Relays

1. Definition

1.1 Relay

A relay is an electromechanical switch that uses an electromagnetic coil to open or close contacts in another circuit.
It allows low-power control signals to operate high-power devices.

Relay examples

Relay Symbol

Relay symbol

The symbol shows the coil (control side) and the contacts (switching side). They’re separate, which is why relays can safely isolate circuits.

1.2 Types of Relays

  • Electromechanical Relay (EMR): Uses moving parts and a coil.
  • Solid State Relay (SSR): No moving parts, uses semiconductors.
  • Reed Relay: Very small, uses magnetic reeds.

If you’re just starting out, EMRs are the ones you’ll most often encounter.

2. Features

  • Coil Voltage: 3V–48V DC or AC. (This is the voltage you supply to activate the relay.)
  • Contact Ratings: up to tens of amps. (How much current the switched device can draw.)
  • Isolation between control and load circuits. (Prevents the high-power side from damaging your low-power microcontroller.)
  • Switching Types: NO (Normally Open), NC (Normally Closed), Changeover.
    (NO = off until activated; NC = on until activated.)

4. How to Use

Safety Note

Avoid exceeding contact voltage/current; arcing can damage contacts.
Arc = tiny spark that jumps when the switch opens/closes. This can wear out the relay or be dangerous at high voltage.

4.1 Identify pins

  • Coil pins: Connect to control voltage (from Arduino or via transistor).
  • Common (COM): The moving part of the switch.
  • Normally Open (NO): COM connects here only when relay is activated.
  • Normally Closed (NC): COM connects here when relay is not activated.

Relay pinout

Tip: On modules, these pins are often labeled right on the board, so double-check before wiring.

4.2 Driving a relay

  • Use transistor or MOSFET to drive coil from microcontroller. (Most microcontrollers can’t power a relay coil directly, the transistor acts as a helper switch.)
  • Add a flyback diode across coil to prevent damage.
    (This diode stops voltage spikes created when the coil turns off from damaging your electronics.)

4.3 Applications

  • Switching high-power loads (lamps, motors, heaters).
  • Isolation between circuits.
  • Automotive control (horns, lights, pumps).

Common beginner project: Turn on a desk lamp using Arduino + relay when a sensor triggers.

Video Explanation

video coming soon