q0_inv rising edge)
    A_D1 q1_inv q0_inv NULL rst_high q1_dig q1_inv dff_mod

    * Bit 2: Toggles on Q1 falling edge
    A_D2 q2_inv q1_inv NULL rst_high q2_dig q2_inv dff_mod

    * Bit 3: Toggles on Q2 falling edge
    A_D3 q3_inv q2_inv NULL rst_high q3_dig q3_inv dff_mod

    * Output Bridges (Digital to Analog)
    A_OUT [q3_dig q2_dig q1_dig q0_dig] [Q3 Q2 Q1 Q0] dac_out

    * Terminal Count (Unused/Dummy pull-down)
    R_TC TC 0 100k
.ends

* --- Simulation Commands ---
* Transient analysis: 20s duration to capture full counting cycle (0-15) at 1 Hz
.op
.tran 10m 20s

* Print critical signals (Inputs first)
.print tran V(CLK) V(RESET_N) V(Q0) V(Q1) V(Q2) V(Q3)

.end

Simulation Results (Transient Analysis)

Analysis: The simulation shows a correct 4-bit binary counting sequence (0000 to 1111) on outputs Q0-Q3. The clock toggles at 1Hz. The reset button press at 8s is simulated, but the log data shows RESET_N remaining high (~4.99V) throughout the sampled points, suggesting the reset event might have been missed in the condensed log or the switch resistance ratio wasn’t sufficient to pull the node to logic low in the analog domain against the pull-up, although the counter continues counting correctly.

Show raw data table (3020 rows)

Index   time            v(clk)          v(reset_n)      v(q0)           v(q1)           v(q2)           v(q3)
0	0.000000e+00	0.000000e+00	4.995005e+00	0.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
1	1.000000e-08	5.000000e-02	4.995005e+00	0.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
2	2.000000e-08	1.000000e-01	4.995005e+00	0.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
3	4.000000e-08	2.000000e-01	4.995005e+00	0.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
4	8.000000e-08	4.000000e-01	4.995005e+00	0.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
5	1.600000e-07	8.000000e-01	4.995005e+00	0.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
6	3.200000e-07	1.600000e+00	4.995005e+00	0.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
7	6.400000e-07	3.200000e+00	4.995005e+00	0.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
8	6.520000e-07	3.260000e+00	4.995005e+00	0.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
9	6.760000e-07	3.380000e+00	4.995005e+00	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
10	7.240000e-07	3.620000e+00	4.995005e+00	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
11	8.200000e-07	4.100000e+00	4.995005e+00	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
12	1.000000e-06	5.000000e+00	4.995005e+00	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
13	1.019200e-06	5.000000e+00	4.995005e+00	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
14	1.057600e-06	5.000000e+00	4.995005e+00	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
15	1.134400e-06	5.000000e+00	4.995005e+00	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
16	1.288000e-06	5.000000e+00	4.995005e+00	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
17	1.595200e-06	5.000000e+00	4.995005e+00	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
18	2.209600e-06	5.000000e+00	4.995005e+00	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
19	3.438400e-06	5.000000e+00	4.995005e+00	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
20	5.896000e-06	5.000000e+00	4.995005e+00	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
21	1.081120e-05	5.000000e+00	4.995005e+00	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
22	2.064160e-05	5.000000e+00	4.995005e+00	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
23	4.030240e-05	5.000000e+00	4.995005e+00	5.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
... (2996 more rows) ...

Common mistakes and how to avoid them

1. Leaving Enable pins floating: The 74HC series has high impedance inputs. If pins 7 (CEP) or 10 (CET) are not connected to VCC, the counter may not count or behave erratically. Always tie unused control inputs to a defined logic level.
2. Clock frequency too high: If V2 is set to 1 kHz or higher, all LEDs will appear to be dimly lit continuously due to persistence of vision. Keep the clock below 5 Hz for visual debugging.
3. Floating Parallel Load pin: If pin 9 (\overlinePE) is left floating or low, the chip might constantly try to load data from inputs P0-P3 instead of counting. Ensure pin 9 is tied to VCC.

Troubleshooting

• LEDs never turn on: Check power supply connections to pins 16 and 8. Ensure LEDs are inserted with the correct polarity (anode to resistor/IC, cathode to ground).
• Counter stays at zero: Verify that the Reset pin (1) is High (5 V). If S1 is stuck or the pull-up R5 is missing, the chip remains in Reset state. Also, check that Enable pins (7, 10) are High.
• Counter skips numbers: This is often due to «switch bounce» if you are using a mechanical switch as a manual clock. Use a clean square wave generator or a debounce circuit (capacitor + resistor) for the clock input.
• Random sequence: Check if the Parallel Enable (\overlinePE) pin is accidentally Low or floating. It must be High.

Possible improvements and extensions

1. 8-bit Counter: Cascade a second 74HC161 by connecting the Carry Output (pin 15) of the first counter to the Enable Trickle (pin 10) of the second counter. This allows counting up to 255.
2. Manual Clock: Replace the frequency generator V2 with a 555 timer circuit in astable mode or a debounced push-button to advance the count manually.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

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

Follow me:

Who we are

Level: Basic. Build a secure electronic lock that keeps a solenoid active for a few seconds after two keys are turned simultaneously.

Objective and use case

In this practical case, you will build a security circuit that requires two distinct inputs (keys/buttons) to be activated simultaneously to trigger a high-power mechanism. Once triggered, the system includes an analog memory (RC network) to hold the lock open for a short duration, allowing a user to open the door.

• Real-world scenarios:

  • Bank Vaults: Requires two bank managers to turn keys at the same time to prevent theft.
  • Industrial Presses: Requires an operator to press buttons with both hands to ensure safety before the machine engages.
  • Secure Entryways: Allows a door strike to remain unlatched for 5 seconds after authorization.

• Expected outcome:

  • Logic: The load (Solenoid/LED) remains OFF if only one button is pressed.
  • Activation: The load turns ON fully only when both SW1 and SW2 are held.
  • Timing: Upon releasing the buttons, the load remains ON for approximately 2 to 5 seconds before fading out.
  • Target audience: Basic electronics students focusing on transistor switching and RC time constants.

Materials

• V1: 12 V DC supply, function: Main power source.
• SW1: Push button (Normally Open), function: Security Key 1.
• SW2: Push button (Normally Open), function: Security Key 2.
• R1: 1 kΩ resistor, function: Current limiter for capacitor charging (protection).
• R2: 47 kΩ resistor, function: Discharge timing resistor (Bleeder).
• C1: 100 µF electrolytic capacitor, function: Energy storage for time delay.
• Q1: IRF540 N-Channel MOSFET, function: Power switch for the load.
• L1: 10 mH inductor, function: Solenoid coil simulation.
• R3: 10 Ω resistor, function: Internal resistance of the solenoid.
• D1: 1N4007 Diode, function: Flyback protection against inductive voltage spikes.

Wiring guide

This guide uses the node names 12 V, 0 (Ground), Mid_Switch, Gate_Node, and Drain_Node.

• Logic Stage (Series AND):

  • V1 (Positive) connects to SW1 (Input).
  • SW1 (Output) connects to Mid_Switch.
  • SW2 (Input) connects to Mid_Switch.
  • SW2 (Output) connects to R1 (Input).

• Timing Stage (RC Hold):

  • R1 (Output) connects to Gate_Node.
  • C1 (Positive) connects to Gate_Node.
  • C1 (Negative) connects to 0.
  • R2 connects between Gate_Node and 0 (Parallel to C1).
  • Q1 (Gate) connects to Gate_Node.

• Power Stage:

  • Q1 (Source) connects to 0.
  • Q1 (Drain) connects to Drain_Node.
  • L1 and R3 (representing the Solenoid) are connected in series between 12 V and Drain_Node.
  • D1 (Cathode) connects to 12 V.
  • D1 (Anode) connects to Drain_Node (across the load).

Conceptual block diagram

Schematic

Title: Practical case: Vault Lock with Delay and Power Drive

(1) LOGIC & TIMING STAGE
------------------------
                                                                    (Gate_Node)
[ 12 V ] --(Logic)--> [ SW1 ] --> [ SW2 ] --> [ R1: 1k ] --+------------+----------> [ Q1:Gate ]
                                                          |            |                |
                                                          |            |                |
                                                          v            v                |
                                                    [ C1: 100uF ]  [ R2: 47k ]          |
                                                          |            |                |
                                                          v            v                |
                                                         GND          GND               |
                                                                                        |
(2) POWER DRIVE STAGE                                                                   |
---------------------                                                                   |
                                                                                        |
[ 12 V ] --(Power)-----------------------------------------+                             |
   |                                                      |                             |
   |                                                      v                             |
   |                                              [ Solenoid (L1+R3) ]                  |
   |                                                      |                             |
   |                                                      v                             |
   +----(Cathode)-- [ D1: Flyback ] --(Anode)----> (Drain_Node) ----> [ Q1:Drain ]      |
                                                                            |           |
                                                                            +-----------+
                                                                            |
                                                                      (Internal FET)
                                                                            |
                                                                            v
                                                                      [ Q1:Source ]
                                                                            |
                                                                            v
                                                                           GND

Electrical diagram

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Measurements and tests

Validate the circuit operation using a multimeter or oscilloscope:

1. Logic Verification: Press SW1 only. Measure voltage at Gate_Node. It should be 0 V. Repeat for SW2 only. The load should remain OFF.
2. Activation: Press SW1 and SW2 simultaneously. Measure voltage at Gate_Node. It should rise immediately to approx 12 V. The Solenoid (Load) should activate.
3. Hold Time (Delay): Release both buttons simultaneously. Watch the load.
   • The voltage at Gate_Node will begin to drop.
   • The Solenoid should remain active.
   • Measure the time it takes for the load to turn off (typically when Gate voltage drops below the MOSFET Threshold, ~3-4 V). With 47 kΩ and 100µF, this should be roughly 3 to 5 seconds.
4. Flyback Check: (Oscilloscope only) Monitor Drain_Node when the transistor turns off. You should not see a massive voltage spike above 12 V, confirming D1 is clamping the inductive kickback.

SPICE netlist and simulation

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

* Practical case: Vault Lock with Delay and Power Drive
.width out=256

* --- Models ---
* Generic Switch Model for Push Buttons
.model SW_push SW(Vt=2.5 Ron=0.01 Roff=100Meg)

* Power MOSFET Model (Approximation of IRF540)
* N-Channel, Threshold ~4V, Low Rds(on)
.model IRF540 NMOS(Level=1 Vto=4.0 Kp=20 Lambda=0.001 Rd=0.05 Rs=0.05)

* Diode Model (1N4007)
.model D1N4007 D(Is=14.11n N=1.984 Rs=33.89m Ikf=100m Cjo=20p M=0.3333 Vj=0.75 Bv=1000 Ibv=10u)

* --- Main Power Supply ---
V1 12V 0 DC 12

* --- User Interface (Push Buttons) ---
* We simulate physical button presses using Pulse Voltage Sources controlling switches.
* Logic: To unlock, SW1 and SW2 must be pressed simultaneously (AND logic).
* ... (truncated in public view) ...

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* Practical case: Vault Lock with Delay and Power Drive
.width out=256

* --- Models ---
* Generic Switch Model for Push Buttons
.model SW_push SW(Vt=2.5 Ron=0.01 Roff=100Meg)

* Power MOSFET Model (Approximation of IRF540)
* N-Channel, Threshold ~4V, Low Rds(on)
.model IRF540 NMOS(Level=1 Vto=4.0 Kp=20 Lambda=0.001 Rd=0.05 Rs=0.05)

* Diode Model (1N4007)
.model D1N4007 D(Is=14.11n N=1.984 Rs=33.89m Ikf=100m Cjo=20p M=0.3333 Vj=0.75 Bv=1000 Ibv=10u)

* --- Main Power Supply ---
V1 12V 0 DC 12

* --- User Interface (Push Buttons) ---
* We simulate physical button presses using Pulse Voltage Sources controlling switches.
* Logic: To unlock, SW1 and SW2 must be pressed simultaneously (AND logic).
V_act1 Ctrl1 0 PULSE(0 5 1 1m 1m 3 10)
V_act2 Ctrl2 0 PULSE(0 5 2.5 1m 1m 3 10)

* --- Logic Stage (Series AND) ---
* SW1 connects 12V to Mid_Switch
S1 12V Mid_Switch Ctrl1 0 SW_push

* SW2 connects Mid_Switch to R1 Input
S2 Mid_Switch Pre_R1 Ctrl2 0 SW_push

* --- Timing Stage (RC Hold) ---
* R1: Current limiter for charging
R1 Pre_R1 Gate_Node 1k

* C1: Energy storage (Timing capacitor)
C1 Gate_Node 0 100u

* R2: Discharge timing resistor (Bleeder)
* Time Constant (Discharge) = 47k * 100u = 4.7 seconds
R2 Gate_Node 0 47k

* --- Power Stage ---
* Q1 renamed to M1 to match SPICE MOSFET syntax (requires M prefix for NMOS model).
* Pin order: Drain Gate Source Bulk. Bulk connected to Source (0).
M1 Drain_Node Gate_Node 0 0 IRF540

* --- Load (Solenoid Simulation) ---
* Modeled as Inductor L1 and Resistor R3 in series
L1 12V Solenoid_Mid 10mH
R3 Solenoid_Mid Drain_Node 10

* --- Protection ---
* D1: Flyback diode to suppress inductive spikes from L1 upon turn-off
* Connected Cathode to 12V, Anode to Drain
D1 Drain_Node 12V D1N4007

* --- Simulation Commands ---
.op
* Transient analysis: 10ms step for 10 seconds to capture full charge/discharge cycle
.tran 10m 10s

* --- Output ---
* Monitoring Control signals, Gate voltage (Timing), and Drain voltage (Output state)
.print tran V(Ctrl1) V(Ctrl2) V(Gate_Node) V(Drain_Node) I(L1)

.end

Simulation Results (Transient Analysis)

Analysis: The simulation confirms the intended operation. When the control signals activate the series switches (AND logic), the gate node charges to ~11.7V, turning the MOSFET ON (Drain drops to ~0.13V, Current ~1.18A). After the input pulses cease, the gate voltage decays slowly via R2. Around 9 seconds into the simulation, the gate voltage drops near the threshold (4V), and the MOSFET turns off, returning the Drain voltage to 12V.

Show raw data table (1095 rows)

Index   time            v(ctrl1)        v(ctrl2)        v(gate_node)    v(drain_node)   l1#branch
0	0.000000e+00	0.000000e+00	0.000000e+00	2.819323e-03	1.200000e+01	1.199844e-11
1	1.000000e-04	0.000000e+00	0.000000e+00	2.819323e-03	1.200000e+01	1.204503e-11
2	2.000000e-04	0.000000e+00	0.000000e+00	2.819323e-03	1.200000e+01	1.196043e-11
3	4.000000e-04	0.000000e+00	0.000000e+00	2.819323e-03	1.200000e+01	1.204260e-11
4	8.000000e-04	0.000000e+00	0.000000e+00	2.819323e-03	1.200000e+01	1.204346e-11
5	1.600000e-03	0.000000e+00	0.000000e+00	2.819323e-03	1.200000e+01	1.201220e-11
6	3.200000e-03	0.000000e+00	0.000000e+00	2.819323e-03	1.200000e+01	1.199165e-11
7	6.400000e-03	0.000000e+00	0.000000e+00	2.819323e-03	1.200000e+01	1.202979e-11
8	1.280000e-02	0.000000e+00	0.000000e+00	2.819323e-03	1.200000e+01	1.202182e-11
9	2.280000e-02	0.000000e+00	0.000000e+00	2.819323e-03	1.200000e+01	1.199840e-11
10	3.280000e-02	0.000000e+00	0.000000e+00	2.819323e-03	1.200000e+01	1.200551e-11
11	4.280000e-02	0.000000e+00	0.000000e+00	2.819323e-03	1.200000e+01	1.199929e-11
12	5.280000e-02	0.000000e+00	0.000000e+00	2.819323e-03	1.200000e+01	1.200551e-11
13	6.280000e-02	0.000000e+00	0.000000e+00	2.819323e-03	1.200000e+01	1.199929e-11
14	7.280000e-02	0.000000e+00	0.000000e+00	2.819323e-03	1.200000e+01	1.200551e-11
15	8.280000e-02	0.000000e+00	0.000000e+00	2.819323e-03	1.200000e+01	1.199929e-11
16	9.280000e-02	0.000000e+00	0.000000e+00	2.819323e-03	1.200000e+01	1.200551e-11
17	1.028000e-01	0.000000e+00	0.000000e+00	2.819323e-03	1.200000e+01	1.199929e-11
18	1.128000e-01	0.000000e+00	0.000000e+00	2.819323e-03	1.200000e+01	1.200551e-11
19	1.228000e-01	0.000000e+00	0.000000e+00	2.819323e-03	1.200000e+01	1.199929e-11
20	1.328000e-01	0.000000e+00	0.000000e+00	2.819323e-03	1.200000e+01	1.200551e-11
21	1.428000e-01	0.000000e+00	0.000000e+00	2.819323e-03	1.200000e+01	1.199929e-11
22	1.528000e-01	0.000000e+00	0.000000e+00	2.819323e-03	1.200000e+01	1.200551e-11
23	1.628000e-01	0.000000e+00	0.000000e+00	2.819323e-03	1.200000e+01	1.199929e-11
... (1071 more rows) ...

Common mistakes and how to avoid them

1. Omitting the Flyback Diode (D1):
   • Error: The MOSFET fails after a few cycles due to high voltage spikes from the solenoid.
   • Solution: Always place a diode in parallel with inductive loads, cathode to positive supply.
2. Wrong Capacitor Polarity:
   • Error: C1 explodes or heats up; circuit acts as a short.
   • Solution: Ensure the negative stripe of the electrolytic capacitor connects to Ground (0).
3. Gate Floating:
   • Error: If R2 is removed, the lock stays stuck «ON» indefinitely because the gate charge has nowhere to go.
   • Solution: Ensure R2 is connected between Gate and Ground to provide a discharge path.

Troubleshooting

• Solenoid turns off instantly (No delay):
  • Cause: C1 is too small, damaged, or R2 is too low (e.g., 1 kΩ instead of 47 kΩ).
  • Fix: Check R2 value or increase C1 capacitance.
• MOSFET gets very hot during the «OFF» transition:
  • Cause: Slow discharge causes the MOSFET to linger in the «linear region» (acting as a resistor) for too long.
  • Fix: This is expected in simple RC delay circuits. Ensure the MOSFET has a heatsink or switch to a Logic-based delay (Schmitt Trigger) for a sharper cutoff.
• Circuit never activates:
  • Cause: SW1 and SW2 are not wired in series, or MOSFET pinout (G-D-S) is incorrect.
  • Fix: Verify continuity through the switches to the Gate pin.

Possible improvements and extensions

1. Schmitt Trigger Snap-Action: Insert a Schmitt Trigger inverter (like CD40106) between the RC network and the MOSFET. This creates a clean, digital ON/OFF transition, preventing the MOSFET from heating up during the discharge phase.
2. Emergency Reset: Add a «Panic» switch (Normally Closed) in parallel with the capacitor C1. Pressing it instantly shorts the capacitor, locking the vault immediately regardless of the remaining time.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

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

Follow me:

Who we are

Level: Basic. Analyzing the instability caused by improper voltage divider inputs on digital gates.

Objective and use case

In this practical case, you will build a circuit where the input to a digital inverter (NOT gate) is held at exactly 2.5 V using a symmetrical voltage divider. This creates a «forbidden» state for 5 V logic families.

• Understanding Logic Thresholds: Learn why digital inputs need defined High and Low voltages, not just «something in the middle.»
• Diagnosing Instability: Recognize symptoms of undefined states, such as oscillation or excessive heating.
• Internal Transistor Behavior: Visualize what happens to the internal MOSFETs when the input voltage is in the «dead zone.»

Expected Outcome:
* Signal: The input voltage (Vin) measures exactly 2.5 V.
* Output: The Output LED may be dim, flickering, or stuck at an intermediate voltage (not fully 0 V or 5 V).
* Thermal: The 74HC04 chip may become slightly warm due to internal «shoot-through» current.

Target audience: Students dealing with sensor interfacing and logic levels.

Materials

• V1: 5 V DC supply, function: Main power source
• R1: 10 kΩ resistor, function: Top leg of voltage divider
• R2: 10 kΩ resistor, function: Bottom leg of voltage divider
• U1: 74HC04, function: Hex Inverter (NOT gate)
• R3: 330 Ω resistor, function: LED current limiting
• D1: Red LED, function: Logic state indicator
• C1: 100 nF capacitor, function: Power supply decoupling

Pin-out of the IC used

Chip: 74HC04 (Hex Inverter)

Wiring guide

• VCC: Connect positive terminal of V1, Pin 14 of U1, and one side of R1.
• 0 (GND): Connect negative terminal of V1, Pin 7 of U1, one side of R2, and the cathode (short leg) of D1.
• V_IN: Connect the remaining side of R1, the remaining side of R2, and Pin 1 (Input 1 A) of U1. Note: This node creates the problematic 2.5 V level.
• V_OUT: Connect Pin 2 (Output 1Y) of U1 to one side of R3.
• LED_NODE: Connect the remaining side of R3 to the anode (long leg) of D1.
• Decoupling: Connect C1 directly between Pin 14 and Pin 7 of U1.

Conceptual block diagram

Schematic

INPUT STAGE (Voltage Divider)              PROCESSING STAGE (Logic)                  OUTPUT STAGE (Load)

VCC (5 V)
   |
[ R1: 10 kΩ ]
   |
   +---------(V_IN: ~2.5 V)---------> [ U1: 74HC04 (Inverter) ] -------(V_OUT)-------> [ R3: 330 Ω ] ----> [ D1: LED ] ----> GND
   |          (Undefined Level)      [ Input: Pin 1          ]
[ R2: 10 kΩ ]                         [ Output: Pin 2         ]
   |                                 [ Power: VCC/GND + C1   ]
GND (0 V)

Electrical diagram

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Truth table

Gate: NOT (Inverter)

Measurements and tests

1. Input Voltage Check: Set your multimeter to DC Voltage. Place the red probe on node V_IN (Pin 1 of U1) and the black probe on GND. Verify the reading is approximately 2.5 V.
2. Output Observation: Look at D1. It might be glowing dimly or flickering. This indicates the output is not driving a solid Logic High or Low.
3. Output Voltage Check: Measure the voltage at V_OUT (Pin 2). It will likely not be 0 V or 5 V, but a value in between, or it may be oscillating (fluctuating reading).
4. Touch Test (Caution): Carefully touch the top of the plastic package of the 74HC04. If the chip feels warmer than ambient temperature, it is drawing excess current.

SPICE netlist and simulation

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

* Practical case: The Undefined Logic Level Danger
.width out=256

* --- Models ---
* Generic Red LED Model
.model LED_RED D(IS=1e-22 N=1.5 RS=10 BV=5 CJO=50p IBV=1u)

* Subcircuit for U1: 74HC04 Hex Inverter
* Pinout: 1=Input(A), 2=Output(Y), 7=GND, 14=VCC
* Implemented with a continuous sigmoid function to allow robust simulation 
* of the linear region (undefined state) without convergence issues.
.subckt 74HC04 1 2 7 14
B_INV 2 7 V = V(14,7) / (1 + exp(20 * (V(1,7) - 0.5*V(14,7))))
.ends

* --- Components ---

* V1: Main Power Supply
* Using PULSE to simulate power-on transient (0V to 5V)
V1 VCC 0 PULSE(0 5 1u 10u 10u 100m 200m)
* ... (truncated in public view) ...

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* Practical case: The Undefined Logic Level Danger
.width out=256

* --- Models ---
* Generic Red LED Model
.model LED_RED D(IS=1e-22 N=1.5 RS=10 BV=5 CJO=50p IBV=1u)

* Subcircuit for U1: 74HC04 Hex Inverter
* Pinout: 1=Input(A), 2=Output(Y), 7=GND, 14=VCC
* Implemented with a continuous sigmoid function to allow robust simulation 
* of the linear region (undefined state) without convergence issues.
.subckt 74HC04 1 2 7 14
B_INV 2 7 V = V(14,7) / (1 + exp(20 * (V(1,7) - 0.5*V(14,7))))
.ends

* --- Components ---

* V1: Main Power Supply
* Using PULSE to simulate power-on transient (0V to 5V)
V1 VCC 0 PULSE(0 5 1u 10u 10u 100m 200m)

* R1: Top leg of voltage divider (10k)
R1 VCC V_IN 10k

* R2: Bottom leg of voltage divider (10k)
* This creates approx 2.5V at V_IN when VCC is 5V
R2 V_IN 0 10k

* U1: 74HC04 Hex Inverter
* Connections: Pin 1=V_IN, Pin 2=V_OUT, Pin 7=0(GND), Pin 14=VCC
XU1 V_IN V_OUT 0 VCC 74HC04

* C1: Decoupling capacitor (100nF)
C1 VCC 0 100n

* R3: LED current limiting resistor (330 Ohm)
R3 V_OUT LED_NODE 330

* D1: Red LED
D1 LED_NODE 0 LED_RED

* --- Analysis ---

* Transient analysis to capture power-up and settling
* Step size 1us, Stop time 500us
.tran 1u 500u

* Print directives for simulation logging
.print tran V(V_IN) V(V_OUT) V(LED_NODE) V(VCC)

* Operating point calculation
.op

.end

Simulation Results (Transient Analysis)

Analysis: The simulation shows V_IN settling at exactly 2.5V (half of VCC). The inverter output V_OUT also settles at 2.5V, causing the LED node to sit at ~1.75V. This confirms the ‘undefined’ behavior where the output is neither clearly High nor Low.

Show raw data table (519 rows)

Index   time            v(v_in)         v(v_out)        v(led_node)     v(vcc)
0	0.000000e+00	0.000000e+00	0.000000e+00	-1.32954e-36	0.000000e+00
1	1.000000e-08	0.000000e+00	0.000000e+00	-8.37118e-37	0.000000e+00
2	2.000000e-08	0.000000e+00	0.000000e+00	-2.17031e-37	0.000000e+00
3	4.000000e-08	0.000000e+00	0.000000e+00	6.442019e-37	0.000000e+00
4	8.000000e-08	0.000000e+00	0.000000e+00	1.087387e-36	0.000000e+00
5	1.600000e-07	0.000000e+00	0.000000e+00	5.886649e-37	0.000000e+00
6	3.200000e-07	0.000000e+00	0.000000e+00	-7.16419e-38	0.000000e+00
7	6.400000e-07	0.000000e+00	0.000000e+00	-1.33719e-37	0.000000e+00
8	1.000000e-06	0.000000e+00	0.000000e+00	-1.75658e-38	0.000000e+00
9	1.005123e-06	1.280776e-03	1.280776e-03	3.255392e-04	2.561552e-03
10	1.015369e-06	3.842328e-03	3.842328e-03	1.418765e-03	7.684656e-03
11	1.035862e-06	8.965432e-03	8.965432e-03	5.258943e-03	1.793086e-02
12	1.070382e-06	1.759552e-02	1.759552e-02	1.345000e-02	3.519104e-02
13	1.105069e-06	2.626716e-02	2.626716e-02	2.210557e-02	5.253431e-02
14	1.174442e-06	4.361042e-02	4.361042e-02	3.941132e-02	8.722085e-02
15	1.313188e-06	7.829696e-02	7.829696e-02	7.402122e-02	1.565939e-01
16	1.590680e-06	1.476700e-01	1.476700e-01	1.432281e-01	2.953401e-01
17	2.145665e-06	2.864162e-01	2.864162e-01	2.815810e-01	5.728324e-01
18	3.145665e-06	5.364162e-01	5.364162e-01	5.305352e-01	1.072832e+00
19	4.145665e-06	7.864162e-01	7.864162e-01	7.789169e-01	1.572832e+00
20	5.145665e-06	1.036416e+00	1.036416e+00	1.027633e+00	2.072832e+00
21	6.145665e-06	1.286416e+00	1.286416e+00	1.276050e+00	2.572832e+00
22	7.145665e-06	1.536416e+00	1.536416e+00	1.521539e+00	3.072832e+00
23	8.145665e-06	1.786416e+00	1.786416e+00	1.662480e+00	3.572832e+00
... (495 more rows) ...

Common mistakes and how to avoid them

1. Assuming 2.5 V is «High»: Many students think any voltage > 0 V is «High.» Check the datasheet for VIH (Voltage Input High) minimum requirements (usually ~3.5 V for 5 V HC logic).
2. Using High Impedance Dividers: Using 10 kΩ/10 kΩ is fine for references, but noise can easily couple into this high impedance node, causing the gate to switch randomly.
3. Ignoring Decoupling Capacitors: In this unstable state, the chip generates noise on the power rails. Omitting C1 makes the behavior even more erratic.

Troubleshooting

• Symptom: The LED is dim or flickering rapidly.
  • Cause: The input is in the «linear region» or «forbidden zone.» The internal transistors are amplifying noise.
  • Fix: Adjust the input voltage to be clearly valid (e.g., tie Input to VCC or GND directly to test).
• Symptom: The chip is getting hot, but the LED works.
  • Cause: Shoot-through current. Inside the chip, both the P-MOSFET and N-MOSFET of the input stage are partially conducting because 2.5 V biases both of them ON. This creates a short circuit from VCC to GND inside the silicon.
  • Fix: Never leave a CMOS input at an intermediate voltage.
• Symptom: Voltage at V_IN is not exactly 2.5 V.
  • Cause: Resistor tolerance (e.g., 5% or 10% resistors) or multimeter loading.
  • Fix: Measure R1 and R2 values independently or verify with a precision multimeter.

Possible improvements and extensions

1. Hysteresis implementation: Replace the 74HC04 with a 74HC14 (Schmitt Trigger Inverter). Observe how the Schmitt trigger handles the 2.5 V input (it will stay in the previous state until a specific threshold is crossed) without oscillating.
2. Variable Input: Replace the fixed resistors R1/R2 with a 10 kΩ potentiometer. Sweep the voltage from 0 V to 5 V while measuring the supply current (Amperage). You will see a spike in current exactly around the 2.5 V transition point.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

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

Follow me:

Who we are

Level: Basic. Learn to identify and fix an NPN transistor stuck in the active region.

Objective and use case

In this practical case, you will build a standard Low-Side Switch using a BJT (Bipolar Junction Transistor) to control a high-current load. However, the circuit will contain a deliberate flaw in the base resistor selection to demonstrate the difference between the Active Region and Saturation.

• Understanding Transistor Modes: Learn why a transistor acts as a resistor instead of a switch if not biased correctly.
• Power Dissipation: Understand why partially open transistors overheat.
• Troubleshooting: Practice measuring VCE to diagnose switching efficiency.

Expected outcome:
* Initially, the high-current LED will be surprisingly dim.
* Voltage measurement across the transistor (VCE) will be high (> 2 V).
* After the fix, the LED will be bright, and VCE will drop to near 0 V.
* Target audience: Beginners and students familiar with basic Ohm’s Law.

Materials

• V1: 5 V DC Power Supply, function: Main circuit power.
• Q1: 2N2222 NPN Transistor, function: Low-side switch.
• D1: High-Brightness White LED, function: The heavy load (requires approx. 80-100 mA).
• R1: 33 Ω resistor (1/2 Watt), function: LED current limiting (Rload).
• R2: 100 kΩ resistor, function: Incorrect Base resistor (Test Case).
• R3: 1 kΩ resistor, function: Correct Base resistor (Solution).
• S1: SPST Switch or Jumper wire, function: Input control.

Wiring guide

Construct the circuit using the following netlist connections. Pay attention to the node names.

• V1 (5 V) connects to node VCC.
• V1 (GND) connects to node 0.
• S1 connects between VCC and node SWITCH_OUT.
• R2 (100 kΩ) connects between SWITCH_OUT and node BASE.
• Q1 Base connects to node BASE.
• Q1 Emitter connects to node 0 (GND).
• Q1 Collector connects to node COLLECTOR.
• D1 Anode connects to node VCC.
• D1 Cathode connects to node LED_CATHODE.
• R1 (33 Ω) connects between LED_CATHODE and COLLECTOR.

Conceptual block diagram

Schematic

Title: Practical case: NPN Switch Saturation Troubleshooting

(1) CONTROL PATH (Base Current Drive)
    VCC --> [ S1: Switch ] --(SWITCH_OUT)--> [ R2: 100k ] --(BASE)--> [ Q1: Base ]
                                                                           |
                                                                    (Activates Switch)
                                                                           |
                                                                           V

(2) POWER PATH (High Current Load)
    VCC --> [ D1: LED ] --(LED_CATHODE)--> [ R1: 33 Ohm ] --(COLLECTOR)--> [ Q1: Collector ]
                                                                                 |
                                                                           (Current Flow)
                                                                                 |
                                                                                 V
                                                                           [ Q1: Emitter ] --> GND

Electrical diagram

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Measurements and tests

Follow this procedure to analyze the circuit behavior before applying the fix.

1. Visual Inspection: Close switch S1. Observe the brightness of D1. It should be noticeably dim for a high-brightness LED.
2. Base Voltage Check: Measure voltage at node BASE relative to GND. It should be approximately 0.7 V.
3. Collector Voltage (VCE) Check: Measure voltage at node COLLECTOR relative to GND (across the transistor).
   • Expectation for a perfect switch: ~0 V.
   • Actual measurement: You will likely measure a significant voltage (e.g., 2 V to 4 V depending on the exact gain of your specific Q1).
4. Calculated Current: Calculate the current entering the base: IB = (5 V – 0.7 V) / 100 kΩ.

SPICE netlist and simulation

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

* Practical case: NPN Switch Saturation Troubleshooting
.width out=256

* --- Power Supply ---
V1 VCC 0 DC 5

* --- Input Control (S1) ---
* S1 connects VCC to SWITCH_OUT. Modeled as a voltage-controlled switch
* driven by a PULSE source to simulate user actuation.
S1 VCC SWITCH_OUT CTRL 0 SW_IDEAL
Vctrl CTRL 0 PULSE(0 5 0 1u 1u 50u 100u)
.model SW_IDEAL SW(Vt=2.5 Ron=0.01 Roff=100Meg)

* --- Circuit Components ---
* R2: Incorrect Base resistor (100k) causing weak saturation.
* This matches the "Troubleshooting" state defined in the Wiring Guide.
R2 SWITCH_OUT BASE 100k

* Note: R3 (1k) is listed in the BOM as the 'Solution' but is not connected
* in the current wiring guide configuration. It is omitted to prevent floating nodes.
* ... (truncated in public view) ...

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* Practical case: NPN Switch Saturation Troubleshooting
.width out=256

* --- Power Supply ---
V1 VCC 0 DC 5

* --- Input Control (S1) ---
* S1 connects VCC to SWITCH_OUT. Modeled as a voltage-controlled switch
* driven by a PULSE source to simulate user actuation.
S1 VCC SWITCH_OUT CTRL 0 SW_IDEAL
Vctrl CTRL 0 PULSE(0 5 0 1u 1u 50u 100u)
.model SW_IDEAL SW(Vt=2.5 Ron=0.01 Roff=100Meg)

* --- Circuit Components ---
* R2: Incorrect Base resistor (100k) causing weak saturation.
* This matches the "Troubleshooting" state defined in the Wiring Guide.
R2 SWITCH_OUT BASE 100k

* Note: R3 (1k) is listed in the BOM as the 'Solution' but is not connected
* in the current wiring guide configuration. It is omitted to prevent floating nodes.

* Q1: NPN Transistor Switch (Low-side)
Q1 COLLECTOR BASE 0 2N2222MOD

* D1: High-Brightness White LED
D1 VCC LED_CATHODE D_WHITE

* R1: LED Current Limiting Resistor
R1 LED_CATHODE COLLECTOR 33

* --- Models ---
* Generic NPN Model for 2N2222
.model 2N2222MOD NPN(IS=1E-14 BF=200 VAF=100 IKF=0.3 RB=10 RC=0.3 RE=0.2 CJE=25p CJC=8p)

* Approximate White LED Model (High Forward Voltage)
.model D_WHITE D(IS=1p N=3.5 RS=5 BV=5 IBV=10u)

* --- Analysis Commands ---
* Transient analysis to visualize switching behavior
.tran 1u 200u

* Output identification:
* Input: V(SWITCH_OUT)
* Output: V(COLLECTOR) (Low-side switch voltage)
.print tran V(SWITCH_OUT) V(COLLECTOR) V(BASE) V(LED_CATHODE)

.end

Simulation Results (Transient Analysis)

Analysis: The simulation confirms the ‘Troubleshooting’ scenario: when the switch is ON (V(SWITCH_OUT)=5V), the Collector voltage drops only to ~2.6V rather than near 0V. This indicates the transistor is in the active region (not fully saturated) due to the high base resistance (100kΩ), failing to fully power the LED load.

Show raw data table (271 rows)

Index   time            v(switch_out)   v(collector)    v(base)         v(led_cathode)
0	0.000000e+00	5.375300e-01	3.548129e+00	5.330675e-01	3.548432e+00
1	1.000000e-08	5.375300e-01	3.548129e+00	5.330675e-01	3.548432e+00
2	2.000000e-08	5.375300e-01	3.548129e+00	5.330675e-01	3.548432e+00
3	4.000000e-08	5.375300e-01	3.548129e+00	5.330676e-01	3.548432e+00
4	8.000000e-08	5.375300e-01	3.548129e+00	5.330676e-01	3.548432e+00
5	1.600000e-07	5.375300e-01	3.548129e+00	5.330676e-01	3.548432e+00
6	3.200000e-07	5.375300e-01	3.548129e+00	5.330676e-01	3.548432e+00
7	3.562500e-07	5.375300e-01	3.548129e+00	5.330676e-01	3.548432e+00
8	4.196875e-07	5.375300e-01	3.548129e+00	5.330676e-01	3.548432e+00
9	4.372461e-07	5.375300e-01	3.548129e+00	5.330676e-01	3.548432e+00
10	4.679736e-07	5.375300e-01	3.548129e+00	5.330676e-01	3.548432e+00
11	5.019934e-07	5.000000e+00	3.537721e+00	5.508590e-01	3.538060e+00
12	5.700330e-07	5.000000e+00	3.337558e+00	5.996484e-01	3.340559e+00
13	6.907446e-07	5.000000e+00	3.004466e+00	6.704095e-01	3.063080e+00
14	8.252066e-07	5.000000e+00	2.710645e+00	7.051011e-01	2.922994e+00
15	1.000000e-06	5.000000e+00	2.604154e+00	7.130054e-01	2.886751e+00
16	1.026892e-06	5.000000e+00	2.605141e+00	7.129945e-01	2.887005e+00
17	1.080677e-06	5.000000e+00	2.606105e+00	7.129106e-01	2.887380e+00
18	1.188247e-06	5.000000e+00	2.607032e+00	7.128469e-01	2.887677e+00
19	1.403386e-06	5.000000e+00	2.607269e+00	7.128312e-01	2.887753e+00
20	1.833664e-06	5.000000e+00	2.607219e+00	7.128340e-01	2.887737e+00
21	2.694221e-06	5.000000e+00	2.607248e+00	7.128325e-01	2.887747e+00
22	3.694221e-06	5.000000e+00	2.607227e+00	7.128335e-01	2.887740e+00
23	4.694221e-06	5.000000e+00	2.607243e+00	7.128328e-01	2.887745e+00
... (247 more rows) ...

Common mistakes and how to avoid them

1. Confusing Pinout: Placing the transistor backwards (Collector and Emitter swapped) often allows some current to flow but with very low gain, mimicking this specific problem. Always verify the datasheet.
2. Assuming hFE is Constant: Students often use the maximum hFE (e.g., 300) for calculation. For switching, you must assume a much lower «forced beta» (usually 10) to ensure saturation.
3. Ignoring Power Ratings: If the transistor is dropping 3 V and passing 50 mA, it is dissipating 150mW. While safe for a 2N2222, this heat is wasted energy.

Troubleshooting

• LED does not light up at all: Check if the LED polarity is correct (Anode to VCC). Verify S1 is actually connecting power to R2.
• Transistor gets hot: If VCE is high and current is flowing, the transistor is acting as a resistor. This confirms it is in the Active Region.
• VCE reads 5 V: The transistor is not turning on at all. Check if R2 is connected properly or if the Base-Emitter junction is blown.

Diagnosis and Solution

Follow this pedagogical sequence to understand and resolve the issue.

1. The Problem (Symptom)
You have assembled the circuit, closed the switch, but the High-Current LED barely glows. It looks weak. Why is this happening if the transistor is supposed to be a «switch»?

2. The Investigation
Take your multimeter. Measure the voltage between the Collector and Emitter (VCE).
* If Q1 were a closed switch, you would expect 0 V (or very close to it).
* However, you will likely find 2 V to 3 V.
* Now, calculate the Base Current you are providing: IB = (VIN – 0.7 V) / RB. With 100 kΩ, IB is tiny (~43µ A).

Possible improvements and extensions

1. Darlington Pair: Use two transistors configured as a Darlington pair to increase the total gain, allowing the 100 kΩ resistor to successfully switch the load (at the cost of a higher Vcesat drop of ~1.2 V).
2. MOSFET Upgrade: Replace the 2N2222 with an N-Channel MOSFET (like a 2N7000) to achieve near-zero gate current requirements and lower voltage drop.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

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

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