| 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 |