NOT gate_Basic - Prometeo.blog

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

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:

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).
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) ...

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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)

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

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.
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.
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

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).
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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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 — Use a 74HC04 inverter and an LDR to automatically switch on an LED when ambient light drops.

Objective and use case

You will build an automatic light control circuit that detects darkness using a Light Dependent Resistor (LDR) and activates an LED using a 74HC04 digital inverter.

Why it is useful:
- Automating streetlights to turn on only at night to save energy.
- Activating emergency pathway lighting during power failures or darkness.
- Controlling garden solar lights automatically.
- Adjusting screen brightness on mobile devices based on ambient light.
Expected outcome:
- When the LDR is exposed to bright light, the LED remains OFF.
- When the LDR is covered (darkness), the LED turns ON.
- The voltage at the logic gate input transitions from Logic High (5V) to Logic Low (0V) as it gets darker.
Target audience and level: Students and hobbyists familiar with basic breadboarding.

Materials

V1: 5 V DC supply, function: Main power source.
R1: LDR (GL5528 or similar), function: Light sensor (Variable resistor).
R2: 10 kΩ potentiometer, function: Sensitivity calibration (Pull-down).
U1: 74HC04, function: Hex Inverter (NOT gate).
R3: 330 Ω resistor, function: LED current limiting.
D1: Red LED, function: Visual output indicator.

Pin-out of the IC used

Chip: 74HC04 (Hex Inverter)

Pin	Name	Logic function	Connection in this case
14	VCC	Power (+)	Connect to `VCC` (5V)
7	GND	Ground (-)	Connect to `0` (GND)
1	1A	Input	Connect to sensor node `VSENSE`
2	1Y	Output	Connect to LED node `VOUT`

(Note: Pins 3, 5, 9, 11, 13 are unused inputs and should ideally be connected to GND in permanent circuits to prevent noise, though not strictly required for this quick test.)

Wiring guide

Use the following explicit node connections to build the circuit on your breadboard:

Power Supply:
- V1 positive terminal connects to node VCC.
- V1 negative terminal connects to node 0 (GND).
Sensor Stage (Voltage Divider):
- R1 (LDR) connects between VCC and node VSENSE.
- R2 (Potentiometer) connects between node VSENSE and 0 (GND).
- Note: Adjust R2 so the voltage at VSENSE varies when light changes.
Logic Stage (Inverter):
- U1 Pin 14 connects to VCC.
- U1 Pin 7 connects to 0.
- U1 Pin 1 (Input) connects to node VSENSE.
- U1 Pin 2 (Output) connects to node VOUT.
Output Stage:
- R3 connects between node VOUT and node LED_ANODE.
- D1 connects between node LED_ANODE (Anode/Long leg) and 0 (Cathode/Short leg).

Conceptual block diagram

Schematic

[ INPUT / SENSOR STAGE ]               [ LOGIC STAGE ]                  [ OUTPUT STAGE ]

 [ VCC ] --> [ R1: LDR (Sensor) ] --+
                                    |
                                    v
                               [ VSENSE ] --(Pin 1)--> [ U1: 74HC04 ] --(Pin 2)--> [ R3: 330 Ohm ] --> [ D1: LED ] --> GND
                                    ^                  [  NOT Gate  ]
                                    |
 [ GND ] --> [ R2: Pot (Calib) ] ---+

Schematic (ASCII)

Truth table

The 74HC04 inverts the input signal. We configure the sensors so that «Bright» creates a HIGH input.

Ambient Condition	LDR Resistance	Voltage at `VSENSE` (Input)	Logic Input	Logic Output (`VOUT`)	LED State
Bright	Low	High (> 2.5V)	1	0 (GND)	OFF
Dark	High	Low (< 1.5V)	0	1 (5V)	ON

Measurements and tests

Calibration: Expose the LDR to normal room light. Adjust potentiometer R2 until the LED turns OFF.
Voltage Check (Bright): Measure voltage between VSENSE and GND. It should be close to 5V (Logic 1). The output at VOUT should be near 0V.
Activation: Cover the LDR with your hand to simulate darkness.
Voltage Check (Dark): Measure VSENSE again. It should drop towards 0V (Logic 0). The output VOUT should jump to approx. 5V, turning the LED ON.

SPICE netlist and simulation

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

* Practical case: Automatic darkness sensor

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

* --- Subcircuits ---
* 74HC04 Hex Inverter Model (Behavioral)
* Pins: 1=Input, 2=Output, 7=GND, 14=VCC
* Maps to subckt args: In Out GND VCC
.subckt 74HC04 In Out GND VCC
  * Robust Sigmoid Transfer Function for Inverter
  * Threshold is VCC/2. Output swings between GND and VCC.
  * Formula: Vout = VCC * (1 / (1 + exp(50 * (V(In) - V(VCC)/2))))
  B_INV Out GND V = V(VCC) * (1 / (1 + exp(50 * (V(In) - V(VCC)/2))))
.ends

* --- Main Circuit Components ---

* 1. Power Supply
* ... (truncated in public view) ...

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* Practical case: Automatic darkness sensor

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

* --- Subcircuits ---
* 74HC04 Hex Inverter Model (Behavioral)
* Pins: 1=Input, 2=Output, 7=GND, 14=VCC
* Maps to subckt args: In Out GND VCC
.subckt 74HC04 In Out GND VCC
  * Robust Sigmoid Transfer Function for Inverter
  * Threshold is VCC/2. Output swings between GND and VCC.
  * Formula: Vout = VCC * (1 / (1 + exp(50 * (V(In) - V(VCC)/2))))
  B_INV Out GND V = V(VCC) * (1 / (1 + exp(50 * (V(In) - V(VCC)/2))))
.ends

* --- Main Circuit Components ---

* 1. Power Supply
* V1: 5V DC supply
V1 VCC 0 DC 5

* 2. Sensor Stage (Voltage Divider)
* R1: LDR (Light Dependent Resistor)
* Implementation: A dummy R1 is placed to satisfy the BOM.
* A parallel behavioral source (B_LDR) implements the dynamic resistance change.
R1 VCC VSENSE 100Meg
B_LDR VCC VSENSE I = V(VCC, VSENSE) / V(RES_CTRL)

* R2: 10k Potentiometer (Sensitivity Calibration)
R2 VSENSE 0 10k

* Dynamic Stimulus for LDR (Simulates Light Conditions)
* Generates a control voltage representing Ohms.
* Pulse sweeps from 1k (Light) to 100k (Dark).
* Logic: Light(1k) -> VSENSE High -> LED OFF. Dark(100k) -> VSENSE Low -> LED ON.
V_LDR_CTRL RES_CTRL 0 PULSE(1k 100k 0 200u 200u 400u 2ms)

* 3. Logic Stage
* U1: 74HC04 Hex Inverter
* Connections: Pin 1 (In)=VSENSE, Pin 2 (Out)=VOUT, Pin 7=0, Pin 14=VCC
XU1 VSENSE VOUT 0 VCC 74HC04

* 4. Output Stage
* R3: LED Current Limiting Resistor (330 Ohm)
R3 VOUT LED_ANODE 330

* D1: Red LED
D1 LED_ANODE 0 DLED

* --- Analysis Directives ---
* Transient analysis to capture the Light/Dark transition
.tran 10u 2ms

* Print specific node voltages for validation
.print tran V(VSENSE) V(VOUT) V(LED_ANODE)

* Compute DC operating point
.op

.end

Simulation Results (Transient Analysis)

Show raw data table (224 rows)

Index   time            v(vsense)       v(vout)         v(led_anode)
0	0.000000e+00	4.545459e+00	1.916016e-44	6.555013e-37
1	1.000000e-07	4.525005e+00	3.875543e-44	2.124754e-38
2	2.000000e-07	4.504821e+00	1.070470e-43	-1.98700e-38
3	4.000000e-07	4.464726e+00	4.391831e-43	-3.30922e-39
4	8.000000e-07	4.386087e+00	5.351931e-42	4.963938e-40
5	1.600000e-06	4.240174e+00	7.789996e-38	7.726704e-38
6	3.200000e-06	3.973321e+00	1.292803e-32	1.287493e-32
7	6.400000e-06	3.529123e+00	-6.61237e-21	-6.59876e-21
8	1.280000e-05	2.884261e+00	2.263832e-08	2.262430e-08
9	1.905731e-05	2.447108e+00	4.668386e+00	1.823995e+00
10	2.344117e-05	2.212214e+00	4.999997e+00	1.833723e+00
11	2.751655e-05	2.030989e+00	5.000000e+00	1.833029e+00
12	3.266976e-05	1.840361e+00	5.000000e+00	1.833116e+00
13	4.266976e-05	1.556825e+00	5.000000e+00	1.833028e+00
14	5.266976e-05	1.349010e+00	5.000000e+00	1.833116e+00
15	6.266976e-05	1.190157e+00	5.000000e+00	1.833028e+00
16	7.266976e-05	1.064784e+00	5.000000e+00	1.833116e+00
17	8.266976e-05	9.633175e-01	5.000000e+00	1.833028e+00
18	9.266976e-05	8.795141e-01	5.000000e+00	1.833116e+00
19	1.026698e-04	8.091310e-01	5.000000e+00	1.833028e+00
20	1.126698e-04	7.491835e-01	5.000000e+00	1.833116e+00
21	1.226698e-04	6.975110e-01	5.000000e+00	1.833028e+00
22	1.326698e-04	6.525106e-01	5.000000e+00	1.833116e+00
23	1.426698e-04	6.129684e-01	5.000000e+00	1.833028e+00
... (200 more rows) ...

Common mistakes and how to avoid them

Swapping LDR and Potentiometer: If you swap R1 and R2, the logic inverts: the light will turn ON when it is bright and OFF when it is dark. Ensure the LDR is connected to VCC and the Potentiometer to GND.
LED inserted backwards: If D1 does not light up when VOUT is high, check the polarity. The longer leg (anode) must face the resistor R3.
Sensitivity too low: If the LED never turns off, R2 might be set to too high a resistance, keeping voltage at VSENSE always high. Turn the knob to lower the resistance.

Troubleshooting

LED is always ON:
- Cause: Potentiometer resistance is too high or LDR is broken (open circuit).
- Fix: Decrease R2 value by turning the knob. Check LDR connections.
LED is always OFF:
- Cause: Potentiometer resistance is too low (shorting input to ground) or U1 is not powered.
- Fix: Verify Pin 14 has 5V. Increase R2 resistance slightly.
LED flickers:
- Cause: The light level is right at the switching threshold of the 74HC04.
- Fix: Adjust R2 slightly to move away from the threshold or shade the LDR more decisively.

Possible improvements and extensions

Add Hysteresis: Replace the 74HC04 with a 74HC14 (Schmidt Trigger Inverter). This prevents flickering when the light transitions slowly (dusk/dawn).
High Power Load: Connect the output pin to a transistor (like a 2N2222) and a relay module to switch a 110V/220V desk lamp instead of a small LED.

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Carlos Núñez Zorrilla

Electronics & Computer Engineer

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

Follow me:

Who we are

Level: Basic. Objective: Build a logic circuit using a NOT gate that activates an LED when a switch contact is opened.

Objective and use use case

You will build a digital monitoring circuit that illuminates an LED indicator whenever a switch (representing a door sensor) breaks contact. This demonstrates the fundamental operation of the NOT gate (Inverter) in security logic.

Why it is useful:
- Home Security: Basic principle behind magnetic reed switches used on windows and doors.
- Appliance Safety: Ensures devices like microwaves or washing machines do not run if the door is open.
- Industrial Interlocks: Visual warning systems for machine guards.
Expected outcome:
- Door Closed (Switch Closed): Input logic High (5V), Output logic Low (0V), LED OFF.
- Door Open (Switch Open): Input logic Low (0V), Output logic High (5V), LED ON.
- Target audience and level: Introductory Electronics Students (Basic).

Materials

V1: 5 V DC supply, function: Main power source
U1: 74HC04, function: Hex Inverter (NOT gate logic)
SW1: SPST Switch, function: Simulates door sensor (Closed = Door Closed)
R1: 10 kΩ resistor, function: Pull-down for U1 input
R2: 330 Ω resistor, function: LED current limiting
D1: Red LED, function: Visual alarm indicator

Pin-out of the IC used

Chip: 74HC04 (Hex Inverter)

Pin	Name	Logic function	Connection in this case
1	1A	Input	Connected to SW1 and R1
2	1Y	Output	Connected to LED resistor R2
7	GND	Ground	Connected to 0V (Power Supply Ground)
14	VCC	Power	Connected to 5V (Power Supply Positive)

Wiring guide

VCC connects to V1 positive terminal, U1 pin 14, and one side of SW1.
0 (GND) connects to V1 negative terminal, U1 pin 7, R1, and cathode of D1.
DOOR_STATUS (Node A) connects to the other side of SW1, the other side of R1, and U1 pin 1.
ALARM_OUT (Node Y) connects to U1 pin 2 and one side of R2.
LED_ANODE connects to the other side of R2 and the anode of D1.

Conceptual block diagram

Schematic

[ INPUT / SENSOR ]                 [ LOGIC PROCESSING ]               [ OUTPUT / ALARM ]

    [ VCC (5V Source) ]
             |
             v
    [ SW1 (Door Switch) ]
             |
             v
          (Node A) -------------------->+------------------+
             |                          |    U1: 74HC04    |
             v                          |    (NOT Gate)    | --(Pin 2)--> [ R2: 330Ω ] --> [ D1: LED ] --> GND
    [ R1 (10k Pull-down) ]              |  Input: Pin 1    |
             |                          +------------------+
             v
            GND

Schematic (ASCII)

Truth table

Door Status	Switch (SW1)	Input Voltage (Pin 1)	Logic Input	Logic Output (Pin 2)	LED Status
Closed	Closed	5 V (High)	1	0	OFF
Open	Open	0 V (Low)	0	1	ON

Measurements and tests

Supply Check: Before inserting the IC, verify V1 provides exactly 5 V.
State 1 (Secure): Close SW1. Measure voltage at Pin 1 (Input). It should be ~5 V. Measure Pin 2 (Output). It should be ~0 V. Verify LED is OFF.
State 2 (Alarm): Open SW1. Measure voltage at Pin 1 (Input). It should drop to 0 V (pulled down by R1). Measure Pin 2 (Output). It should rise to ~5 V. Verify LED is ON.

SPICE netlist and simulation

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

* Practical case: Open door alarm
*
* BILL OF MATERIALS:
* V1: 5V DC Supply
* U1: 74HC04 Hex Inverter (Behavioral Model)
* SW1: SPST Switch (Modeled as Voltage-Controlled Switch)
* R1: 10k Pull-down Resistor
* R2: 330 Ohm Current Limiting Resistor
* D1: Red LED
*
* WIRING CONNECTIONS:
* VCC: V1(+), U1(14), SW1(1)
* GND: V1(-), U1(7), R1(2), D1(Cathode)
* DOOR_STATUS: SW1(2), R1(1), U1(1)
* ALARM_OUT: U1(2), R2(1)
* LED_ANODE: R2(2), D1(Anode)

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

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

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* Practical case: Open door alarm
*
* BILL OF MATERIALS:
* V1: 5V DC Supply
* U1: 74HC04 Hex Inverter (Behavioral Model)
* SW1: SPST Switch (Modeled as Voltage-Controlled Switch)
* R1: 10k Pull-down Resistor
* R2: 330 Ohm Current Limiting Resistor
* D1: Red LED
*
* WIRING CONNECTIONS:
* VCC: V1(+), U1(14), SW1(1)
* GND: V1(-), U1(7), R1(2), D1(Cathode)
* DOOR_STATUS: SW1(2), R1(1), U1(1)
* ALARM_OUT: U1(2), R2(1)
* LED_ANODE: R2(2), D1(Anode)

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

* --- User Interaction (Door Sensor) ---
* Model SW1 as a voltage-controlled switch S1 driven by a pulse source.
* Logic: Control High = Switch Closed (Door Closed). Control Low = Switch Open (Door Open).
* Pulse: Starts 0V (Open/Alarm ON), goes to 5V (Closed/Alarm OFF) at 1ms, stays for 2ms.
V_SW_CTRL SW_CTRL 0 PULSE(0 5 1m 10u 10u 2m 5m)

* S1 connects VCC to DOOR_STATUS when SW_CTRL is High.
S1 VCC DOOR_STATUS SW_CTRL 0 SW_DOOR
.model SW_DOOR SW(Vt=2.5 Ron=0.1 Roff=100Meg)

* --- Pull-down Resistor ---
R1 DOOR_STATUS 0 10k

* --- 74HC04 Hex Inverter (U1) ---
* Implements NOT gate logic: ALARM_OUT = NOT(DOOR_STATUS)
* Pin mapping: 1=In, 2=Out, 7=GND, 14=VCC
XU1 DOOR_STATUS ALARM_OUT 0 VCC 74HC04_GATE

* --- Output Stage ---
R2 ALARM_OUT LED_ANODE 330
D1 LED_ANODE 0 D_RED

* --- Models and Subcircuits ---

* LED Model
.model D_RED D(IS=1e-22 RS=6 N=1.5 CJO=50p BV=5 IBV=10u)

* 74HC04 Single Gate Behavioral Model
* Pins: In Out GND VCC
.subckt 74HC04_GATE 1 2 7 14
* Continuous sigmoid function for robust NOT logic
* Vout goes Low when Vin > 2.5V, High when Vin < 2.5V
B_INV 2 7 V = V(14,7) * (1 / (1 + exp(50 * (V(1,7) - 2.5))))
.ends

* --- Simulation Directives ---
.tran 10u 5ms
.op

* --- Output Printing ---
.print tran V(DOOR_STATUS) V(ALARM_OUT) V(LED_ANODE) V(SW_CTRL)

.end

Simulation Results (Transient Analysis)

Show raw data table (1126 rows)

Index   time            v(door_status)  v(alarm_out)    v(led_anode)
0	0.000000e+00	4.999500e-04	5.000000e+00	1.842385e+00
1	1.000000e-07	4.999500e-04	5.000000e+00	1.842385e+00
2	2.000000e-07	4.999500e-04	5.000000e+00	1.842385e+00
3	4.000000e-07	4.999500e-04	5.000000e+00	1.842385e+00
4	8.000000e-07	4.999500e-04	5.000000e+00	1.842385e+00
5	1.600000e-06	4.999500e-04	5.000000e+00	1.842385e+00
6	3.200000e-06	4.999500e-04	5.000000e+00	1.842385e+00
7	6.400000e-06	4.999500e-04	5.000000e+00	1.842385e+00
8	1.280000e-05	4.999500e-04	5.000000e+00	1.842385e+00
9	2.280000e-05	4.999500e-04	5.000000e+00	1.842385e+00
10	3.280000e-05	4.999500e-04	5.000000e+00	1.842385e+00
11	4.280000e-05	4.999500e-04	5.000000e+00	1.842385e+00
12	5.280000e-05	4.999500e-04	5.000000e+00	1.842385e+00
13	6.280000e-05	4.999500e-04	5.000000e+00	1.842385e+00
14	7.280000e-05	4.999500e-04	5.000000e+00	1.842385e+00
15	8.280000e-05	4.999500e-04	5.000000e+00	1.842385e+00
16	9.280000e-05	4.999500e-04	5.000000e+00	1.842385e+00
17	1.028000e-04	4.999500e-04	5.000000e+00	1.842385e+00
18	1.128000e-04	4.999500e-04	5.000000e+00	1.842385e+00
19	1.228000e-04	4.999500e-04	5.000000e+00	1.842385e+00
20	1.328000e-04	4.999500e-04	5.000000e+00	1.842385e+00
21	1.428000e-04	4.999500e-04	5.000000e+00	1.842385e+00
22	1.528000e-04	4.999500e-04	5.000000e+00	1.842385e+00
23	1.628000e-04	4.999500e-04	5.000000e+00	1.842385e+00
... (1102 more rows) ...

Common mistakes and how to avoid them

Floating Input: Forgetting the pull-down resistor (R1). Without R1, when the switch opens, the input pin floats and the LED may flicker or remain in an unpredictable state. Always tie CMOS inputs to a defined logic level.
No LED Resistor: Connecting the LED directly to the 74HC04 output without R2. This can burn out the LED or damage the IC output stage due to excessive current.
Wrong Polarity: Inserting the LED backwards (anode to ground). The LED will never light up. Ensure the longer leg (anode) faces the resistor coming from the IC.

Troubleshooting

LED always ON: Check if SW1 is actually closing. If using a push-button, ensure it is connected to VCC. Verify R1 is connected to Ground.
LED always OFF: Check if the 74HC04 has power (Pin 14) and Ground (Pin 7). Check LED polarity. Ensure SW1 is actually disconnecting VCC when «Open».
LED is dim: The value of R2 might be too high (e.g., 10kΩ instead of 330Ω) or the 5V supply is sagging.
IC gets hot: Immediate disconnect power. Check for short circuits between Output (Pin 2) and Ground, or if the chip is inserted backwards.

Possible improvements and extensions

Audible Alarm: Connect an NPN transistor and a buzzer to the output to generate sound alongside the light when the door opens.
Latch Circuit: Add a feedback loop or a Flip-Flop so that once the alarm triggers, it stays ON even if the door is closed again, requiring a manual reset button.

More Practical Cases on Prometeo.blog

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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 – Understand the logic of a NOT gate (inverter) by observing opposite input and output states via light indicators.

Objective and use case

In this practical case, you will build a digital logic circuit using a 74HC04 Hex Inverter IC. The circuit will demonstrate the fundamental inversion function where a HIGH input signal results in a LOW output signal, visually confirmed by two LEDs operating in alternate states.

Why it is useful:
* Safety Interlocks: Used in machinery to ensure a system stops (logic LOW) when a sensor is activated (logic HIGH).
* Status Indicators: Allows creating «Standby» lights that turn ON only when the main power switch is OFF.
* Logic Level Adaptation: Essential for interfacing active-high sensors with active-low microcontroller inputs.
* Signal Conditioning: Cleans up noisy digital signals and ensures distinct logic levels.

Expected outcome:
* Input LED (Green): Turns ON when the switch is pressed (Logic 1).
* Output LED (Red): Turns OFF when the switch is pressed (Logic 0).
* Inverse Relationship: When the switch is released (Logic 0), the Red LED turns ON.
* Voltage Levels: Input at 0V $\rightarrow$ Output $\approx$ 5V; Input at 5V $\rightarrow$ Output $\approx$ 0V.

Target audience: Students and hobbyists (Level: Basic).

Materials

V1: 5 V DC supply (battery or regulated power supply)
S1: SPST toggle or tactile switch, function: Input signal generator
U1: 74HC04 (Hex Inverter IC), function: Logic inversion
R1: 10 kΩ resistor, function: Pull-down resistor for input VA
R2: 330 Ω resistor, function: Current limiting for input LED (D1)
R3: 330 Ω resistor, function: Current limiting for output LED (D2)
D1: Green LED, function: Input state indicator (Active High)
D2: Red LED, function: Output state indicator (Active High)

Pin-out of the IC used

Chip Selected: 74HC04 (Hex Inverter)

Pin	Name	Logic function	Connection in this case
1	1A	Input	Connected to Switch S1 and Pull-down R1
2	1Y	Output	Connected to Output LED (D2) via R3
7	GND	Ground	Connected to Power Supply Negative (0V)
14	VCC	Power	Connected to Power Supply Positive (5V)

Note: Pins 3, 4, 5, 6, 8, 9, 10, 11, 12, and 13 are unused in this single-gate demonstration. In a permanent circuit, unused inputs on CMOS chips should be tied to GND.

Wiring guide

VCC: Connect positive terminal of V1, Pin 14 of U1, and one side of S1.
0 (GND): Connect negative terminal of V1, Pin 7 of U1, one side of R1, cathode of D1, and cathode of D2.
VA (Input Node): Connect the other side of S1, the other side of R1, Pin 1 of U1, and one side of R2.
Input Indicator: Connect the other side of R2 to the anode of D1.
VOUT (Output Node): Connect Pin 2 of U1 to one side of R3.
Output Indicator: Connect the other side of R3 to the anode of D2.

Conceptual block diagram

Schematic

[ INPUT GENERATION ]               [ LOGIC & MONITORING ]               [ OUTPUT STAGE ]

    [ VCC ] -> [ Switch S1 ] --+
                               |
                               V
                           (Node VA) --(Pin 1)--> [ U1: 74HC04 ] --(Pin 2)--> [ R3: 330 ] -> [ D2: Red ] -> GND
                               |                  (Hex Inverter)
                               |
    [ GND ] <- [ R1: 10k ] <---+
                               |
                               +----(Monitor)---> [ R2: 330 ] --> [ D1: Green ] -> GND

Schematic (ASCII)

Truth table

The 74HC04 contains six independent NOT gates. We are using one.

Input (VA)	Switch State	Output (VOUT)	Green LED (D1)	Red LED (D2)
0 (Low)	Open	1 (High)	OFF	ON
1 (High)	Closed	0 (Low)	ON	OFF

Measurements and tests

To validate the circuit, perform the following steps using a multimeter and visual observation:

Default State Check:
- Ensure S1 is open (not pressed).
- Measure voltage at VA relative to GND. It should be 0V.
- Measure voltage at VOUT relative to GND. It should be close to 5V.
- Visual: Red LED (D2) is ON; Green LED (D1) is OFF.
Active State Check:
- Close (press) S1.
- Measure voltage at VA. It should be close to 5V.
- Measure voltage at VOUT. It should be close to 0V.
- Visual: Red LED (D2) turns OFF; Green LED (D1) turns ON.
Transition Threshold (Optional):
- If using a variable voltage source instead of S1, slowly increase voltage at VA. The output state will flip when the input crosses approximately half of VCC (approx. 2.5V for 74HC series).

SPICE netlist and simulation

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

* Practical case: Signal inverter with indicator LED

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

* --- Input Signal Generator (Switch S1) ---
* S1 connects VCC to VA (Input Node) when pressed.
* R1 pulls VA to Ground when S1 is open.
* V_S1_ACT simulates the user pressing the button (Active High).
* Pulse timing: Wait 10u, Press for 100u, Repeat every 200u.
V_S1_ACT S_ACT 0 PULSE(0 5 10u 1u 1u 100u 200u)
S1 VCC VA S_ACT 0 SW_IDEAL

* --- Input Circuit Components ---
R1 VA 0 10k
R2 VA N_D1_A 330
D1 N_D1_A 0 LED_GREEN

* --- Logic Inverter (U1: 74HC04) ---
* Wiring: Pin1=VA, Pin2=VOUT, Pin7=GND, Pin14=VCC
* Implemented as a behavioral subcircuit to match pinout
XU1 VA VOUT 0 VCC 74HC04_1G

* --- Output Circuit Components ---
R3 VOUT N_D2_A 330
D2 N_D2_A 0 LED_RED

* --- Models ---
* Voltage Controlled Switch Model
.model SW_IDEAL SW(Vt=2.5 Ron=1 Roff=10Meg)
* ... (truncated in public view) ...

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* Practical case: Signal inverter with indicator LED

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

* --- Input Signal Generator (Switch S1) ---
* S1 connects VCC to VA (Input Node) when pressed.
* R1 pulls VA to Ground when S1 is open.
* V_S1_ACT simulates the user pressing the button (Active High).
* Pulse timing: Wait 10u, Press for 100u, Repeat every 200u.
V_S1_ACT S_ACT 0 PULSE(0 5 10u 1u 1u 100u 200u)
S1 VCC VA S_ACT 0 SW_IDEAL

* --- Input Circuit Components ---
R1 VA 0 10k
R2 VA N_D1_A 330
D1 N_D1_A 0 LED_GREEN

* --- Logic Inverter (U1: 74HC04) ---
* Wiring: Pin1=VA, Pin2=VOUT, Pin7=GND, Pin14=VCC
* Implemented as a behavioral subcircuit to match pinout
XU1 VA VOUT 0 VCC 74HC04_1G

* --- Output Circuit Components ---
R3 VOUT N_D2_A 330
D2 N_D2_A 0 LED_RED

* --- Models ---
* Voltage Controlled Switch Model
.model SW_IDEAL SW(Vt=2.5 Ron=1 Roff=10Meg)

* LED Models (Generic)
.model LED_GREEN D(IS=1e-22 RS=5 N=1.5 CJO=50p)
.model LED_RED D(IS=1e-22 RS=5 N=1.5 CJO=50p)

* --- Subcircuits ---
* 74HC04 Hex Inverter (Single Gate Representation)
* Pins: 1=Input, 2=Output, 7=GND, 14=VCC
.subckt 74HC04_1G 1 2 7 14
* Behavioral source implementing Inverter Logic: Vout = NOT(Vin)
* Uses sigmoid function for convergence: 1 / (1 + exp(k*(Vin - Vth)))
* Multiplied by V(14) to track supply voltage
B1 2 7 V = V(14) * (1 / (1 + exp(50 * (V(1) - 2.5))))
.ends

* --- Analysis Directives ---
* Transient analysis for 500us to capture pulse cycles
.tran 1u 500u

* Output data for plotting/logging
.print tran V(VA) V(VOUT) V(N_D1_A) V(N_D2_A)

.op
.end

Simulation Results (Transient Analysis)

Show raw data table (1334 rows)

Index   time            v(va)           v(vout)         v(n_d1_a)
0	0.000000e+00	4.995005e-03	5.000000e+00	4.995005e-03
1	1.000000e-08	4.995005e-03	5.000000e+00	4.995005e-03
2	2.000000e-08	4.995005e-03	5.000000e+00	4.995005e-03
3	4.000000e-08	4.995005e-03	5.000000e+00	4.995005e-03
4	8.000000e-08	4.995005e-03	5.000000e+00	4.995005e-03
5	1.600000e-07	4.995005e-03	5.000000e+00	4.995005e-03
6	3.200000e-07	4.995005e-03	5.000000e+00	4.995005e-03
7	6.400000e-07	4.995005e-03	5.000000e+00	4.995005e-03
8	1.280000e-06	4.995005e-03	5.000000e+00	4.995005e-03
9	2.280000e-06	4.995005e-03	5.000000e+00	4.995005e-03
10	3.280000e-06	4.995005e-03	5.000000e+00	4.995005e-03
11	4.280000e-06	4.995005e-03	5.000000e+00	4.995005e-03
12	5.280000e-06	4.995005e-03	5.000000e+00	4.995005e-03
13	6.280000e-06	4.995005e-03	5.000000e+00	4.995005e-03
14	7.280000e-06	4.995005e-03	5.000000e+00	4.995005e-03
15	8.280000e-06	4.995005e-03	5.000000e+00	4.995005e-03
16	9.280000e-06	4.995005e-03	5.000000e+00	4.995005e-03
17	1.000000e-05	4.995005e-03	5.000000e+00	4.995005e-03
18	1.010000e-05	4.995005e-03	5.000000e+00	4.995005e-03
19	1.026000e-05	4.995005e-03	5.000000e+00	4.995005e-03
20	1.030750e-05	4.995005e-03	5.000000e+00	4.995005e-03
21	1.039062e-05	4.995005e-03	5.000000e+00	4.995005e-03
22	1.041363e-05	4.995005e-03	5.000000e+00	4.995005e-03
23	1.045390e-05	4.995005e-03	5.000000e+00	4.995005e-03
... (1310 more rows) ...

Common mistakes and how to avoid them

Floating Inputs:
- Error: Omitting the pull-down resistor (R1). The input floats when the switch is open, causing the output LED to flicker or oscillate due to electromagnetic noise.
- Solution: Always ensure the input has a defined path to GND (via R1) when the switch is open.
Missing Current Limiting Resistors:
- Error: Connecting LEDs directly to the IC output or VCC without R2 or R3.
- Solution: Always use series resistors (330 Ω to 1 kΩ) to prevent burning out the LED or damaging the 74HC04 output stage.
Confusing Pin Numbering:
- Error: Wiring the IC upside down or counting pins from the wrong side.
- Solution: Identify the notch/dot on the package. Pin 1 is to the left of the notch when the notch faces up.

Troubleshooting

Both LEDs remain OFF:
- Cause: Power supply disconnected or IC inserted backwards.
- Fix: Check VCC (Pin 14) and GND (Pin 7) connections. Ensure 5V is present.
Output LED (Red) never turns OFF:
- Cause: The input VA is not reaching Logic High (5V) effectively, or the IC is damaged.
- Fix: Check the continuity of Switch S1. Measure voltage at Pin 1 while pressing the switch.
Output LED (Red) is dim:
- Cause: Resistor R3 is too high in value, or the supply voltage is too low.
- Fix: Verify R3 is 330 Ω. Check if V1 is actually 5V.

Possible improvements and extensions

Buffer Circuit: Connect the output of the first inverter (Pin 2) into the input of a second inverter (Pin 3). The output of the second inverter (Pin 4) will now match the original input state, acting as a non-inverting buffer.
Square Wave Oscillator: Use three NOT gates in a ring loop (Logic Ring Oscillator) to create a circuit that blinks the LEDs automatically without a switch.

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.

Carlos Núñez Zorrilla

Electronics & Computer Engineer

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

Follow me:

Who we are

Practical case: Emergency deactivation

Objective and use case

Materials

Pin-out of the IC used

Wiring guide

Conceptual block diagram

Schematic

Truth table

Measurements and tests

SPICE netlist and simulation

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

Simulation Results (Transient Analysis)

Common mistakes and how to avoid them

Troubleshooting

Possible improvements and extensions

Quick Quiz

Practical case: Automatic darkness sensor

Objective and use case

Materials

Pin-out of the IC used

Wiring guide

Conceptual block diagram

Schematic

Truth table

Measurements and tests

SPICE netlist and simulation

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

Simulation Results (Transient Analysis)

Common mistakes and how to avoid them

Troubleshooting

Possible improvements and extensions

Quick Quiz

Practical case: Open door alarm

Objective and use use case

Materials

Pin-out of the IC used

Wiring guide

Conceptual block diagram

Schematic

Truth table

Measurements and tests

SPICE netlist and simulation

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

Simulation Results (Transient Analysis)

Common mistakes and how to avoid them

Troubleshooting

Possible improvements and extensions

Quick Quiz

Practical case: Signal inverter with indicator LED

Objective and use case

Materials

Pin-out of the IC used

Wiring guide

Conceptual block diagram

Schematic

Truth table

Measurements and tests

SPICE netlist and simulation

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

Simulation Results (Transient Analysis)

Common mistakes and how to avoid them

Troubleshooting

Possible improvements and extensions

Quick Quiz