Transistor

With three terminals, a bipolar transistor lets a small base current control a much larger collector current. Circuits on this page use NPN and PNP transistors as switches that turn a lamp, LED, relay or motor on from a weak signal, as common-emitter amplifiers, in pairs with capacitors as astable multivibrators that flash two LEDs alternately, and with an LDR in light-sensing circuits. For example, "Using Transistor as a switch" turns a lamp on and off through the transistor's base.