Transistors as Switches

Transistors do more than just amplify, they can also be used as switches. Can you think of how they could be used this way? (No fair peeking at the answer below)

You press the Key, you'll see the LED Digital Display show "8".

Release the Key and the Display will become dark again. Look at the schematic for this Project, do you see what is going on?

In our last Project Meet the Transistor, we saw how a small change in the emitter-to-base current can cause a big change in the emitter-to-collector currents. But did you realize that in most PNP transistor circuits there can't be any collector-to-emitter current flow unless the base-to-emitter current is at a certain PNP transistor circuit just by lowering the base-to emitter current. (Or turn it on by increasing the base-to-emitter current.)

In future Projects, watch for transistors being used as switches. And when you build circuits on your own, think of ways you can make use of a transistor's switching ability (wouldn't the CdS Cell in the emitter-to-base current path be interesting? ).