A relay is an electrically controlled switch that allows a low-power microcontroller (like Arduino, ESP32, Raspberry Pi, or STM32) to control high-power devices (motors, lights, heaters, etc.). 🚀

⚡ 1. Understanding Relay Modules

Types of Relays Used with Microcontrollers

✅ Electromechanical Relays (EMR) – Clicky, with physical contacts, handles AC/DC loads.
✅ Solid-State Relays (SSR) – No moving parts, faster switching, best for AC loads.
✅ Relay Modules (with built-in transistors & protection diodes) – Plug-and-play!

🔧 2. How to Connect a Relay to Microcontrollers

A microcontroller cannot drive a relay directly because it outputs low voltage (3.3V or 5V), while the relay coil may require higher current (70-150mA at 5V or 12V). Solution? Use a transistor!

🔹 Method 1: Connecting a Relay Module (Easy Way)

💡 If you have a ready-made relay module, it already has a transistor & diode built-in. Just connect it directly to your microcontroller.

🛠️ Wiring for a Relay Module

✅ When IN = HIGH, the relay turns ON (NO closes).
✅ When IN = LOW, the relay turns OFF (NO opens).

🔹 Method 2: Using a Transistor (For a Raw Relay)

If you have a bare relay, you’ll need a transistor (e.g., NPN 2N2222, BC547, TIP120) to switch it ON/OFF.

🛠️ Circuit Diagram

• Microcontroller GPIO → 1kΩ resistor → Transistor Base
• Transistor Collector → Relay Coil
• Transistor Emitter → GND
• Diode (1N4007) across the relay coil (for protection)
• Relay Common & NO/NC connected to the load

🤖 3. Connecting a Relay to Different Microcontrollers

🔹 Arduino (5V Logic)

✅ Directly connect IN to an Arduino digital pin.
✅ Works with 5V relay modules.

int relayPin = 7;
void setup() {
  pinMode(relayPin, OUTPUT);
}
void loop() {
  digitalWrite(relayPin, HIGH);  // Turns ON the relay
  delay(1000);
  digitalWrite(relayPin, LOW);   // Turns OFF the relay
  delay(1000);
}

🔹 ESP32 (3.3V Logic)

❌ Problem: Most relay modules require 5V signal, but ESP32 runs on 3.3V logic.
✅ Solution: Use a transistor circuit or a 3.3V relay module.

#define relayPin 2  // ESP32 GPIO2
void setup() {
  pinMode(relayPin, OUTPUT);
}
void loop() {
  digitalWrite(relayPin, HIGH);  // Relay ON
  delay(2000);
  digitalWrite(relayPin, LOW);   // Relay OFF
  delay(2000);
}

🔹 Raspberry Pi (3.3V Logic)

✅ Use a 3.3V relay module or an NPN transistor (like ESP32 setup).

import RPi.GPIO as GPIO
import time
relayPin = 17  # GPIO17
GPIO.setmode(GPIO.BCM)
GPIO.setup(relayPin, GPIO.OUT)
while True:
    GPIO.output(relayPin, GPIO.HIGH)  # Relay ON
    time.sleep(2)
    GPIO.output(relayPin, GPIO.LOW)   # Relay OFF
    time.sleep(2)

🔹 STM32 (3.3V Logic)

✅ Same as ESP32, use a transistor circuit if needed.
✅ Some STM32 boards have 5V pins, allowing direct connection.

⚠️ Safety Tips When Using Relays

⚡ Relays control HIGH VOLTAGE – Always be careful!
✅ Use optocouplers if controlling AC loads for isolation.
✅ Use a diode across the relay coil (if not using a module) to prevent voltage spikes.
✅ NEVER directly switch AC mains with a microcontroller – use relays rated for it.

🔥 Conclusion: Which Method is Best?

🔹 For Beginners → Use a relay module (easiest, no extra components needed).
🔹 For Low-Power Loads → Use a transistor to drive a relay coil.
🔹 For High-Voltage Loads → Use optocouplers & high-quality relays.