Proximity Alarm I Circuit

IC1 contains several oscillators and an amplifier. The low-frequency audio-signal oscillator is used to supply an input to the amplifier. That signal is the audio tone that is amplified, then supplied to the speaker by the amplifier. The high-frequency oscillator is purposely set to be very unstable. It is dormant or off` until the resistor-capacitor (RC) network is changed. The resistance (R) in this case is made up of R2 and PL As the resistance of PI is decreased, the unit becomes more sensitive (more unstable), and less capacitance (C) is needed to cause the oscillator to oscillate.
Proximity Alarm I Circuit - schematic

The capacitance required is provided by C2 and by any capacitance introduced via the antenna loop. When you come near that loop, your inherent body capacitance causes the high-frequency oscillator to begin to oscillate, which then causes the low-frequency oscillator to be switched on internally. Once the alarm is sounding, the IC is designed so that it latches, that is, it stays on until the power to it is switched off.

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