Inverters Ula and Ulb are connected in a simple RC oscillator circuit. The frequency is determined by the values of Rl, Cl, 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-coupler circuit: C3, Dl, D2 and C4. The dc voltage is applied to the input of Ulc-the third inverter amplifier-keeping its output in a low state, which keeps Ql turned off so that no sound is produced by BZL With Cl 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 Cl 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 Cl and C2, one at a time, to a lower value until the circuit just ceases oscillation: Buzzer BZl should sound off. Back off either Cl or C2 just a smidgen until the oscillator starts up again-that is the most sensitive setting of the circuit.

   Schematic

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