ESP32 Relay Module Control

1. Introduction

A relay is an electromechanically or solid-state operated switch that allows a low-power microcontroller signal — such as the 3.3 V logic output of an ESP32 — to safely switch a completely separate, higher-power circuit, including mains AC appliances like bulbs, fans, or pumps. Since the ESP32 GPIO pins cannot directly drive the current required by a relay coil, an intermediate switching stage (commonly a transistor) is used, or a ready-made relay module that already contains this driver circuitry is used instead.

This guide walks through both approaches: building a discrete transistor-relay driver from scratch, and using an off-the-shelf single-channel relay module — the more common and beginner-friendly method.

2. Why a Transistor Driver Is Needed

An ESP32 GPIO pin can typically supply only a few milliamps to a few tens of milliamps safely. A small 5 V or 12 V relay coil, however, may require 50–150 mA to energize. Connecting the coil directly to a GPIO pin would exceed the pin's current limits and could damage the microcontroller.

The solution is to use a transistor (typically an NPN type such as the 2N2222 or BC547) as an electronic switch:

• The ESP32 GPIO pin connects — through a current-limiting base resistor — to the base of the transistor.
• The relay coil is connected between the external supply (5 V/12 V) and the transistor's collector.
• The transistor's emitter connects to ground, which must be common with the ESP32's ground.
• A flyback diode (e.g. 1N4007) is placed across the relay coil to protect the transistor from the voltage spike generated when the coil is de-energized.

3. Circuit Diagram

The diagram below shows the discrete transistor driver stage feeding a relay, which in turn switches an AC bulb.

Note that the ESP32 side (blue wires, low-voltage logic) and the AC side (orange wires, mains voltage) are fully isolated by the relay contacts — only the grounds of the ESP32 and the transistor/relay driver stage are shared, never the AC wiring.

4. Using a Ready-Made Relay Module

Instead of building the transistor driver circuit yourself, a pre-assembled relay module is widely available and already contains the transistor (or optocoupler), flyback diode, and status LED on a small PCB. These modules are sold as single-channel or multi-channel (e.g., 2-channel, 4-channel, 8-channel) boards, but the underlying switching logic is identical regardless of channel count.

Typical Single-Channel Relay Module Pinout

5. Step-by-Step Wiring Instructions

Follow the exact order below when wiring an ESP32 to a single-channel relay module and an AC bulb.

1. Step 1: Connect relay module GND to ESP32 GND
2. Step 2: Connect relay module VCC to external 5V or 12V power supply
3. Step 3: Connect relay module IN pin to ESP32 GPIO (e.g., GPIO 2)
4. Step 4: Connect one AC mains wire to relay COM terminal
5. Step 5: Connect relay NO terminal to one terminal of the bulb holder
6. Step 6: Connect the other terminal of the bulb holder to the AC mains return wire
7. Step 7: Double-check that ESP32 GND and relay module GND share the same reference
8. Step 8: Power the ESP32 via USB or a separate 5V supply
9. Step 9: Power the relay module's VCC from the external supply only
10. Step 10: Plug the AC cable into the wall socket only after all connections are verified

6. Sample Arduino Code — Blinking the Relay

The following simple sketch toggles the relay (and therefore the bulb) on and off every second, equivalent to the basic blink program demonstrated on the ESP32 board.

// ESP32 Relay Blink Example
#define RELAY_PIN 2   // GPIO connected to relay IN pin

void setup() {
  pinMode(RELAY_PIN, OUTPUT);
}

void loop() {
  digitalWrite(RELAY_PIN, HIGH);  // Energize relay -> bulb ON
  delay(1000);
  digitalWrite(RELAY_PIN, LOW);   // De-energize relay -> bulb OFF
  delay(1000);
}

Uploading the Code

1. Step 1: Open the Arduino IDE and select Tools > Board > ESP32 Dev Module
2. Step 2: Select the correct COM port under Tools > Port
3. Step 3: Click the Upload button (right-arrow icon)
4. Step 4: Wait for the message "Connecting..." to appear
5. Step 5: Press and hold the BOOT button if upload does not start automatically
6. Step 6: Wait for "Done uploading" to appear in the console
7. Step 7: Observe the relay module's onboard LED and audible click as it switches

7. Grounding and Safety Notes

A frequent point of confusion is why the ESP32's ground must be connected to the relay module's ground even though the relay is switching a completely separate AC circuit. The reason is that the relay's coil-driving side (VCC, GND, IN) is a low-voltage DC circuit that must share a common reference with the ESP32 so that the GPIO's HIGH/LOW signal is correctly interpreted by the module's onboard transistor or optocoupler.

8. Testing the Circuit

1. Step 1: Power the ESP32 and confirm the relay module's power LED turns on
2. Step 2: Listen for an audible click each time the relay switches state
3. Step 3: Verify the bulb turns on when the relay contact closes
4. Step 4: Verify the bulb turns off when the relay contact opens
5. Step 5: Adjust the delay() values in the sketch to change blink timing
6. Step 6: Re-upload the sketch after any code changes

If the relay clicks but the bulb does not light, recheck the AC-side wiring (COM/NO) and confirm the bulb and socket are functional.

9. Future Enhancements

Once the basic ESP32-to-relay circuit is verified, the same hardware setup can be extended with additional software features without changing any wiring, such as:

• Controlling the relay over Wi-Fi using a simple web server hosted on the ESP32
• Integrating with voice assistants such as Alexa or Google Home via IoT platforms
• Scheduling automatic ON/OFF times using an NTP-synced clock
• Expanding to multi-channel relay modules to control several appliances independently