Metal Detector
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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.
Title: Metal Detector
electronic circuit
Source: www.electronics-lab.com
Published on: 2005-02-10
Reads: 1916
Print version: 
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