Current switch
Electronic

A current switch is an electronic switch to turn the output voltage ON or OFF according to the input current.

We're going the use this switch to build a circuit that turns LED on when it becomes dark and off when it becomes bright. As you might have imagined, we use the CdS cell. We use two transistors to make up the current switch.

When it becomes bright and the resistance of the CdS cell decreases, Q1 's basic current Ib flows to turn Q1 on and Q2 off, then LED goes out. When the surroundings become dark, the current switch works in exactly the opposite way.

After switching the power ON, see how the LED lights up and goes out as you change the surrounding brightness by cupping your hand around the CdS cell and removing it.