A relay is an electrically operated switch that harnesses the power of an electromagnet to either open or close a circuit, while a switch is a mechanical device that opens or closes a circuit physically. Switches can be activated manually, but relays are typically controlled by electrical signals.

Why Relay

You can connect an LED directly to ESP32 but you cannot connect a high voltage devices (such as an electric lamp, pump, electromagnetic lock, linear actuator, or even a big machine…) directly to ESP32. We need a relay in between ESP32 and the high voltage devices. Without relay, the high voltage will destroy ESP32.

Some relays can work with both DC and AC voltage, we extremely recommend you NOT to use AC voltage. Use a DC device (≤24V) only.



esp32 Relay Pinout

How Relay Works (esp32 relay)



The relay pins can be categorized two groups: input (low voltage) and output (high voltage).

Normally, we do NOT use all high voltage pins. We usually use only two, depending on the operation mode we choose:

Additionally, if the relay supports both LOW and HIGH level triggers, there is usually a jumper to select one of two: LOW level trigger or HIGH level trigger.


The pin order of the relay can be different between manufacturers. Please check the labels printed on the relay carefully!

How to Connect the high-voltage Device to Relay

There are several ways that a relay module can work, depending on our choices.

The “normally” means “if the IN/signal pin is connected to LOW (0V)”.

Before going into detail, let’s see some quick information:

The combination of the input modes mode and output modes modes creates many use cases. If you are a beginner, we recommend using HIGH level trigger mode and normally open mode

Because the LOW level trigger and HIGH level trigger mode work oppositely, The next will explain the HIGH level trigger mode in detail. The LOW level trigger works oppositely.

HIGH Level Trigger - Normally Open Mode

Connect the high-voltage device to the COM pin and NO pin. Just like a switch:

HIGH Level Trigger - Normally Closed Mode

Connect the the high-voltage device to the COM pin and NC pin. Just like a switch:

Which mode should we use?

⇒ It depends on application.

Relay - Explained and animated - how relay works https://www.youtube.com/watch?v=Ca20ktPygY8