Use Relays to Control High-Voltage Circuits with an Arduino

To control high-voltage or high-power circuits with an Arduino, you have to isolate them from the Arduino with a relay. Here's how!

Circuits that operate at high voltages or at high currents cannot be controlled directly by an Arduino. Instead, you use a low-voltage control signal from the Arduino to control a relay, which is capable of handling and switching high-voltage or high-power circuits. A relay consists of an electromagnet that, when energized, causes a switch to close or open. Relays provide complete electrical isolation between the control circuit and the circuit being controlled.
A relay typically has five pins:

1. Coil input pin1. This is generally connected to the positive terminal of your signal source.
   
2. Coil input pin2. This is generally connected to the negative terminal of your signal source.
   
3. Normally Open pin(NO). This pin is normally not connected to the common pin, it is connected when the relay is activated.
   
4. Normally closed pin(NC). This pin is normally connected to common pin and is disconnected when relay is activated.
   
5. Common. In most of the cases, this pin is connected to the ground of the source we use to drive the appliance.

Experiment

In this tutorial, we will drive a 12V DC motor using relays.

Hardware Required

• 1 x 12 VCD power supply
• 1 x 12V DC motor
  
• 1 x relay
• 1 x Arduino Mega2560
  
• 1 x NPN transistor
• 1 x Diode
  
• 1 x jumper wires

Wiring Diagram

The circuit shown in the diagram below uses a relay to switch 12 V across a DC motor. To turn on the motor, the program writes a HIGH value to pin 3, which turns on the transistor. When the transistor turns on,  current flows through the relay coil causing the relay to close, which connects 12 V across the motor, causing it to spin.