Mobile phone sniffer - gsm telephone detector
With this mobile phone sniffer will track down mobile phones. Use this handy directional finder in situations and locations where the use of a mobile phone is forbidden. This neat unit will not only detect the presence of mobile phone activity (voice or text), it can home in on the culprit!
Apart from mobile phones this unit can also pick-up the signals sent out by modern digital cordless (DECT) phones. These operate around the same frequency as cell phones. As with cell phones you will not be able to eavesdrop on conversations because the speech is digitally encoded but you will be able to hear the telltale RF pulses.
Microwave ovens operate in the SHF band at about 2,400 MHz and you can use this unit to check for RF leaks around the door seal. Adding a few turns to the two coils and increasing the antennas length will allow you to experiment at lower frequencies in the short wave radio spectrum (not in stereo however).
This circuit for the Sniffer unit is quite simple. Looking at just one half of the circuit, the dipole antenna rms A1 and A2 form a resonant circuit with L1. Diode D1 detects the RF pulses and the signal is amplified by IC1. A dual operational amplifier type LM358 forms the headphone amplifier. The ‘power-on’ LED D3 also provides a reference voltage for the virtual earth inputs for IC1. The output signal is direct coupled to the standard Walkman type headphones via R4; this means that the amplifier operates in class ‘A’ mode giving lower distortion even at high output signal levels.
The elements for each dipole antenna are made up of two lengths of wire from 3 to 7.5 cm long arranged in-line. These two antennae are fitted at 90° to each other at the corners of the PCB. This arrangement works rather like your ears and gives you spatial awareness of the transmitter direction (handy for pin-pointing a cellphone if many phones are sending simultaneously).
Construction
Detector diodes D1 and D2 are shown in the parts list as BAT43 or BAT45 Schottky diodes. Three alternative germanium diodes are also listed, and during tests these actually improved the Sniffer units sensitivity! The popular BAT85 Schottky diode is unsuitable in this application because it has poor high frequency characteristics. The construction of L1 and L2 is not critical. In the prototype, we wound the ten turns around a 5mm diameter LED before removing the LED and fitting the coils to the PCB. The on/off switch (S1) for the unit is connected to two pads on the PCB (Figure 2). If the switch is fitted with changeover contacts, the unused connections can simply be clipped off. A press-to-make pushbutton can be used for S1; this would ensure that you never accidentally leave the unit switched on.
Make certain that the battery connector is correctly connected to the PCB. The red cable will carry +9 V from the battery but if you are in any doubt check with a voltmeter. Fit all the components to the PCB but do not plug IC1 into its socket yet. Before connecting power to the Sniffer unit, double-check that all the polarised components are correctly fitted. If the LED lights when the battery is fitted, S1 is switched on and you are confident that all is in order, turn off the unit and fit IC1 carefully into its socket.
Fit the Sniffer unit into any suitable case that will accept the PCB and battery. The phone Sniffer’s sensitivity is dependent on three factors: The type of diodes used, the length of the antennae and the voltage gain of amplifiers IC1A and IC1B. The antenna length is matched to the radio frequency of interest. GSM phones transmit at around 950 MHz so each arm of the dipole should be half the wavelength or 7.5 cm (3 cm for phones operating in the new 1800 MHz band). Try experimenting with aerial length.
The wire type is not critical but its best to choose a fairly thick wire gauge that will not bend too easily. It’s important to make strong soldered joints for the aerials so it is best to fit solder posts to the PCB aerial pads and solder the aerials to the posts.
Trying it out
Test the unit with a mobile phone. Turn the ringer off on the phone and then ring up the mobile from another phone. There is no need to answer the mobile because it emits RF pulses whenever it receives an incoming call request. Start off 1 to 2 metres away from the phone switch on the Sniffer and make sure that the bleeping sounds in the headset are equal volume when the Sniffer is pointing directly at the phone. Point the Sniffer off to the left of the phone and the bleeps in the left headphone should get softer. If the opposite occurs you have mixed up the wiring to the headphone socket. Don’t panic, the problem can be fixed without even warming up the soldering iron: If it’s not comfortable wearing the headphones round the other way, then just flip the Sniffer unit over and use it upside-down!
The range at which the Sniffer can detect a mobile is dependent on the RF power output of the phone. GSM phones alter their radiated power dynamically. The nearer the phone is to the base station, the less transmit power it will need to reach it. This is partly done to conserve battery life.
The amplifier gain can be increased to improve the Sniffer’s detection range. Feedback resistors R3 and R7 set the gain of amplifiers IC1.A and IC1.B. Try increasing both of these up to 1 MΩ or even 10 MΩ. It will not cause any damage and you will soon know when the amplification factor is too high because the headphone noise level increases and eventually the amplifier will start to whistle.
I’m sure I’m not the only one who rings my mobile number to find out where I have left the phone but what if the mobile’s ringer is turned off? No problem for the Sniffer, it can simply pin-point the (silently) ringing mobile by homing in on its RF pulses! Sounds like a good idea for a party game…
- Cell phone detector circuit picture
- Mobile phone sniffer schematic:
- Mobile telephone sniffer pcb
Random schematics
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[...] Mobile phone sniffer - gsm telephone detector With this mobile phone sniffer will track down mobile phones. Use this handy directional finder in situations and locations where the use of a mobile phone is forbidden. This neat unit will not only detect the presence of mobile phone … [...]