Outputs and switching

5 V relay module with ESP32

A relay module lets an ESP32 switch a load that it could never drive itself, through a clicking mechanical contact. Most modules have an optocoupler or a transistor, a coil at 5 V, and three contact terminals: common, normally open and normally closed.

Most modules are active-low: the relay turns on when the input goes LOW. That is deliberate, so a floating pin during boot does not switch the load. On an ESP32 there is a catch: 3.3 V high may not be high enough to hold some 5 V modules fully off. Pulling the pin up to 5 V is not an option either, because an ESP32 pin cannot take it. So remove the JD-VCC jumper if your module has one, or use an NPN transistor stage. Choose a pin that stays quiet during boot.

Mains voltage can kill. If you switch 230 V, use a certified enclosure, keep low voltage and mains apart, and let a qualified electrician do the wiring. Do not trust the printed rating on a cheap relay for its full load: motors and heaters need margin. Inductive DC loads need a flyback diode across the load. LCD Ultra Pro can switch a pin directly from a button or rule, and the Time Switch components can drive it on a schedule.

Power and logic levels

3.3 V: check your module first

Supply	Often 5 V coil
Signal level	Input level varies, an active-low input can sit near 5 V
On an ESP32	Use one of the two circuits below. Modules with a JD-VCC jumper can use 3V3 on VCC. Other modules need an NPN transistor.

ESP32 pins, including those on a display board's connectors, work at 3.3 V and are not 5 V tolerant. Modules differ, so if in doubt, measure the signal before you connect it.

const int RELAY_PIN = 5;    // choose a pin that stays quiet during boot
const bool ACTIVE_LOW = true;

void relay(bool on) {
  digitalWrite(RELAY_PIN, on == ACTIVE_LOW ? LOW : HIGH);
}

void setup() {
  relay(false);             // set the level before enabling the output
  pinMode(RELAY_PIN, OUTPUT);
}

void loop() {
  relay(true);
  delay(2000);
  relay(false);
  delay(2000);
}

Use this information at your own risk. It is for general guidance only: the example code compiles, but it has not been tested on every part, and modules differ between sellers. Check the manufacturer's data before you build. Product names belong to their owners, and LCD Ultra Pro is not affiliated with them. See our Terms.