This is a mobile phone sniffer circuit that can detect the signals being used in the GSM (Global System for Mobile Communication) 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 media. 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. If they are small like about 100k, this time it becomes harder to hear the signal. Component List (Malzeme Listesi) R1, R5 : 100K 1/4W Resistor R2, R6 : 1k 1/4W Resistor R3, R7 : 8.2M 1/4W Resistor R4, R8 : 220 Ohm 1/4W Resistor R9 : 2.2K 1/4W Resistor D1, D2 : BAT43 Schottky Diode C1, C2, C4 : 100nF Polyester Capacitor C3 : 100uF 16V Electrolytic Capacitor L1, L2 : See Text U1 : LM358 J1 : 8 Pin Socket J2 : Stereo JAck 1 x 9V Battery 1 x 9V Battery Socket 1 x LED 1 x On/Off Switch The diodes, gain of the amplifiers and the length of the antenna is critical in this circuit. Calculating the length of the antenna is simple. The formulation is given below. λ=c/f = (300.000km/h)/900MHz =33.3 cm Then; Antenna Length = λ / 2 = 16.6 cm 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. The chokes are 10 turn molded chokes. The wire diameter should be about 0.5 - 06 mm and wound around a 5 mm cylindrical object. Diodes are very critical. You should use one of BAT43, BAT45, AA112, AA116 or AA119. When a silisium diode is used the circuit also works but the detecting area becomes very very narrow.