Here is a very sensitive long distance cell phone detector circuit that can detect the mobile phone signals from 30 to 40 feet away. It is a very useful circuit and can be used to detect cell phones at the places where they are not allowed for example examination halls etc. The circuit can also be used to
detect a spy cell phone or also for finding the lost cell phone when the phone is on vibrate alert.

The circuit cannot detect the speech or text message content, it can only detect the encoded signal sound due to which we can understand that someone is using a cell phone.

The circuit is very simple and using very few components, which are two 1N34 diodes, CA3140 IC, transistor, LED, three resistors and a buzzer that is optional, you can also remove the buzzer and use only LED with its 470 Ohms current limiting resistor.

Working of the circuit is simple, when we dial a number or send an SMS or MMS through our cell phone then our cell phone generate an RF signal to connect to its nearest cell phone tower, the two diodes shown in the circuit detects these RF signals which are amplified by the CA3140 IC. The output is fed to the transistor, which become switch on and activate the LED and the peizo buzzer. The operating voltage of the circuit is 9 to 12V DC but it can also be operated with 6 volt DC.  

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Long Range Cell Phone Detector Circuit Diagram