The circuit described here is that of a metal detector. The opera- tion of the circuit is based on
superheterodyning principle which is commonly used in superhet receivers. The circuit utilises
two RF oscillators. The frequencies of both oscillators are fixed at 5.5 MHz. The first RF oscillator
comprises transistor T1 (BF 494) and a 5.5MHz ceramic filter commonly used in TV sound-IF
section. The second oscillator is a Colpitts oscillator realised with the help of transistor T3
(BF494) and inductor L1 (whose construction details follow) shunted by trimmer capacitor VC1.
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These two oscillators frequencies (say Fx and Fy) are mixed in the mixer transistor T2 (another
BF 494) and the difference or the beat frequency (Fx-Fy) output from collector of transistor T2 is
connected to detector stage comprising diodes D1 and D2 (both OA 79). The output is a pulsating
DC which is passed through a low-pass filter realised with the help of a 10k resistor R12 and two
15nF capacitors C6 and C10. It is then passed to AF amplifier IC1 (2822M) via volume control
VR1 and the output is fed to an 8-ohm/1W speaker. The inductor L1 can be constructed using 15
turns of 25SWG wire on a 10cm (4-inch) diameter air-core former and then cementing it with
insulating varnish. For proper operation of the circuit it is critical that frequencies of both the
oscillators are the same so as to obtain zero beat in the absence of any metal in the near vicinity
of the circuit. The alignment of oscillator 2 (to match oscillator 1 frequency) can be done with the
help of trimmer capacitor VC1. When the two frequencies are equal, the beat frequency is zero,
i.e. beat frquency=Fx-Fy=0, and thus there is no sound from the loudspeaker. When search coil
L1 passes over metal, the metal changes its inductance, thereby changing the second oscillators
frequency. So now Fx-Fy is not zero and the loudspeaker sounds. Thus one is able to detect
presence of metal
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