Two-Transistor Light Alarm

Project Number: 105

OBJECTIVE: To compare transistor circuits.

Description: This light alarm uses two transistors (Q1 and Q2) and both sets of batteries. Build the circuit with the jumper connected as shown, and turn it on. Nothing happens. Break the jumper connection and the lamp (L2) turns on. You could replace the jumper with a longer wire and run it across a doorway to signal an alarm when someone enters.