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.