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. 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 no sound should
be heard. Using a non-metallic screwdriver, carefully adjust C1 and C2, one at a time, to a lower value until the
circuit just ceases oscillation: Buzzer BZ1 should sound off. Back off either C1 or C2 just a smidgen until the
oscillator starts up again-that is the most sensitive setting of the circuit.
[] A note from a visitor :
This alarm is very sensitive to minor capacitance changes. If you use a protoboard before final soldering into
a permenent board take in account the capacitance created by the proto board.
Reference : " Zeus Electronics "
Last
Updated : February 15, 2002 