Transistor, and logic probe

Transistors work just like relay switches, it has three sections: the collector which recieves the current, the base which controls the current, and the emitter which sends it out. 

 When you combine the transistor with a switch, it is possible to switch the flow of the current.  With the setup to the right we can create a logic probe.

 A logic probe works to help the user find faults in the electronic design. Not only does it do fault finding but it helps in understanding how the electronics work in a system.

Normally the LED is dim when you make the probe touch a "hot flow" the LED brightens up,and when you touch ground the light shuts off; this results in a proper current flow.