Practical case: Emergency deactivation

Emergency deactivation prototype (Maker Style)

Level: Basic. Build a safety circuit that cuts a ‘Ready’ signal instantly when a stop button is pressed.

Objective and use case

In this tutorial, you will build a digital logic circuit that inverts an input signal. Specifically, a «System Ready» indicator (Green LED) will remain active by default and will immediately turn off when an emergency pushbutton is pressed.

  • Why it is useful:

    • Industrial safety: Simulates an emergency kill switch where the active state shuts down machinery.
    • Security systems: Sensors (like door contacts) often break a circuit to trigger an alarm or change a status.
    • Fail-safe logic: Ensures a system defaults to «safe» (off) when an active intervention occurs.

  • Expected outcome:

    • Idle State: When the button is NOT pressed (Logic 0), the Green LED is ON (Logic 1).
    • Active State: When the button IS pressed (Logic 1), the Green LED turns OFF (Logic 0).
    • Signal Voltage: Input transitions between 0 V and 5 V; Output inverts logically.

  • Target audience and level: Students and hobbyists learning basic digital inversion.

Materials

  • V1: 5 V DC supply, function: main power source.
  • U1: 74HC04 Hex Inverter IC, function: logic inversion (NOT gate).
  • S1: Pushbutton (Normally Open), function: emergency signal trigger.
  • R1: 10 kΩ resistor, function: pull-down resistor for input stability.
  • R2: 330 Ω resistor, function: current limiting for the LED.
  • D1: Green LED, function: ‘System Ready’ indicator.

Pin-out of the IC used

Selected Chip: 74HC04 (Hex Inverter)

Pin Name Logic function Connection in this case
14 VCC Power Supply (+5V) Connect to 5 V rail
7 GND Ground (0V) Connect to 0 V rail
1 1A Input 1 Connect to Pushbutton and Pull-down resistor
2 1Y Output 1 Connect to LED resistor (R2)

Wiring guide

Construct the circuit following these node connections (Nodes: VCC, 0, V_IN, V_OUT):

  • Power Supply:
    • V1 connects between VCC (positive) and 0 (negative/GND).
    • U1 Pin 14 connects to VCC.
    • U1 Pin 7 connects to 0.
  • Input Stage (Button Logic):
    • S1 connects between VCC and V_IN.
    • R1 connects between V_IN and 0 (This pulls the input to 0 V when the button is open).
    • U1 Pin 1 (Input 1A) connects to V_IN.
  • Output Stage (Indicator):
    • U1 Pin 2 (Output 1Y) connects to V_OUT.
    • R2 connects between V_OUT and node LED_ANODE.
    • D1 Anode connects to LED_ANODE.
    • D1 Cathode connects to 0.

Conceptual block diagram

Conceptual block diagram — 74HC04 NOT gate

Schematic

[ INPUT STAGE ]                          [ LOGIC STAGE ]                       [ OUTPUT STAGE ]

    [ V1: 5V Supply ] --(Power VCC)--------> [ U1 Power: Pin 14 ]

    [ S1: Pushbutton ] --(Press = 5V)--+
    (Emergency Trig)                   |
                                       v
                                  [ Node V_IN ] --(Pin 1)--> [   U1: 74HC04   ] --(Pin 2)--> [ R2: 330 Ohm ] --> [ D1: Green LED ] --> [ GND ]
                                       ^                     [ Hex Inverter IC]              (Current Limit)     (System Ready)
                                       |                     [   (NOT Gate)   ]
    [ R1: 10k Resistor ] --(Open = 0V)-+                     [  GND: Pin 7    ]
    (Pull-down to GND)                                             |
                                                                   v
                                                                [ GND ]
Schematic (ASCII)

Truth table

The 74HC04 implements the Boolean NOT function ($Y = \overline{A}$).

Button State Input Voltage (V_IN) Logic Input (A) Logic Output (Y) LED State
Released 0 V (Pulled down) 0 1 ON
Pressed 5 V (VCC) 1 0 OFF

Measurements and tests

Follow these steps to validate the emergency deactivation logic:

  1. Idle Check:

    • Ensure the power supply is on. Do not touch the button.
    • Visual: The Green LED should be lit.
    • Measurement: Use a multimeter to measure voltage at V_IN (Pin 1). It should be approx 0 V.
    • Measurement: Measure voltage at V_OUT (Pin 2). It should be approx 5 V (Logic High).

  2. Activation Check:

    • Press and hold the pushbutton S1.
    • Visual: The Green LED must turn OFF immediately.
    • Measurement: Voltage at V_IN should rise to 5 V.
    • Measurement: Voltage at V_OUT should drop to approx 0 V (Logic Low).

SPICE netlist and simulation

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

* Practical case: Emergency deactivation
* Circuit: Inverter Logic (NOT Gate) with LED Indicator

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

* --- Input Stage (Button Logic) ---
* Components: S1 (Pushbutton), R1 (Pull-down)
* Connectivity: S1 connects VCC to V_IN. R1 connects V_IN to 0.
* Logic: 
*   - Button Released (Default): S1 Open -> V_IN pulled to 0V by R1.
*   - Button Pressed (Emergency): S1 Closed -> V_IN pulled to 5V (VCC).

* Simulation of S1 (Normally Open Pushbutton):
* Modeled as a Voltage-Controlled Switch (S1) driven by SW_CTRL.
* Vt=2.5V ensures switch closes when control signal is 5V.
S1 VCC V_IN SW_CTRL 0 SW_BTN
.model SW_BTN SW(Vt=2.5 Vh=0.1 Ron=1 Roff=10Meg)

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

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

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* Practical case: Emergency deactivation
* Circuit: Inverter Logic (NOT Gate) with LED Indicator

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

* --- Input Stage (Button Logic) ---
* Components: S1 (Pushbutton), R1 (Pull-down)
* Connectivity: S1 connects VCC to V_IN. R1 connects V_IN to 0.
* Logic: 
*   - Button Released (Default): S1 Open -> V_IN pulled to 0V by R1.
*   - Button Pressed (Emergency): S1 Closed -> V_IN pulled to 5V (VCC).

* Simulation of S1 (Normally Open Pushbutton):
* Modeled as a Voltage-Controlled Switch (S1) driven by SW_CTRL.
* Vt=2.5V ensures switch closes when control signal is 5V.
S1 VCC V_IN SW_CTRL 0 SW_BTN
.model SW_BTN SW(Vt=2.5 Vh=0.1 Ron=1 Roff=10Meg)

* Control Signal (User Finger Simulation):
* Generates a pulse: 0V (Released) -> 5V (Pressed) -> 0V (Released).
* Timeline: Idle for 100us, Press for 300us, then Release.
V_BTN_CTRL SW_CTRL 0 PULSE(0 5 100u 1u 1u 300u 1000u)

* R1: 10k Pull-down resistor
R1 V_IN 0 10k

* --- Logic Stage (U1) ---
* Component: 74HC04 Hex Inverter
* Connectivity: Pin 1 (Input) -> V_IN, Pin 2 (Output) -> V_OUT.
* Power: Pin 14 -> VCC, Pin 7 -> 0.
XU1 V_IN V_OUT 0 VCC 74HC04_INV

* Subcircuit for 74HC04 Inverter
* Behavioral model: Output is High when Input is Low.
* Uses a sigmoid function for smooth switching and convergence.
.subckt 74HC04_INV In Out Gnd Vcc
B1 Out Gnd V = V(Vcc,Gnd) / (1 + exp(50 * (V(In,Gnd) - V(Vcc,Gnd)/2)))
.ends

* --- Output Stage (Indicator) ---
* Components: R2 (Resistor), D1 (Green LED)
* Connectivity: V_OUT -> R2 -> LED_ANODE -> D1 -> 0
* Logic: 
*   - V_IN=0 (Ready) -> V_OUT=5 -> LED ON.
*   - V_IN=5 (Emergency) -> V_OUT=0 -> LED OFF.

R2 V_OUT LED_ANODE 330

* D1: Green LED
D1 LED_ANODE 0 LED_GREEN
.model LED_GREEN D(Is=1e-22 Rs=5 N=1.5 Cjo=10p Vj=0.75 M=0.33 BV=5 Ibv=10u)

* --- Simulation Directives ---
* Transient analysis to observe the button press event
.tran 10u 600u

* Output data for analysis
.print tran V(V_IN) V(V_OUT) V(LED_ANODE) V(SW_CTRL)

* Calculate DC operating point
.op

.end

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)
Show raw data table (260 rows) 
Index   time            v(v_in)         v(v_out)        v(led_anode)
0	0.000000e+00	4.995005e-03	5.000000e+00	1.833072e+00
1	6.000000e-08	4.995005e-03	5.000000e+00	1.833072e+00
2	1.200000e-07	4.995005e-03	5.000000e+00	1.833072e+00
3	2.400000e-07	4.995005e-03	5.000000e+00	1.833072e+00
4	4.800000e-07	4.995005e-03	5.000000e+00	1.833072e+00
5	9.600000e-07	4.995005e-03	5.000000e+00	1.833072e+00
6	1.920000e-06	4.995005e-03	5.000000e+00	1.833072e+00
7	3.840000e-06	4.995005e-03	5.000000e+00	1.833072e+00
8	7.680000e-06	4.995005e-03	5.000000e+00	1.833072e+00
9	1.536000e-05	4.995005e-03	5.000000e+00	1.833072e+00
10	2.536000e-05	4.995005e-03	5.000000e+00	1.833072e+00
11	3.536000e-05	4.995005e-03	5.000000e+00	1.833072e+00
12	4.536000e-05	4.995005e-03	5.000000e+00	1.833072e+00
13	5.536000e-05	4.995005e-03	5.000000e+00	1.833072e+00
14	6.536000e-05	4.995005e-03	5.000000e+00	1.833072e+00
15	7.536000e-05	4.995005e-03	5.000000e+00	1.833072e+00
16	8.536000e-05	4.995005e-03	5.000000e+00	1.833072e+00
17	9.536000e-05	4.995005e-03	5.000000e+00	1.833072e+00
18	1.000000e-04	4.995005e-03	5.000000e+00	1.833072e+00
19	1.001000e-04	4.995005e-03	5.000000e+00	1.833072e+00
20	1.002750e-04	4.995005e-03	5.000000e+00	1.833072e+00
21	1.003234e-04	4.995005e-03	5.000000e+00	1.833072e+00
22	1.004082e-04	4.995005e-03	5.000000e+00	1.833072e+00
23	1.004317e-04	4.995005e-03	5.000000e+00	1.833072e+00
... (236 more rows) ...

Common mistakes and how to avoid them

  1. Floating Input: Omitting R1 (pull-down resistor) causes the input to float when the button is released.
    • Solution: Always ensure the input pin is connected to GND via a resistor (e.g., 10 kΩ) when the switch is open.
  2. LED Reversed: The LED does not light up even when the output is High.
    • Solution: Check D1 polarity. The longer leg (Anode) must face the resistor/IC output; the shorter leg (Cathode) goes to Ground.
  3. Short Circuiting Power: Connecting the button directly between VCC and GND without the gate input in between or wiring the button in parallel with the supply.
    • Solution: Follow the node list carefully. The button connects VCC to the Input Pin, not directly to Ground.

Troubleshooting

  • Symptom: LED is always ON, pressing the button does nothing.
    • Cause: The button is not connected to VCC, or the input pin is permanently grounded.
    • Fix: Check continuity across S1 when pressed. Ensure S1 connects to Pin 1.
  • Symptom: LED is always OFF.
    • Cause: IC not powered, LED reversed, or input permanently connected to VCC.
    • Fix: Measure Pin 14 (VCC) and Pin 7 (GND). Check V_IN voltage; it should be 0 V when the button is released.
  • Symptom: LED flickers when your hand gets close to the wire.
    • Cause: Floating input (Missing R1).
    • Fix: Install the 10 kΩ pull-down resistor securely between Pin 1 and Ground.

Possible improvements and extensions

  1. Add a «Stop» Indicator: Add a second inverter (or use another gate on the same chip) to drive a Red LED that turns ON when the system is stopped (Output High when Input High).
  2. Latching Circuit: Replace the simple NOT gate with a Flip-Flop logic circuit so that once the emergency button is pressed, the system stays off even if the button is released, requiring a separate «Reset» button.

More Practical Cases on Prometeo.blog

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Quick Quiz

Question 1: What is the primary function of the digital logic circuit described in the tutorial?




Question 2: What is the state of the Green LED when the pushbutton is NOT pressed (Idle State)?




Question 3: What happens to the 'System Ready' indicator when the emergency pushbutton is pressed?




Question 4: Which component is specified as the main power source (V1) for this circuit?




Question 5: In the context of industrial safety, what does this circuit simulate?




Question 6: What is the logic level of the Green LED when the button is pressed (Active State)?




Question 7: What is the voltage range for the input signal transitions described?




Question 8: What concept ensures a system defaults to a 'safe' or off state during an intervention?




Question 9: Who is the target audience for this tutorial?




Question 10: Which type of security system component is mentioned as a similar use case?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

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

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