This handy, pocket-size mobile transmission detector or sniffer can sense the presence of an activated mobile cell phone from a distance of one and-a-half meters. So it can be used to prevent use of mobile phones in examination halls, confidential rooms, etc. It is also useful for detecting the use of mobile phone for Spying and unauthorized video transmission. The circuit can detect the incoming and outgoing calls, SMS and video transmission even if the mobile phone is kept in the silent mode. The moment the Bug detects RF transmission signal from an activated mobile phone, it starts sounding a beep alarm and the LED blinks. The alarm continues until the signal transmission ceases. Assemble the circuit on a general purpose PCB as compact as possible and enclose in a small box like junk mobile case. As mentioned earlier, capacitor C3 should have a lead length of 18 mm with lead spacing of 8 mm. Carefully solder the capacitor in standing position with equal spacing of the leads. The response can be optimized by trimming the lead length of C3 for the desired frequency. You may use a short telescopic type antenna.
Use the miniature 12V battery of a remote control and a small buzzer to make the gadget pocket-size. The unit will give the warning indication if someone uses Mobile phone within a radius of 1.5 meters.
IC1 is designed as a differential amplifier Non inverting input is connected to the potential divider R1, R2. Capacitor C2 keeps the non inverting input signal stable for easy swing to + or – R3 is the feedback resistor
IC1 functions as a current to voltage converter, since it converts the tiny current released by the 0.22 capacitor as output voltage.
At power on output go high and LED lights for a short period. This is because + input gets more voltage than the – input. After a few seconds, output goes low because the output current passes to the – input through R2. Meanwhile, capacitor C1 also charges. So that both the inputs gets almost equal voltage and the output remains low. 0.22 capacitor (no other capacitor can be substituted) remains fully charged in the standby state.
When the high frequency radiation from the mobile phone is sensed by the circuit, 0.22 cap discharges its stored current to the + input of IC1 and its output goes high momentarily. (in the standby state, output of the differential amplifier is low since both inputs get equal voltage of 0.5 volts or more). Any increase in voltage at + input will change the output state to high.
Purpose of the circuit
This circuit is intended to detect unauthorized use of mobile phones in examination halls, confidential rooms etc. It also helps to detect unauthorized video and audio recordings. It detects the signal from mobile phones even if it is kept in the silent mode. It also detects SMS.
Mobile phone uses RF with a wavelength of 30cm at 872 to 2170 MHz. That is the signal is high frequency with huge energy. When the mobile phone is active, it transmits the signal in the form of sine wave which passes through the space. The encoded audio/video signal contains electromagnetic radiation which is picked up by the receiver in the base station. Mobile phone system is referred to as “Cellular Telephone system” because the coverage area is divided into “cells” each of which has a base station. The transmitter power of the modern 2G antenna in the base station is 20-100 watts.