|
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
|
