Using BC547 Transistor to Drive a 5V Relay (Arduino Tutorial)

 Introduction

In this tutorial we will study how to drive a relay using a transistor

🛠️ Components Needed

• Arduino Uno R3

• LU-5-R Relay (5V)

• BC547 Transistor (NPN)

• 1kΩ Resistor (for the Transistor Base)

• 1N4007 Diode (Flyback/Freewheeling diode)

• External Power Source (Battery or Power Supply)

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🔌 Circuit and Connections

The BC547 acts as a "bridge" that allows a small signal from the Arduino to control the heavy current of the relay coil.

1. The Transistor Connections:

• Emitter (E): Connect directly to Arduino GND.

• Base (B): Connect through a 1kΩ resistor to Arduino Digital Pin 13.

• Collector (C): Connect to one side of the Relay Coil.

2. The Relay & Diode:

• Coil Side A: Connect to the Transistor Collector.

• Coil Side B: Connect to the 5V supply.

• Flyback Diode (1N4007): Place this across the relay coil pins (Silver stripe toward the 5V side). This prevents the "star" warning sparks from inductive kickback.

3. The Load Side (LU-5-R Pins):

• Connect your light bulb or motor to the COM and NO pins as usual.

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💻 Arduino Code

You can use the simple Blink code to test this circuit.

void setup() {

  pinMode(13, OUTPUT); // Sends signal to BC547 Base
}

void loop() {
  digitalWrite(13, HIGH); // Transistor turns ON, Relay clicks
  delay(1000);
  digitalWrite(13, LOW);  // Transistor turns OFF, Relay releases
  delay(1000);
}

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🔍 Why do we need the BC547?

An Arduino pin can only output about 20mA of current. A typical LU-5-R relay coil needs about 70mA to 100mA to stay closed.

• If you connect it directly, you might damage your Arduino 

• The BC547 amplifies the small current from Pin 13 to a level high enough to power the coil safely.

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💡 Pro-Tips for Kerala Students

• Check the Pins: For the BC547, looking at the flat side, the pins are Collector-Base-Emitter (CBE) from left to right.

• Safety: This transistor circuit is the industry standard for driving loads. Whether you are in a lab in Kottayam or building a home automation system, always use a transistor driver for relays.