Cell Phone Detector


This handy, pocket size mobile transmission detector ca sense the presence of an activated mobile cell phone from a distance of one and a half meter. So it can be used to prevent use of mobile phones in examination halls, confidential rooms, etc. It also useful for detecting the use of mobile for spying and unauthorized video transmission. The circuit can detect the incoming and outgoing calls, SMS and video transmission even if the transmission signal from an activated mobile phone, its LED starts blinking. The alarm continue until the signal transmission ceases. Assemble the circuit on Vero board as compact as possible and enclose in small box like junk mobile case.

The LED will give the warning indication if someone uses Mobile phone within a radius of 1.5 meters.


  1. Battery (9V)
  2. Resistors
  3. Transistor (BC548)
  4. Vero board
  5. IC (CA3130)
  6. Capacitors
  7. LED


Ordinary LC circuits are used to detect low frequency radiation in the AM and FM bands. The tuned tank circuit having a coil and a variable capacitor retrieve the signal from the carrier wave. But such LC circuit cannot detect high frequency waves near the microwave region. Hence in the circuit, a capacitor is used to detect RF from mobile phone considering that, a capacitor can store energy even from an outside source and oscillate like LC circuit.

Figure 3.1 Schematics

One lead of the capacitor gets DC from the positive rail and the other leads gone to the negative input of IC. So the capacitor gets energy for storage. This energy is applied to the inputs of IC so that the inputs of IC are almost balanced with 1.4 volts. In this state output is zero. But at any time IC can give a high output if a small current is induced to its inputs. There a natural electromagnetic field around the capacitor caused by the 50 Hz from electrical wiring. When the mobile phone radiates high energy pulsations, capacitor oscillates and release energy in the inputs of IC. This oscillation indicated by the flashing of the LED. In short, capacitor carries energy and is in an electromagnetic field. So a slight change in field caused by the RF from phone will disturb the field and forces the capacitor to release energy.


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