Relays are electrically operated switches that open and close the circuits by receiving electrical signals from outside sources. They turn something on/off based on input from another electrical signal. This could be from a sensor, microcontroller, or other device. The remote on your TV is an example of signal that turns on the relay switch inside of the TV when the power button is pressed. Relays are useful for controlling devices that operate at high current or different power supply.
Relays can be of different types like electromechanical and solid state. Electromechanical relays are frequently used. Let us see how this type of relay works.
Single Pole Double Throw (SPDT)
A single pole double throw relay connects one input circuit to one of the two outputs. This relay is also called as changeover relay.
The arrow in the picture will switch between top and bottom output when the input signal is generated. Let’s look at a practical example of how this type of relay would be used. The arrow is represented by the COM in the picture below. The COM is attached to the + power of the device you wish to control. It will switch between NO and NC or Normally Open and Normally Closed. Normally Open means the switch is open or the device is powered off when there is no signal trigger. Normally closed is vice versa. The opposite side of the relay is your control board power supply. This is typically 5V, 12V, or 24V. You would choose your relay based on what type of output your signal trigger will have.
In the diagram below you will see an example of the relay turning on a lightbulb when a sensor is activated.
