ESP32-C3 · Relay

ESP32-C3 Relay Simulator: Wiring, Code and a Live Circuit

This is a single-channel 5 V relay module wired to an ESP32-C3 DevKitM-1, 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 5V, GND to GND, and IN to GPIO 4. The pin only gives the signal; the module powers the coil.

Run it in the editor Read the code

3.3 V logic
No library
IN on GPIO 4
Coil on 5V

ESP32-C3 · Relaylive0.000 s 0.00x

Click to open it in the editor

The circuit itself, running on the simulator with the sketch below. Press what can be pressed; click anything else to open it in the editor.

Wiring the Relay to the ESP32-C3

Every connection in the circuit above, with the board’s own pin names.

Relay pin	ESP32-C3 pin	Why
VCC	5V	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	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 5V 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-C3

GPIO 8 carries the onboard LED and GPIO 9 is the BOOT button, and both are also the default I2C pins. GPIO 2, 8 and 9 are strapping pins, and only GPIO 0 to 4 can read an analog voltage.

On a real bench

The ESP32-C3 DevKitM-1 is small enough to sit on one breadboard with room for the Relay beside it. Its USB port goes through a USB-to-serial chip; the cheaper SuperMini boards use the C3’s own USB, where Serial needs USB CDC on boot turned on in the IDE.

The code

The sketch the circuit above runs. Open it in the editor, change a line and press Run: it compiles in your browser.

Relay on ESP32-C3 GPIO 4

const int RELAY_PIN = 4;   // IN, GPIO 4

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, at 115200 baud in this sketch.

Common mistakes with the Relay on the ESP32-C3

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-C3 DevKitM-1?

Yes. The circuit at the top of this page is an ESP32-C3 DevKitM-1 with a relay module, running in your browser on a simulated ESP32-C3 (RISC-V). 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-C3?

Usually, yes: power it from 5V 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.

Start building free Open the editor