Practical case: Data transfer synchronization

Data transfer synchronization prototype (Maker Style)

Level: Advanced. Implement a gated clock circuit allowing synchronization pulses only when Data Ready and System Enable are active.

Objective and use case

In this session, you will design and analyze a clock gating circuit using cascaded AND gates to control the flow of high-speed clock pulses to a shift register. You will validate that the clock signal only propagates when two distinct control flags (‘Data Ready’ and ‘System Enable’) are simultaneously high.

Why it is useful:
* Power Management: Disabling the clock tree to idle shift registers or sub-systems reduces dynamic power consumption in CMOS circuits.
* Data Integrity: Ensures data is only clocked into the buffer when the source indicates valid data (Data Ready) and the controller allows reception (System Enable).
* Bus Arbitration: Prevents bus contention by synchronizing multiple peripherals sharing a common data line.

Expected outcome:
* The output GATED_CLK mirrors the input CLK only when DATA_RDY = 1 and SYS_EN = 1.
* Propagation delay between input clock edge and output clock edge is measured (typically 7–15 ns for 74HC series).
* Identification of «runt pulses» or glitches if enable signals change state while the clock is high.

Target audience and level:
Electronics engineering students and embedded system designers (Advanced).

Materials

  • U1: 74HC08 Quad 2-input AND gate IC, function: logic gating
  • V_CLK: Pulse generator, function: Master Clock (1 MHz, 0V-5V)
  • V_DR: DC voltage source or Switch, function: Data Ready signal
  • V_SE: DC voltage source or Switch, function: System Enable signal
  • V1: 5 V DC supply, function: Main power
  • C1: 100 nF ceramic capacitor, function: U1 decoupling
  • R_LOAD: 10 kΩ resistor, function: Simulates input impedance of shift register
  • C_LOAD: 15 pF capacitor, function: Simulates input capacitance and probe load

Pin-out of the IC used

Selected Chip: 74HC08 (Quad 2-Input AND Gate)

Pin Name Logic function Connection in this case
1 1A Input Gate 1 Connects to Node DATA_RDY
2 1B Input Gate 1 Connects to Node SYS_EN
3 1Y Output Gate 1 Connects to Node ENABLE_COMBINED (Internal)
4 2A Input Gate 2 Connects to Node ENABLE_COMBINED
5 2B Input Gate 2 Connects to Node CLK_IN
6 2Y Output Gate 2 Connects to Node GATED_CLK
7 GND Ground Connects to Node 0
14 VCC Power Supply Connects to Node VCC

Wiring guide

Construct the circuit following these node connections. Ensure the power supply is off while wiring.

  • Power Supply:

    • V1 positive terminal connects to node VCC.
    • V1 negative terminal connects to node 0 (GND).
    • C1 connects between VCC and 0 (placed close to U1).
    • U1 Pin 14 connects to VCC.
    • U1 Pin 7 connects to 0.

  • Control Logic (Gate 1):

    • V_DR (Data Ready) positive terminal connects to node DATA_RDY.
    • V_SE (System Enable) positive terminal connects to node SYS_EN.
    • U1 Pin 1 (1A) connects to node DATA_RDY.
    • U1 Pin 2 (1B) connects to node SYS_EN.
    • U1 Pin 3 (1Y) connects to node ENABLE_COMBINED.

  • Clock Gating (Gate 2):

    • V_CLK (Clock Source) positive terminal connects to node CLK_IN.
    • U1 Pin 4 (2A) connects to node ENABLE_COMBINED.
    • U1 Pin 5 (2B) connects to node CLK_IN.
    • U1 Pin 6 (2Y) connects to node GATED_CLK.

  • Output Loading:

    • R_LOAD connects between GATED_CLK and 0.
    • C_LOAD connects between GATED_CLK and 0.

Conceptual block diagram

Conceptual block diagram — 74HC08 AND gate

Schematic

[ INPUT SIGNALS ]                       [ LOGIC PROCESSING (U1: 74HC08) ]                     [ OUTPUT STAGE ]

                                             +-----------------------------------+
                                             |    POWER SUPPLY & DECOUPLING      |
                                             |  V1 (5V) -> Pin 14, GND -> Pin 7  |
                                             |  C1 (100nF) across VCC/GND        |
                                             +-----------------------------------+
                                                               |
    [ V_DR: Data Ready ] --(Pin 1)-->+                         |
                                     |                         v
                                     +-----> [ AND Gate 1 ] ---+
                                     |       (Control Logic)   |
    [ V_SE: Sys Enable ] --(Pin 2)-->+                         |
                                                               |
                                                               | (Pin 3: ENABLE_COMBINED)
                                                               |
                                                               v
                                                          (Pin 4)
                                                               +---> [ AND Gate 2 ] --(Pin 6)--> [ R_LOAD (10k) ] --+
                                                               |     (Clock Gating)              [ C_LOAD (15pF) ]  |
    [ V_CLK: Master Clk ] --(Pin 5)----------------------------+     (Signal: GATED_CLK)                            v
                                                                                                                   GND
Schematic (ASCII)

Truth table

This table represents the cascaded logic: GATED_CLK = (DATA_RDY AND SYS_EN) AND CLK_IN.

DATA_RDY SYS_EN ENABLE_COMBINED (Internal) CLK_IN GATED_CLK State Description
0 X 0 X 0 Blocked: Data not ready
X 0 0 X 0 Blocked: System disabled
1 1 1 0 0 Active: Clock Low phase
1 1 1 1 1 Active: Clock High phase passed

(X = Don’t Care)

Measurements and tests

Perform the following validation steps using an oscilloscope (Dual Channel recommended).

  1. Static Logic Validation:

    • Set V_CLK to 0V. Toggle V_DR and V_SE. Ensure GATED_CLK remains 0V.
    • Set V_CLK to 5V (DC). Ensure GATED_CLK is High ONLY when both V_DR and V_SE are High.

  2. Dynamic Clock Gating:

    • Configure V_CLK to a 1 MHz square wave (50% duty cycle).
    • Enable Channel 1 on CLK_IN and Channel 2 on GATED_CLK.
    • Activate both V_DR and V_SE. Verify Channel 2 replicates Channel 1.
    • Deactivate V_DR. Verify Channel 2 goes flat Low.

  3. Propagation Delay Analysis (Advanced):

    • With the clock running and passing through, maximize horizontal zoom (timebase ~10ns/div).
    • Measure the time difference between the 50% voltage point of the rising edge of CLK_IN and the rising edge of GATED_CLK.
    • Expected Result: A delay of approximately 15ns–25ns (sum of delays through Gate 1 and Gate 2).

  4. Glitch/Hazard Observation:

    • While CLK_IN is High, manually toggle DATA_RDY.
    • Observe if truncated pulses («runts») appear on the output. These are hazards caused by asynchronous gating.

SPICE netlist and simulation

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

* Practical case: Data transfer synchronization

* --- Power Supply ---
* V1 positive terminal connects to node VCC. Negative to node 0 (GND).
V1 VCC 0 DC 5
* C1 connects between VCC and 0 (placed close to U1).
C1 VCC 0 100n

* --- Input Stimuli (Dynamic) ---
* V_CLK: Master Clock (1 MHz, 0V-5V). 
* PULSE(V1 V2 TD TR TF PW PER) -> 1us Period, 0.49us Width
V_CLK CLK_IN 0 PULSE(0 5 0 10n 10n 490n 1u)

* V_DR: Data Ready signal.
* Simulates a data packet ready signal. Pulses High from 5us to 15us.
V_DR DATA_RDY 0 PULSE(0 5 5u 10n 10n 10u 40u)

* V_SE: System Enable signal.
* Simulates system enable window. Pulses High from 2us to 22us.
V_SE SYS_EN 0 PULSE(0 5 2u 10n 10n 20u 50u)

* --- Control Logic (U1: 74HC08) ---
* Instantiation of the IC using a behavioral subcircuit.
* Mapping pins according to Wiring Guide:
* Pin 1 (1A) -> DATA_RDY
* Pin 2 (1B) -> SYS_EN
* Pin 3 (1Y) -> ENABLE_COMBINED
* Pin 4 (2A) -> ENABLE_COMBINED
* Pin 5 (2B) -> CLK_IN
* Pin 6 (2Y) -> GATED_CLK
* ... (truncated in public view) ...

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

🔒 Part of this section is premium. With the 7-day pass or the monthly membership 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 

* Practical case: Data transfer synchronization

* --- Power Supply ---
* V1 positive terminal connects to node VCC. Negative to node 0 (GND).
V1 VCC 0 DC 5
* C1 connects between VCC and 0 (placed close to U1).
C1 VCC 0 100n

* --- Input Stimuli (Dynamic) ---
* V_CLK: Master Clock (1 MHz, 0V-5V). 
* PULSE(V1 V2 TD TR TF PW PER) -> 1us Period, 0.49us Width
V_CLK CLK_IN 0 PULSE(0 5 0 10n 10n 490n 1u)

* V_DR: Data Ready signal.
* Simulates a data packet ready signal. Pulses High from 5us to 15us.
V_DR DATA_RDY 0 PULSE(0 5 5u 10n 10n 10u 40u)

* V_SE: System Enable signal.
* Simulates system enable window. Pulses High from 2us to 22us.
V_SE SYS_EN 0 PULSE(0 5 2u 10n 10n 20u 50u)

* --- Control Logic (U1: 74HC08) ---
* Instantiation of the IC using a behavioral subcircuit.
* Mapping pins according to Wiring Guide:
* Pin 1 (1A) -> DATA_RDY
* Pin 2 (1B) -> SYS_EN
* Pin 3 (1Y) -> ENABLE_COMBINED
* Pin 4 (2A) -> ENABLE_COMBINED
* Pin 5 (2B) -> CLK_IN
* Pin 6 (2Y) -> GATED_CLK
* Pin 7 (GND)-> 0
* Pin 14(VCC)-> VCC
XU1 DATA_RDY SYS_EN ENABLE_COMBINED ENABLE_COMBINED CLK_IN GATED_CLK 0 VCC 74HC08_BEHAVIORAL

* --- Output Loading ---
* R_LOAD connects between GATED_CLK and 0.
R_LOAD GATED_CLK 0 10k
* C_LOAD connects between GATED_CLK and 0.
C_LOAD GATED_CLK 0 15p

* --- Subcircuit Model: 74HC08 ---
* Robust behavioral implementation using sigmoid functions for convergence.
* Only the gates used in the wiring are modeled to save complexity.
.subckt 74HC08_BEHAVIORAL 1A 1B 1Y 2A 2B 2Y GND VCC
    * Gate 1: 1Y = 1A AND 1B
    * Function: V(VCC) * Sigmoid(A) * Sigmoid(B)
    B_G1 1Y_INT GND V = V(VCC) * (1 / (1 + exp(-50*(V(1A)-2.5)))) * (1 / (1 + exp(-50*(V(1B)-2.5))))
    R_G1 1Y_INT 1Y 100

    * Gate 2: 2Y = 2A AND 2B
    B_G2 2Y_INT GND V = V(VCC) * (1 / (1 + exp(-50*(V(2A)-2.5)))) * (1 / (1 + exp(-50*(V(2B)-2.5))))
    R_G2 2Y_INT 2Y 100
.ends

* --- Analysis Directives ---
.op
* Transient analysis: 10ns step, 25us duration to capture full logic sequence
.tran 10n 25u

* --- Output Directives ---
.print tran V(CLK_IN) V(DATA_RDY) V(SYS_EN) V(ENABLE_COMBINED) V(GATED_CLK)

.end

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)
Show raw data table (6072 rows) 
Index   time            v(clk_in)       v(data_rdy)     v(sys_en)
0	0.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
1	1.000000e-10	5.000000e-02	0.000000e+00	0.000000e+00
2	2.000000e-10	1.000000e-01	0.000000e+00	0.000000e+00
3	4.000000e-10	2.000000e-01	0.000000e+00	0.000000e+00
4	8.000000e-10	4.000000e-01	0.000000e+00	0.000000e+00
5	1.600000e-09	8.000000e-01	0.000000e+00	0.000000e+00
6	3.200000e-09	1.600000e+00	0.000000e+00	0.000000e+00
7	6.400000e-09	3.200000e+00	0.000000e+00	0.000000e+00
8	1.000000e-08	5.000000e+00	0.000000e+00	0.000000e+00
9	1.064000e-08	5.000000e+00	0.000000e+00	0.000000e+00
10	1.192000e-08	5.000000e+00	0.000000e+00	0.000000e+00
11	1.448000e-08	5.000000e+00	0.000000e+00	0.000000e+00
12	1.960000e-08	5.000000e+00	0.000000e+00	0.000000e+00
13	2.960000e-08	5.000000e+00	0.000000e+00	0.000000e+00
14	3.960000e-08	5.000000e+00	0.000000e+00	0.000000e+00
15	4.960000e-08	5.000000e+00	0.000000e+00	0.000000e+00
16	5.960000e-08	5.000000e+00	0.000000e+00	0.000000e+00
17	6.960000e-08	5.000000e+00	0.000000e+00	0.000000e+00
18	7.960000e-08	5.000000e+00	0.000000e+00	0.000000e+00
19	8.960000e-08	5.000000e+00	0.000000e+00	0.000000e+00
20	9.960000e-08	5.000000e+00	0.000000e+00	0.000000e+00
21	1.096000e-07	5.000000e+00	0.000000e+00	0.000000e+00
22	1.196000e-07	5.000000e+00	0.000000e+00	0.000000e+00
23	1.296000e-07	5.000000e+00	0.000000e+00	0.000000e+00
... (6048 more rows) ...

Common mistakes and how to avoid them

  1. Leaving unused inputs floating:
    • Issue: Unused inputs on the 74HC08 (e.g., Pins 9, 10, 12, 13) pick up noise, causing high power consumption or oscillation.
    • Solution: Connect all unused AND gate inputs directly to GND or VCC.
  2. Ignoring propagation delay accumulation:
    • Issue: Assuming the output happens instantly. In this cascaded setup (Gate 1 -> Gate 2), the delay is double that of a single gate.
    • Solution: Account for this delay in timing diagrams; signals may arrive too late for the setup time of the subsequent shift register.
  3. Gating the clock asynchronously:
    • Issue: Changing SYS_EN while the clock is High clips the pulse width, violating the minimum pulse width requirement of the shift register.
    • Solution: Ideally, synchronize the Enable signal to the falling edge of the clock (using a Flip-Flop) before feeding it to the AND gate.

Troubleshooting

  • Symptom: Output is permanently Low, even when all inputs are High.
    • Cause: Missing power to Pin 14 or GND to Pin 7.
    • Fix: Check VCC/GND continuity with a multimeter.
  • Symptom: «Ghosting» or noisy edges on the oscilloscope.
    • Cause: Lack of decoupling capacitor or long ground leads on probes.
    • Fix: Install C1 (100nF) extremely close to the IC; use the ground spring on the probe tip.
  • Symptom: Signal edges are very rounded (slow rise time).
    • Cause: Capacitive loading is too high (long wires or breadboard stray capacitance).
    • Fix: Shorten wires or add a buffer if driving a heavy load.

Possible improvements and extensions

  1. Glitch-free Gating: Add a D-Flip-Flop (e.g., 74HC74) to synchronize the ENABLE_COMBINED signal so it only changes state when the Clock is Low.
  2. Wait-State Insertion: Expand the circuit to assert a «BUSY» signal back to the controller whenever the clock is successfully gated, confirming data transfer is active.

More Practical Cases on Prometeo.blog

Find this product and/or books on this topic on Amazon

Go to Amazon

As an Amazon Associate, I earn from qualifying purchases. If you buy through this link, you help keep this project running.

Quick Quiz

Question 1: What is the primary function of the 74HC08 IC in this circuit design?




Question 2: Under which specific condition will the output `GATED_CLK` mirror the input `CLK`?




Question 3: Which of the following is NOT listed as a benefit of using this clock gating circuit?




Question 4: What potential signal integrity issue is associated with enable signals changing state while the clock is high?




Question 5: What is the typical expected propagation delay for the 74HC series logic gates mentioned?




Question 6: How does clock gating contribute to power management in CMOS circuits?




Question 7: In the context of Bus Arbitration, what does this circuit help prevent?




Question 8: Which logic gate configuration is used to control the flow of the high-speed clock pulses?




Question 9: What is the primary purpose of the 'Data Integrity' benefit in this circuit?




Question 10: What specific type of circuit is being designed and analyzed in this session?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

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

Follow me:

Practical case: Safety interlock in a chemical reactor

Safety interlock in a chemical reactor prototype (Maker Style)

Level: Advanced. Design a redundant safety system enabling catalyst injection only when three critical variables are within range.

Objective and use case

In this case, you will build a hardware-based safety interlock logic circuit that monitors three simulated analog parameters (Temperature, Pressure, Level). It uses comparators to digitize these signals and a cascaded logic gate arrangement to control a high-power relay.

Why it is useful:
* Industrial Safety: Prevents chemical reactions from starting unless environmental conditions are perfect, avoiding run-away thermal events.
* Machine Guarding: Ensures guards are closed, pressure is released, and motors are stopped before unlocking maintenance doors.
* Medical Devices: Prevents laser or radiation emission unless all interlocks (key switch, door contact, patient sensor) are active.

Expected outcome:
* Logic Output: The final control signal V_SAFE goes HIGH (Logic 1, ~5V) only when all three inputs are within the «Safe» zone simultaneously.
* Visual Indication: The Relay activates (closing the circuit for the catalyst valve) and a Green LED lights up only in the 1-1-1 state.
* Hysteresis: The system maintains stability near switching thresholds (dependent on comparator configuration).

Target audience: Electronics engineering students and industrial automation technicians.

Materials

  • V1: 5 V DC power supply, function: Logic and sensor power.
  • U1: 74HC08, function: Quad 2-input AND gate (used to create 3-input logic).
  • U2: LM339, function: Quad open-collector comparator (digitizes analog sensors).
  • R1: 10 kΩ potentiometer, function: Simulator for Temperature Sensor.
  • R2: 10 kΩ potentiometer, function: Simulator for Pressure Sensor.
  • R3: 10 kΩ potentiometer, function: Simulator for Tank Level.
  • R_REF: 10 kΩ resistor array (or pots), function: Voltage dividers for reference thresholds (2.5 V).
  • R_PU1: 4.7 kΩ resistor, function: Pull-up for Comparator 1 output.
  • R_PU2: 4.7 kΩ resistor, function: Pull-up for Comparator 2 output.
  • R_PU3: 4.7 kΩ resistor, function: Pull-up for Comparator 3 output.
  • R_BASE: 1 kΩ resistor, function: Transistor base current limiting.
  • Q1: 2N2222 NPN Transistor, function: Relay driver.
  • K1: 5 V Relay, function: Actuator for catalyst valve.
  • D1: 1N4007 Diode, function: Flyback protection for Q1.
  • D2: Green LED, function: Indicator for «Injection Active».
  • R_LED: 330 Ω resistor, function: LED current limiting.

Pin-out of the IC used

Selected Chip: 74HC08 (Quad 2-Input AND Gate)

Pin Name Logic function Connection in this case
1 1A Input A (Gate 1) Connected to Temperature Status (SIG_TEMP)
2 1B Input B (Gate 1) Connected to Pressure Status (SIG_PRES)
3 1Y Output (Gate 1) Intermediate result (Temp AND Pres)
4 2A Input A (Gate 2) Connected to 1Y (Intermediate result)
5 2B Input B (Gate 2) Connected to Level Status (SIG_LEV)
6 2Y Output (Gate 2) Final Safety Signal (V_SAFE)
7 GND Ground Connected to power supply 0
14 VCC Power Connected to VCC (+5V)

Wiring guide

Power Supply
* V1 connects between VCC and 0 (GND).
* U1 (74HC08) Pin 14 connects to VCC, Pin 7 to 0.
* U2 (LM339) Pin 3 (V+) connects to VCC, Pin 12 (GND) to 0.

Analog Inputs & Comparators (Signal Conditioning)
* R1 (Temp Pot) connects between VCC and 0; wiper connects to U2 Input 1- (NODE_T_SENS).
* Reference divider connects to U2 Input 1+ (NODE_REF).
* U2 Output 1 connects to SIG_TEMP.
* R_PU1 connects between VCC and SIG_TEMP (required for open-collector).
* R2 (Pres Pot) connects between VCC and 0; wiper connects to U2 Input 2- (NODE_P_SENS).
* Reference divider connects to U2 Input 2+ (NODE_REF).
* U2 Output 2 connects to SIG_PRES.
* R_PU2 connects between VCC and SIG_PRES.
* R3 (Level Pot) connects between VCC and 0; wiper connects to U2 Input 3- (NODE_L_SENS).
* Reference divider connects to U2 Input 3+ (NODE_REF).
* U2 Output 13 connects to SIG_LEV.
* R_PU3 connects between VCC and SIG_LEV.

Digital Logic (Cascading for 3-Input AND)
* U1 Pin 1 (1A) connects to SIG_TEMP.
* U1 Pin 2 (1B) connects to SIG_PRES.
* U1 Pin 3 (1Y) connects to U1 Pin 4 (2A).
* U1 Pin 5 (2B) connects to SIG_LEV.
* U1 Pin 6 (2Y) connects to V_SAFE.

Output Stage
* R_BASE connects between V_SAFE and NODE_BASE.
* Q1 Base connects to NODE_BASE.
* Q1 Emitter connects to 0.
* Q1 Collector connects to NODE_RELAY.
* K1 (Relay coil) connects between VCC and NODE_RELAY.
* D1 connects between NODE_RELAY (Anode) and VCC (Cathode) Note: Check polarity, cathode to positive for flyback.
* R_LED connects between NODE_RELAY and NODE_LED_A.
* D2 connects between NODE_LED_A and 0.

Conceptual block diagram

Conceptual block diagram — 74HC08 AND gate

Schematic

Title: Practical case: Safety interlock in a chemical reactor

      [ SENSORS / INPUTS ]           [ SIGNAL CONDITIONING ]               [ LOGIC PROCESSING ]                  [ OUTPUT ACTUATOR STAGE ]

                                         (VCC / Pull-ups)                                                         (VCC 5V Source)
                                                |                                                                        |
    [ R1: Temp Pot ] --(Analog)--> [ U2: LM339 Comp 1 ] --(SIG_TEMP)-->+                                    +------------+------------+
    (Simulates Sensor)             [ Ref: 2.5V Divider]                |                                    |            |            |
                                                                       v                                [ K1 Relay ] [ D1 Diode ] [ R_LED ]
                                                                [ U1: 74HC08 ]                          [  Coil    ] [ Cathode^ ] [   +     ]
    [ R2: Pres Pot ] --(Analog)--> [ U2: LM339 Comp 2 ] --(SIG_PRES)-->[  AND Gate A  ] --(Intermed)--> |            [ Anode v  ] [ D2 LED  ]
    (Simulates Sensor)             [ Ref: 2.5V Divider]                [              ]       |         |            |            |
                                                                                              |         +------------+------------+
                                                                                              v                      |
                                                                                        [ U1: 74HC08 ]               | (NODE_RELAY)
                                                                                        [  AND Gate B  ]             v
    [ R3: Levl Pot ] --(Analog)--> [ U2: LM339 Comp 3 ] --(SIG_LEV)--->(Pin 5)--------->[              ] --(V_SAFE)--> [ R_BASE ] --> [ Q1: 2N2222 ]
    (Simulates Sensor)             [ Ref: 2.5V Divider]                                 [              ]                              [ NPN Base   ]
                                                                                                                                      [            ]
                                                                                                                                      [ Collector  ] --< (Sinks Current)
                                                                                                                                      [ Emitter    ]
                                                                                                                                             |
                                                                                                                                            GND
Schematic (ASCII)

Truth table

This table represents the cascaded logic (Temp AND Pressure AND Level).

Temp OK (A) Pres OK (B) Level OK (C) Intermediate (A·B) Final Output (V_SAFE) Action
0 0 0 0 0 Safe Mode (Off)
0 0 1 0 0 Safe Mode (Off)
0 1 0 0 0 Safe Mode (Off)
0 1 1 0 0 Safe Mode (Off)
1 0 0 0 0 Safe Mode (Off)
1 0 1 0 0 Safe Mode (Off)
1 1 0 1 0 Safe Mode (Off)
1 1 1 1 1 Inject Catalyst

Measurements and tests

  1. Individual Sensor Test: Adjust R1 (Temp) below the threshold. Verify SIG_TEMP goes HIGH (approx 5V). Repeat for R2 and R3.
  2. Logic Logic Validation: Set Temp and Pressure to «Safe» (High logic), but Level to «Unsafe» (Low logic). Measure U1 Pin 3 (Intermediate); it should be HIGH. Measure U1 Pin 6 (V_SAFE); it should be LOW.
  3. Full Activation: Set all three potentiometers to the «Safe» range. Verify V_SAFE is HIGH, Q1 saturates, and the Relay (K1) clicks «On».
  4. Response Time: Connect an oscilloscope channel 1 to SIG_LEV and channel 2 to V_SAFE. Toggle the Level switch and measure the propagation delay (typically nanoseconds for the gate, milliseconds for the relay).

SPICE netlist and simulation

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

* Practical case: Safety interlock in a chemical reactor

* --- Models ---
* Generic NPN Transistor
.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=46.91E-9 TF=411.1E-12 ITF=0.6 VTF=1.7 XTF=3 RB=10 RC=1 RE=0.1)
* Flyback Diode
.model D1N4007 D(IS=7.02767n RS=0.03415 N=1.2686 EG=1.11 XTI=3 BV=1000 IBV=10m CJO=10p VJ=0.7 M=0.5 FC=0.5 TT=100n)
* Green LED Indicator
.model LED_GREEN D(IS=1e-22 RS=5 N=1.5 CJO=50p BV=5 IBV=10u EG=2.1)
* Voltage Controlled Switch for Open Collector Comparator
* Vt=0: Switch state changes when control voltage crosses 0V
* Ron=10: Low resistance when closed (Logic 0 / Low)
* Roff=100Meg: High resistance when open (Logic 1 / High via Pull-up)
.model SW_OC SW(Vt=0 Vh=0.001 Ron=10 Roff=100Meg)

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

* --- Reference Voltage (R_REF) ---
* Function: Voltage dividers for reference thresholds (2.5 V)
* Wiring: Reference divider connects to U2 Input + (NODE_REF)
R_REF_A VCC NODE_REF 10k
R_REF_B NODE_REF 0 10k

* --- Sensors (Simulated with PWL Voltage Sources) ---
* R1, R2, R3 Potentiometers simulated by PWL sources at the wiper nodes.
* Logic: Low Voltage (<2.5V) = Safe. High Voltage (>2.5V) = Unsafe/Alarm.
* Sequence: T=Safe, then Temp Fault, then Pres Fault, then Level Fault.

* ... (truncated in public view) ...

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

🔒 Part of this section is premium. With the 7-day pass or the monthly membership 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 

* Practical case: Safety interlock in a chemical reactor

* --- Models ---
* Generic NPN Transistor
.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=46.91E-9 TF=411.1E-12 ITF=0.6 VTF=1.7 XTF=3 RB=10 RC=1 RE=0.1)
* Flyback Diode
.model D1N4007 D(IS=7.02767n RS=0.03415 N=1.2686 EG=1.11 XTI=3 BV=1000 IBV=10m CJO=10p VJ=0.7 M=0.5 FC=0.5 TT=100n)
* Green LED Indicator
.model LED_GREEN D(IS=1e-22 RS=5 N=1.5 CJO=50p BV=5 IBV=10u EG=2.1)
* Voltage Controlled Switch for Open Collector Comparator
* Vt=0: Switch state changes when control voltage crosses 0V
* Ron=10: Low resistance when closed (Logic 0 / Low)
* Roff=100Meg: High resistance when open (Logic 1 / High via Pull-up)
.model SW_OC SW(Vt=0 Vh=0.001 Ron=10 Roff=100Meg)

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

* --- Reference Voltage (R_REF) ---
* Function: Voltage dividers for reference thresholds (2.5 V)
* Wiring: Reference divider connects to U2 Input + (NODE_REF)
R_REF_A VCC NODE_REF 10k
R_REF_B NODE_REF 0 10k

* --- Sensors (Simulated with PWL Voltage Sources) ---
* R1, R2, R3 Potentiometers simulated by PWL sources at the wiper nodes.
* Logic: Low Voltage (<2.5V) = Safe. High Voltage (>2.5V) = Unsafe/Alarm.
* Sequence: T=Safe, then Temp Fault, then Pres Fault, then Level Fault.

* R1: Temp Sensor Simulator
V_SENS_T NODE_T_SENS 0 PWL(0 1 100u 1 101u 4 200u 4 201u 1)

* R2: Pressure Sensor Simulator
V_SENS_P NODE_P_SENS 0 PWL(0 1 300u 1 301u 4 400u 4 401u 1)

* R3: Tank Level Simulator
V_SENS_L NODE_L_SENS 0 PWL(0 1 500u 1 501u 4 600u 4 601u 1)

* --- U2: LM339 Quad Comparator ---
* Function: Digitizes analog sensors. Open Collector Outputs.
* Logic: If V(In-) > V(In+), Switch closes to Ground (Output Low).
*        Else Switch opens (Output High via Pull-up).

* Comparator 1 (Temperature)
* Wiring: Wiper (NODE_T_SENS) to Input 1-, Ref to Input 1+, Output to SIG_TEMP
S_COMP1 SIG_TEMP 0 NODE_T_SENS NODE_REF SW_OC
* R_PU1: Pull-up for Comparator 1
R_PU1 VCC SIG_TEMP 4.7k

* Comparator 2 (Pressure)
* Wiring: Wiper (NODE_P_SENS) to Input 2-, Ref to Input 2+, Output to SIG_PRES
S_COMP2 SIG_PRES 0 NODE_P_SENS NODE_REF SW_OC
* R_PU2: Pull-up for Comparator 2
R_PU2 VCC SIG_PRES 4.7k

* Comparator 3 (Level)
* Wiring: Wiper (NODE_L_SENS) to Input 3-, Ref to Input 3+, Output to SIG_LEV
S_COMP3 SIG_LEV 0 NODE_L_SENS NODE_REF SW_OC
* R_PU3: Pull-up for Comparator 3
R_PU3 VCC SIG_LEV 4.7k

* --- U1: 74HC08 Quad 2-input AND ---
* Function: Safety Logic.
* Modeled using Behavioral Sources (Sigmoid function for HC logic levels).

* Gate 1 (Pins 1, 2, 3)
* Inputs: SIG_TEMP, SIG_PRES. Output: NODE_AND_INT
B_AND1 NODE_AND_INT 0 V = 5 * (1 / (1 + exp(-20*(V(SIG_TEMP)-2.5)))) * (1 / (1 + exp(-20*(V(SIG_PRES)-2.5))))

* Gate 2 (Pins 4, 5, 6)
* Inputs: NODE_AND_INT (Pin 4 connects to Pin 3), SIG_LEV. Output: V_SAFE
B_AND2 V_SAFE 0 V = 5 * (1 / (1 + exp(-20*(V(NODE_AND_INT)-2.5)))) * (1 / (1 + exp(-20*(V(SIG_LEV)-2.5))))

* --- Output Stage ---

* R_BASE: Transistor base current limiting
R_BASE V_SAFE NODE_BASE 1k

* Q1: 2N2222 NPN Transistor, Relay driver
* Wiring: Base to NODE_BASE, Emitter to 0, Collector to NODE_RELAY
Q1 NODE_RELAY NODE_BASE 0 2N2222MOD

* K1: 5 V Relay Coil
* Wiring: Connects between VCC and NODE_RELAY
* Modeled as Inductor and Series Resistor
L_K1 VCC NODE_RELAY_INT 10m
R_K1 NODE_RELAY_INT NODE_RELAY 100

* D1: Flyback protection
* Wiring: Anode to NODE_RELAY, Cathode to VCC
D1 NODE_RELAY VCC D1N4007

* D2 & R_LED: Indicator "Injection Active"
* Corrected Wiring: LED should be parallel to Relay Coil to indicate Activation.
* Path: VCC -> Resistor -> LED Anode -> LED Cathode -> Collector (NODE_RELAY).
* When Q1 is ON (Relay Active), NODE_RELAY is Low (~0.2V), LED turns ON.
R_LED VCC NODE_LED_A 330
D2 NODE_LED_A NODE_RELAY LED_GREEN

* --- Analysis Directives ---
* Transient analysis to observe the sequence of sensor faults
.tran 1u 800u

* Print required signals
.print tran V(NODE_T_SENS) V(NODE_P_SENS) V(NODE_L_SENS)
.print tran V(SIG_TEMP) V(SIG_PRES) V(SIG_LEV)
.print tran V(NODE_AND_INT) V(V_SAFE)
.print tran V(NODE_RELAY) V(NODE_BASE)

.end

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)
Show raw data table (3828 rows) 
Index   time            v(node_t_sens)  v(node_p_sens)  v(node_l_sens)
0	0.000000e+00	1.000000e+00	1.000000e+00	1.000000e+00
1	1.000000e-08	1.000000e+00	1.000000e+00	1.000000e+00
2	2.000000e-08	1.000000e+00	1.000000e+00	1.000000e+00
3	4.000000e-08	1.000000e+00	1.000000e+00	1.000000e+00
4	8.000000e-08	1.000000e+00	1.000000e+00	1.000000e+00
5	1.600000e-07	1.000000e+00	1.000000e+00	1.000000e+00
6	3.200000e-07	1.000000e+00	1.000000e+00	1.000000e+00
7	6.400000e-07	1.000000e+00	1.000000e+00	1.000000e+00
8	1.280000e-06	1.000000e+00	1.000000e+00	1.000000e+00
9	2.280000e-06	1.000000e+00	1.000000e+00	1.000000e+00
10	3.280000e-06	1.000000e+00	1.000000e+00	1.000000e+00
11	4.280000e-06	1.000000e+00	1.000000e+00	1.000000e+00
12	5.280000e-06	1.000000e+00	1.000000e+00	1.000000e+00
13	6.280000e-06	1.000000e+00	1.000000e+00	1.000000e+00
14	7.280000e-06	1.000000e+00	1.000000e+00	1.000000e+00
15	8.280000e-06	1.000000e+00	1.000000e+00	1.000000e+00
16	9.280000e-06	1.000000e+00	1.000000e+00	1.000000e+00
17	1.028000e-05	1.000000e+00	1.000000e+00	1.000000e+00
18	1.128000e-05	1.000000e+00	1.000000e+00	1.000000e+00
19	1.228000e-05	1.000000e+00	1.000000e+00	1.000000e+00
20	1.328000e-05	1.000000e+00	1.000000e+00	1.000000e+00
21	1.428000e-05	1.000000e+00	1.000000e+00	1.000000e+00
22	1.528000e-05	1.000000e+00	1.000000e+00	1.000000e+00
23	1.628000e-05	1.000000e+00	1.000000e+00	1.000000e+00
... (3804 more rows) ...

Common mistakes and how to avoid them

  1. Missing Pull-up Resistors: The LM339 comparator has an open-collector output. If you forget R_PU1/2/3, the logic gate inputs will float or remain LOW. Always tie outputs to VCC via a resistor (e.g., 4.7kΩ).
  2. Floating Unused Inputs: The 74HC08 is a CMOS device. If pins 9, 10, 12, 13 (Gates 3 and 4) are left floating, they pick up noise and increase power consumption. Connect unused inputs to GND.
  3. Inductive Kickback: Omitting D1 (Flyback diode) across the relay coil. This will generate a high voltage spike when the relay turns off, destroying transistor Q1 immediately.

Troubleshooting

  • Relay chatters (buzzing sound): The analog inputs are hovering exactly at the threshold voltage. Fix: Add a feedback resistor (hysteresis) between the comparator output and the non-inverting input.
  • Logic output is always HIGH: Check the comparator inputs. If the reference voltage is reversed (e.g., Ref > Signal vs Signal > Ref), the logic might be inverted.
  • Transistor gets hot but relay doesn’t switch: Q1 might be receiving insufficient base current, or the pinout (E-B-C) is incorrect. Fix: Verify R_BASE value and transistor pinout.

Possible improvements and extensions

  1. Latch Circuit: Add a feedback loop (or a Set-Reset latch) so that if safety is breached, the system shuts down and requires a manual «Reset» button press to restart, even if conditions return to normal.
  2. Fault Identification: Add red LEDs to the output of each comparator (inverted) to indicate exactly which variable (Temperature, Pressure, or Level) caused the shutdown.

More Practical Cases on Prometeo.blog

Find this product and/or books on this topic on Amazon

Go to Amazon

As an Amazon Associate, I earn from qualifying purchases. If you buy through this link, you help keep this project running.

Quick Quiz

Question 1: What is the primary function of the hardware-based safety interlock logic circuit described?




Question 2: Which component is used to digitize the analog sensor signals in this circuit?




Question 3: What logic state is required from the final control signal `V_SAFE` to activate the system?




Question 4: Which logic gate arrangement is used to process the digitized signals to ensure all conditions are met?




Question 5: In the context of industrial safety, what specific hazard does this system help prevent?




Question 6: What visual indicator confirms that the system is in the safe `1-1-1` state?




Question 7: What role do the simulated analog parameters (Temperature, Pressure, Level) play in this system?




Question 8: Which of the following is NOT listed as a use case for this type of safety interlock?




Question 9: What happens to the catalyst valve circuit when the relay activates?




Question 10: Why is this system described as a 'redundant' safety system?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

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

Follow me: