Solid State Relay G3MB-202P-5VDC 4PIN 2A240VAC China Quality

To use this Relay you need to connect the control input (number 3) pin to the 5V GPIO pin of the microcontroller board you are using and number 4 pin to GND and if 5V supply is on the there will be contact between number 1 and number 2 load pins.

Relays are used to control high voltage high current systems with your low voltage control inputs or to provide protection and electric separation between the systems. The normal Electromagnetic relays operate on principal of electromagnetism which is when a coil is energized it will behave as magnet to make connections in control line and when it is de-energized the connection is open. But this is not the case with Solid State relay which uses photocoupling for switching on and off the load. Its operation is faster than the normal electromagnetic relays and it can be used in applications like Switched Mode Power Supply and PWM control of motors and Servo Motor applications. It also smaller in size than conventional relays and can be packaged much more efficiently.