Proximity Alarm

Posted on Oct 30, 2012

Inverters U1a and U1b are connected in a simple RC oscillator circuit. The frequency is determined by the values of R1, C1 C2 and the internal characteristics of the integrated circuit. As long as the circuit is oscillating, a positive dc voltage is developed at the output of the voltage-couple circuit: C3, D2 and C4. The dc voltage is applied to the input of U1c-the third inverter amplifier-keeping its output in a low state, which keeps Q1 turned off so that no sound is produced by BZ1.

Proximity Alarm
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With C1 and C2 adjusted to the most sensitive point, the pickup plate will detect a hand 3 to 5-inches away and sound an alert. Set C1 and C2 to approximately one-half of their maximum value and apply power to the circuit. The circuit should oscillate and

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