description source: http://www.electroschematics.com/1035/mobile-bug-detector-sniffer/
This handy mobile phone detector can sense the presence of an activated mobile cellphone from a distance of one and-a-half metre. 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 both the incoming and outgoing calls, SMS and video transmission even if the mobile phone is kept in the silent mode. The moment the mobile phone detector 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.