This is a mobile phone sniffer circuit that can detect the signals being used in the GSM band at about 900 MHz. Since the signals are digitally encoded, it can detect only the signal activity, not the speech or the message contents. A headphone is used to hear the detected signals.
The circuit schematic is given in the .rar archive attachment. There are two separate detector units. Every detector unit consists of a dipole antenna, a choke and a diode. The antenna receives the GSM signals in the medium. Then a small amount of charge is induced in the choke. The diode demodulates the signal and finishes detecting. The diodes must be schottky diodes or germanium diodes. Since the forward voltage of a silisium diode is high, it won’t give a sufficient result in this circuit.
LM358 amplifies the received signal. It contains two separate op-amps that are supplied by a common power source. R3 and R7 resistors determine the gain of the amplifiers. When the resistor values are greater than 10M then the noise level increases.
So there are four pieces of antenna and each one is about 8.3 cm long. The wire type is not critical but its better to choose a fairly thick wire that will not bend too easily. It is a 1.5 mm diameter wire seen in the photo that we used. The two antennas must be positioned perpendicularly.
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