Practical case: Standby mode indicator

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 Schematic

Electrical diagram

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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.

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* 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)

.end

Simulation 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) ...

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

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Carlos Núñez Zorrilla

Electronics & Computer Engineer

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

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