Posted on Jul 25

How to use a microcontroller to control a relay?

#microcontroller #relay #arduinouno #esp32

Controlling a relay with a microcontroller (e.g., Arduino, ESP32, Raspberry Pi Pico) is a fundamental skill for switching high-voltage devices (like lights, motors, or appliances) safely. Here’s a step-by-step guide:

1. Key Components

Microcontroller (Arduino Uno, ESP32, etc.)
Relay Module (Optically isolated, e.g., 5V or 3.3V compatible)
Power Supply (For the high-voltage side, if needed)
Load (Bulb, fan, etc.)
Breadboard & Jumper Wires

2. Relay Wiring
Most relay modules have:

Control Pins: VCC, GND, IN (signal)
Load Pins: COM (common), NO (Normally Open), NC (Normally Closed)

Connections

Load Wiring
COM → High-voltage power (e.g., 120V AC)
NO → Load (e.g., bulb) → Return to neutral
https://i.imgur.com/JQ8WzVl.png
(Always keep high-voltage wiring insulated!)

3. Example Code (Arduino)

cpp const int relayPin = 2; // GPIO D2 void setup() { pinMode(relayPin, OUTPUT); Serial.begin(9600); } void loop() { // Turn relay ON (closes circuit) digitalWrite(relayPin, HIGH); Serial.println("Relay ON - Circuit CLOSED"); delay(2000); // Turn relay OFF (opens circuit) digitalWrite(relayPin, LOW); Serial.println("Relay OFF - Circuit OPEN"); delay(2000); }

4. Example Code (ESP32/ESP8266 - MicroPython)

python from machine import Pin import time relay = Pin(2, Pin.OUT) # GPIO2 while True: relay.value(1) # ON print("Relay ON") time.sleep(2) relay.value(0) # OFF print("Relay OFF") time.sleep(2)

5. Important Considerations
A. Relay Module Types
Active LOW vs. Active HIGH:

Some relays trigger when IN is LOW (LOW = ON). Check your module’s datasheet.
Adjust code: digitalWrite(relayPin, LOW) to turn ON.

B. Power Isolation

Use an optocoupler-based relay module to protect your microcontroller from high-voltage noise.
Avoid sharing GND between high-voltage and microcontroller circuits.

C. Flyback Diode
If controlling inductive loads (motors, solenoids), add a diode across the relay coil to prevent voltage spikes.

6. Advanced Control
A. PWM Control (For Solid-State Relays)

cpp analogWrite(relayPin, 128); // 50% duty cycle (for SSR)

B. WiFi/Bluetooth Control (ESP32)

python # ESP32 with Web Server (MicroPython) import network import socket from machine import Pin relay = Pin(2, Pin.OUT) sta_if = network.WLAN(network.STA_IF) sta_if.active(True) sta_if.connect("WiFi_Name", "Password") def web_page(): html = """<html><body> <a href="/on"><button>ON</button></a> <a href="/off"><button>OFF</button></a> </body></html>""" return html s = socket.socket() s.bind(('0.0.0.0', 80)) s.listen(5) while True: conn, addr = s.accept() request = conn.recv(1024) if "/on" in request: relay.value(1) elif "/off" in request: relay.value(0) conn.send(web_page()) conn.close()

7. Troubleshooting