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Burglar Alarm
Électronique / Electronic

This burglar alarm makes a buzzing sound when anyone sneaking into your house trips over a wire and breaks it off or disconnects it from a terminal. Instead of stretching out the wire, try to figure out how to connect a switch to the door of your house, so that the alarm sounds when a burglar opens the door.

Start by sliding the switch to position OFF and assembling the circuit. When you finish the wiring, connect the terminals 13 and 14 to the long wire, and slide the switch to position ON to turn on the power. At this time, no sound comes from the speaker.

To test the alarm, detach the wire from terminal 13. The speaker gives out a beep. This beep is the alarm that tells you a burglar is about to break into your house. As you can see in the schematic, this burglar alarm uses the dual operational amplifier as an astable multivibrator, as the electronic buzzer in the last project did.

You can change its frequency by using different values for the 10K ohm resistor and the 0.1 μF capacitor. Note how the tone of the buzzer changes when you set the 10K ohm resistor to 47K ohms or switch the 100K ohm and 220K ohm resistors with each other.







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