ESP32 Relay Simulator: Wiring, Code and a Live Circuit
This is a single-channel 5 V relay module wired to an ESP32 DevKit, clicking on and off once a second live in your browser. The arm on the drawing moves between NC and NO as the board switches it.
VCC to VIN (5 V from USB), GND to GND, and IN to GPIO 4 (D4). The pin only gives the signal; the module powers the coil.
- 3.3 V logic
- No library
- IN on GPIO 4 (D4)
- Coil on VIN (5 V from USB)
Wiring the Relay to the ESP32
Every connection in the circuit above, with the board’s own pin names.
| Relay pin | ESP32 pin | Why |
|---|---|---|
| VCC | VIN (5 V from USB) | The coil is a 5 V coil; it takes about 70 mA, more than any pin can give. |
| GND | GND | Common ground. |
| IN | GPIO 4 (D4) | A logic signal only: the module’s transistor switches the coil. |
| COM, NO, NC | The load | The switched side: COM is the arm, NO is open until the relay pulls in, NC is closed until then. |
The coil runs from VIN (5 V from USB) and IN comes from a 3.3 V pin. Most single-channel modules switch fine from 3.3 V; some optocoupler modules with a high-side jumper need closer to 5 V, and then a small transistor or a 3.3 V relay module solves it. The simulated relay is set to trigger on HIGH.
Pins to leave alone on the ESP32
GPIO 34, 35, 36 (VP) and 39 (VN) are input only: they cannot drive a pin or take an internal pull-up. GPIO 0, 2, 5, 12 and 15 are strapping pins, read at power-on, and GPIO 6 to 11 are the flash chip.
On a real bench
The thirty-pin DevKit straddles a breadboard with one row of holes free on each side, which is where the Relay’s jumpers go. Many ESP32 boards are wider and cover both rows; then use two breadboards side by side. Hold BOOT while uploading if the IDE cannot connect.
The code
The sketch the circuit above runs. Open it in the editor, change a line and press Run: it compiles in your browser.
const int RELAY_PIN = 4; // IN, GPIO 4 (D4)
void setup() {
Serial.begin(115200);
pinMode(RELAY_PIN, OUTPUT);
digitalWrite(RELAY_PIN, LOW); // released at start
}
void loop() {
digitalWrite(RELAY_PIN, HIGH); // pull in: COM to NO
Serial.println("Relay on");
delay(1000);
digitalWrite(RELAY_PIN, LOW); // release: COM back to NC
Serial.println("Relay off");
delay(1000);
}
digitalWrite(RELAY_PIN, HIGH) turns on the module’s transistor, which energizes the coil and pulls the arm from NC over to NO about 10 ms later. LOW lets it fall back.
The relay is off at the start because setup() writes LOW before anything else. Do the same in a real project, so the load does not flick on while the board boots.
Serial.begin(115200) opens the serial monitor. Serial is UART0 on the USB bridge. ESP32 sketches usually run it at 115200 baud, which this one does.
Common mistakes with the Relay on the ESP32
Active low modules
Many cheap relay boards switch when IN goes LOW, so the sketch seems inverted and the relay is on at boot. Check the board, and in the editor set the relay’s trigger property to low to match.
Driving a coil straight from a pin
A bare relay coil wants 70 mA or more and kicks back a voltage spike when it releases. On a module the transistor and the flyback diode are already there; a bare relay needs both, and never a direct pin.
Switching mains without care
The contacts are rated for mains, but the board and the screw terminals are not a safe enclosure. Learn on a 12 V lamp or a motor, and leave mains to a properly rated, enclosed module.
Questions and answers
Can I simulate a relay with an ESP32 DevKit?
Yes. The circuit at the top of this page is an ESP32 DevKit with a relay module, running in your browser on a simulated ESP32 (Xtensa LX6). The simulated relay has a pull-in and release time, and its contacts really connect COM to NO or NC, so a lamp wired through it switches. Press Run it in the editor to change the wiring or the code; it is free and needs no account.
Will a 5 V relay module work with the ESP32?
Usually, yes: power it from VIN (5 V from USB) and drive IN from a GPIO. If a module will not switch from 3.3 V, use a 3.3 V relay module or a transistor between them.
What do NO and NC mean?
Normally open and normally closed. With the coil off, COM is connected to NC; with it on, COM is connected to NO. Wire a load to NO to have it off until the relay switches.
How fast can a relay switch?
A few times a second at most. It takes about 10 ms to pull in and its contacts wear with every click, so for anything faster, or for dimming, use a MOSFET.
Build your own Relay project
Open this circuit in the editor, change the wiring or the code, and keep your version in a free account.