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 Colpitt’s oscillator realised with the
help of transistor T3 (BF494) and inductor L1 (whose construction details follow)
shunted by trimmer capacitor VC1. 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 oscillator’s frequency. So now Fx-Fy
is not zero and the loudspeaker sounds. Thus one is able to detect presence of
metal.
