Level: Basic – Understand logical inversion using a NOT gate to activate a standby LED when the main system turns off.
Objective and use case
You will build a digital logic circuit using a 74HC04 NOT gate that monitors a main power switch. When the switch is turned off, the NOT gate logically inverts the signal to activate a «standby» indicator LED.
Why this is useful:
* It perfectly replicates how household appliances (like televisions or microwaves) indicate they are plugged in but currently turned off.
* It provides clear visual feedback in industrial panels when a machine is safe to approach.
* It serves as a foundational example of how to invert control signals for active-low indicators and logic translation.
Expected outcome:
* When the main switch is closed (HIGH logic state, near 5 V), the standby LED remains strictly OFF.
* When the main switch is open (LOW logic state, near 0 V), the standby LED turns ON.
* The circuit accurately demonstrates the inversion of logic states (V_in vs. V_out) through practical voltage measurements.
Target audience and level: Beginners in digital electronics learning basic logic gates.
Materials
- V1: 5 V DC supply, function: main power source
- SW1: SPST switch, function: main system power switch simulator
- R1: 10 kΩ resistor, function: pull-down for VA node
- U1: 74HC04 hex inverter IC, function: logical NOT gate
- R2: 330 Ω resistor, function: LED current limiting
- D1: red LED, function: standby mode indicator
Pin-out of the 74HC04 IC
The 74HC04 is a Hex Inverter IC, meaning it contains six independent NOT gates. We will use the first gate.
| Pin | Name | Logic function | Connection in this case |
|---|---|---|---|
| 1 | 1 A | Data Input | Connects to switch output (VA) |
| 2 | 1Y | Data Output | Connects to LED resistor (VOUT) |
| 7 | GND | Ground | Connects to system ground (0) |
| 14 | VCC | Positive Supply | Connects to positive voltage (VCC) |
(Note: The other input pins [3, 5, 9, 11, 13] should ideally be tied to ground in a permanent circuit to prevent floating inputs and reduce power consumption, but are omitted here for simplicity).
Wiring guide
- V1: connects between VCC and 0.
- SW1: connects between VCC and VA.
- R1: connects between VA and 0.
- U1: Pin 14 connects to VCC, Pin 7 connects to 0, Pin 1 (1 A) connects to VA, Pin 2 (1Y) connects to VOUT.
- R2: connects between VOUT and VLED.
- D1: anode connects to VLED, cathode connects to 0.
Conceptual block diagram
Schematic
VCC --> [ SW1: SPST Switch ] --(Node VA)--> [ U1: 74HC04 Inverter ] --(VOUT)--> [ R2: 330 Ω Resistor ] --(VLED)--> [ D1: Red LED ] --> GND
|
V
[ R1: 10 kΩ Pull-down ]
|
V
GND
Electrical diagram
Truth table
Input (VA) |
Output (VOUT) |
Standby LED State |
|---|---|---|
| 0 (LOW) | 1 (HIGH) | ON |
| 1 (HIGH) | 0 (LOW) | OFF |
Measurements and tests
- Test the Input Signal (V_in): Connect your multimeter between node VA and ground (0). Open SW1 and verify the voltage is near 0 V. Close SW1 and verify the voltage is near 5 V.
- Test the Inverted Output (V_out): Connect your multimeter between node VOUT and ground (0). Observe the voltage invert: it should be near 5 V when SW1 is open, and near 0 V when SW1 is closed.
- Verify the Logic State: Physically observe D1. Ensure it lights up only when the simulated main system (SW1) is powered down.
SPICE netlist and simulation
Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)
* Practical case: Standby mode indicator
.width out=256
* Power Supply
V1 VCC 0 DC 5
* Switch SW1 (Main system power switch simulator)
* Starts closed (system ON, standby OFF), opens at 50us (system OFF, standby ON)
S1 VCC VA SW_CTRL 0 SWMOD
VSW_CTRL SW_CTRL 0 PULSE(5 0 50u 1u 1u 100u 250u)
.model SWMOD SW(VT=2.5 RON=0.1 ROFF=100MEG)
* Pull-down resistor for switch node VA
R1 VA 0 10k
* U1: 74HC04 Hex Inverter IC
* Pin 1 (1A) = VA, Pin 2 (1Y) = VOUT, Pin 14 = VCC, Pin 7 = 0
XU1 VA VOUT VCC 0 74HC04_INV
* Current limiting resistor for LED
* ... (truncated in public view) ...Copy this content into a .cir file and run with ngspice.
* Practical case: Standby mode indicator
.width out=256
* Power Supply
V1 VCC 0 DC 5
* Switch SW1 (Main system power switch simulator)
* Starts closed (system ON, standby OFF), opens at 50us (system OFF, standby ON)
S1 VCC VA SW_CTRL 0 SWMOD
VSW_CTRL SW_CTRL 0 PULSE(5 0 50u 1u 1u 100u 250u)
.model SWMOD SW(VT=2.5 RON=0.1 ROFF=100MEG)
* Pull-down resistor for switch node VA
R1 VA 0 10k
* U1: 74HC04 Hex Inverter IC
* Pin 1 (1A) = VA, Pin 2 (1Y) = VOUT, Pin 14 = VCC, Pin 7 = 0
XU1 VA VOUT VCC 0 74HC04_INV
* Current limiting resistor for LED
R2 VOUT VLED 330
* D1: Red LED (Standby mode indicator)
D1 VLED 0 DLED
.model DLED D(IS=1e-15 N=1.8 RS=10)
* Subcircuit for 74HC04 Inverter Gate
.subckt 74HC04_INV A Y VCC GND
B1 Y_int GND V=V(VCC,GND)*0.5*(1-tanh(10*(V(A,GND)-2.5)))
Rin A GND 100Meg
Rout Y_int Y 50
.ends
* Simulation Directives
.tran 1u 300u
.op
* Output Directives (Input and Output nodes listed first)
.print tran V(VA) V(VOUT) V(VLED) V(VCC)
.endSimulation Results (Transient Analysis)
Analysis: The simulation shows that when the switch is closed (VA ≈ 5V), the inverter output VOUT is 0V and the LED is off. When the switch opens at 50us (VA ≈ 0V due to pull-down R1), VOUT goes HIGH (≈ 4.5V) and the LED turns on (VLED ≈ 1.48V). This perfectly matches the intended truth table.
Show raw data table (340 rows)
Index time v(va) v(vout) v(vled) v(vcc) 0 0.000000e+00 4.999950e+00 1.082004e-19 8.223227e-19 5.000000e+00 1 1.000000e-08 4.999950e+00 9.063787e-31 6.888478e-30 5.000000e+00 2 2.000000e-08 4.999950e+00 -9.06379e-31 -6.88848e-30 5.000000e+00 3 4.000000e-08 4.999950e+00 -3.79630e-41 -2.88519e-40 5.000000e+00 4 8.000000e-08 4.999950e+00 1.518521e-41 1.154076e-40 5.000000e+00 5 1.600000e-07 4.999950e+00 1.017634e-51 7.734020e-51 5.000000e+00 6 3.200000e-07 4.999950e+00 -2.54409e-52 -1.93351e-51 5.000000e+00 7 6.400000e-07 4.999950e+00 -2.34426e-62 -1.78164e-61 5.000000e+00 8 1.280000e-06 4.999950e+00 4.262287e-63 3.239338e-62 5.000000e+00 9 2.280000e-06 4.999950e+00 3.983291e-73 3.027301e-72 5.000000e+00 10 3.280000e-06 4.999950e+00 -3.57046e-74 -2.71355e-73 5.000000e+00 11 4.280000e-06 4.999950e+00 -3.93493e-84 -2.99055e-83 5.000000e+00 12 5.280000e-06 4.999950e+00 2.990920e-85 2.273099e-84 5.000000e+00 13 6.280000e-06 4.999950e+00 3.797323e-95 2.885965e-94 5.000000e+00 14 7.280000e-06 4.999950e+00 -2.50545e-96 -1.90414e-95 5.000000e+00 15 8.280000e-06 4.999950e+00 -3.60072e-106 -2.73655e-105 5.000000e+00 16 9.280000e-06 4.999950e+00 2.098779e-107 1.595072e-106 5.000000e+00 17 1.028000e-05 4.999950e+00 3.367893e-117 2.559599e-116 5.000000e+00 18 1.128000e-05 4.999950e+00 -1.75812e-118 -1.33617e-117 5.000000e+00 19 1.228000e-05 4.999950e+00 -3.11579e-128 -2.36800e-127 5.000000e+00 20 1.328000e-05 4.999950e+00 1.472749e-129 1.119289e-128 5.000000e+00 21 1.428000e-05 4.999950e+00 2.856788e-139 2.171159e-138 5.000000e+00 22 1.528000e-05 4.999950e+00 -1.23370e-140 -9.37613e-140 5.000000e+00 23 1.628000e-05 4.999950e+00 -2.59978e-150 -1.97583e-149 5.000000e+00 ... (316 more rows) ...
Reference SPICE netlist (ngspice)
* Practical case: Standby mode indicator
.width out=256
* Power Supply
V1 VCC 0 DC 5
* Switch SW1 (Main system power switch simulator)
* Starts closed (system ON, standby OFF), opens at 50us (system OFF, standby ON)
S1 VCC VA SW_CTRL 0 SWMOD
VSW_CTRL SW_CTRL 0 PULSE(5 0 50u 1u 1u 100u 250u)
.model SWMOD SW(VT=2.5 RON=0.1 ROFF=100MEG)
* Pull-down resistor for switch node VA
R1 VA 0 10k
* U1: 74HC04 Hex Inverter IC
* Pin 1 (1A) = VA, Pin 2 (1Y) = VOUT, Pin 14 = VCC, Pin 7 = 0
XU1 VA VOUT VCC 0 74HC04_INV
* Current limiting resistor for LED
R2 VOUT VLED 330
* D1: Red LED (Standby mode indicator)
D1 VLED 0 DLED
.model DLED D(IS=1e-15 N=1.8 RS=10)
* Subcircuit for 74HC04 Inverter Gate
.subckt 74HC04_INV A Y VCC GND
B1 Y_int GND V=V(VCC,GND)*0.5*(1-tanh(10*(V(A,GND)-2.5)))
Rin A GND 100Meg
Rout Y_int Y 50
.ends
* Simulation Directives
.tran 1u 300u
.op
* Output Directives (Input and Output nodes listed first)
.print tran V(VA) V(VOUT) V(VLED) V(VCC)
.endSimulation Results (Transient Analysis)
Analysis: The simulation shows that when the switch is closed (VA ≈ 5V), the inverter output VOUT is 0V and the LED is off. When the switch opens at 50us (VA ≈ 0V due to pull-down R1), VOUT goes HIGH (≈ 4.5V) and the LED turns on (VLED ≈ 1.48V). This perfectly matches the intended truth table.
Common mistakes and how to avoid them
- Omitting the pull-down resistor (R1): Without R1, opening SW1 leaves the input pin (VA) floating, which can cause the NOT gate to oscillate unpredictably or pick up stray noise. Always secure the LOW state with a pull-down resistor.
- Forgetting IC power pins: It is common to wire the input and output of a logic gate but forget to connect VCC (Pin 14) and GND (Pin 7) on the U1 chip itself. The gate will not function without power.
- Reversing the LED polarity: If D1 is installed backwards (cathode to VLED, anode to 0), it will block current and never light up, even when VOUT correctly outputs 5 V.
Troubleshooting
- Symptom: The standby LED is always OFF.
- Cause: The LED might be backwards, R2 might be too high in value, or the IC is missing power.
- Fix: Check LED orientation (long leg to VLED). Verify U1 pins 14 and 7 are securely connected to VCC and 0.
- Symptom: The standby LED is always ON, regardless of the switch.
- Cause: The switch is not properly connected to VCC, or the switch contacts are faulty, leaving the input permanently pulled LOW by R1.
- Fix: Measure node VA. If it stays at 0 V when the switch is closed, check the wiring from VCC to SW1.
- Symptom: The standby LED flickers when the switch is open.
- Cause: Node VA is floating. R1 is likely disconnected or incorrectly placed.
- Fix: Ensure R1 firmly connects node VA directly to ground (0).
Possible improvements and extensions
- Add a «Main System ON» indicator: Connect a green LED and a 330 Ω resistor directly to node VA to show when the main system is actively running, creating a dual-state visual indicator.
- Drive multiple standby indicators: Use another of the unused NOT gates in the 74HC04 (e.g., input on pin 3 connected to VA, output on pin 4) to drive a secondary standby indicator or a low-power piezo buzzer.
More Practical Cases on Prometeo.blog
Find this product and/or books on this topic on Amazon
As an Amazon Associate, I earn from qualifying purchases. If you buy through this link, you help keep this project running.
Quick Quiz
Carlos Núñez Zorrilla
Electronics & Computer Engineer
Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).
