Posted on Dec 29, 2012

The monitor senses an ajar door and, if the situation isn`t corrected within 20 seconds, sounds a beeping alarm. The circuit is controlled by a magnetic reed switch and magnet on the door. With the door closed, the switch is closed and the alarm is disarmed. Opening the door opens switch, Cl starts charging up through Rl. Approximately 20 seconds later, the voltage at pin 9 is high enough to tum on the oscillator formed from C, D, R2, R3, and C2.

Click here to download the full size of the above Circuit.

That pulses the piezoelectric transducer"s 3-kHz oscillator. For lower standby drain on the battery, change Rl to 66 M{} and Cl to 1 mF (film).

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