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Proximity Alarm

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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


Parts List:
Component: Value: Datasheet: Qty:
Resistor

470

Not Available 01
Resistor

1K

Not Available 01
Resistor

100K

Not Available 01
Capacitor 9-50pF Variable Not Available 02
Capacitor 680pF Not Available 01
Capacitor 0.1uF Not Available 01
Capacitor 100uF Not Available 01
Capacitor 39pF Not Available 01
Diode 1N914 Available 02
Transistor 2N2222 Available 01
IC 4049 Available 01