Relay is an electromechanical switch. It is an electrically operated switch. A relay is a special type of switch turned ON and OFF by an electromagnet (the diagram given below).

Relay Circuit


When current flows 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 has two switching positions, one is normally closed NC and other is Normally open NO.

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

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.

In other words, 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 is also known as a relay switch.


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.

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


In general, It has 5 pins described below:

1.Input 1Apply Signal to trigger relay, either positive or negative signal.
2.Input 2Apply Signal to trigger relay, signal opposite to input 1 (if positive to input 1 then negative here or vice versa).
3.COM (common)Com point where you connect the one terminal of ground or supply voltage (recommended) you want to give to the load either 5V/12V/24V DC or 110V/220V AC etc.
4.NO (Normally open)Switching terminal, when relay gets triggered then arm of relay trip to NO terminal. So the signal from com will be directed to NO terminal of the relay.
5.NC(Normally close)When the relay doesn't get triggered, a signal connected to the common point will be directed to the NC terminal of the relay


You can apply a positive signal to either of the input (input 1/ input 2) but on the other input must be a ground (negative) signal. Keep in mind the potential difference between the inputs must be the voltage of relays rating.

For example, we have a 5V DC relay, apply +5V to input 1 and ground to input 2 (makes 5V potential difference) or you can give +12 V to input 1 and +7 V to input 2 it will also work because of the potential difference of 5V.

When a current flows through the relays coil an electromagnetic field is set up. This electromagnetic field attracts an iron armature, whose other end pushes the contacts together, completing the circuit.


  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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