What makes Relay different from types of switches available in the market like push button, SPST, momentary, diode, transistor.

You already read: Relays pin Configuration and Working Example

Is that, the voltage needed to throw the switch doesn’t have to be the same as the voltage being switched (in simple words a less amount of voltage required for switching the lever may be 5V/12V DC and switching voltage can be 120V/220V AC). Actually it helps us to interface between microcontrollers and AC appliances.


A relay is an electrically operated switch.

When current flow through the coil of the relays it creates a magnetic field which attracts the lever and changes the switching contact. So there are two possibilities

  • either current is flowing through coil of the relay
  • or current is not flowing through coil of the relay

So it have two switching positions, one is normally closed NC and other is Normally open NO.

It can switch from one circuit to another circuit which can be completely separate from the first with some circuitry we will discuss later in this chapter. That’s why it also known as relay switch.

Relay 5V


Generally it is a 5 pin electrical component, two out of these five pins are for coil connection namely coil-1 and coil-2, one is NC – Normally closed contact and another is NO – normally open switching contact and the last pin is COM used for the supply voltage for the appliance.

Relay Pinout

The relay diagram shown below is a example of SPDT – single pole double through switch.

Relay diagram


  1. Compact 5-pin configuration with plastic moulding
  2. Operating time: 10msec Release time: 5msec
  3. Maximum AC load voltage is 250V/125V AC and current is 10A
  4. Maximum DC load voltage is 30V/28 DC and current is 10A
  5. Trigger/Operating Voltage is 5V DC
  6. Maximum switching: 300 operating/minute (mechanically)
  7. Trigger Current (Nominal current) : 70mA

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