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L85, Mobile Signal Detector

This mobile signal detector can sense the presence of an activated mobile cell phone from a distance of one and-a-half meters.

If a RF signal is detected the cell phone detector circuit will inform you using a sound alarm (beep sound) and a blinking LED.


  • IC1 = CA313

  • IC2 = 555 Timer

  • T1 = BC548

  • R1, R3 = 2.2M, R2 = 100K, R4 = 1K

  • R5 = 12K, R6 = 15K

  • C1, C2 = 22pf, C3 = 0.22uf, C4 = 100uf 16v,

  • C5 = 47pf, C6, C7 = 0.1uf, C8 = 0.01uf, C9 = 4.7uf

  • Piezo Buzzer

  • LED (Red)

  • Battery 9-12volt

  • Antenna (5 inch)

  • S1 = Switch

In this type of circuit (that use high frequency 0.9-3Ghz) you can’t use an LC circuit for frequency tuning.

The circuit uses a 0.22μF disk capacitor (C3) to according the RF circuit to ensure the capability of the circuit to capture the mobile phone signal.

This cellular phone detector can detect any activity of a mobile phone incoming or outgoing SMS video transmission or voice transmission.

The C3 capacitor must have leads length of 18 mm with 8 mm spacing between leads to obtain the desired frequency. This small disk capacitor acts as a small gigahertz loop antenna to collect the RF signals.

The CA3130 operational amplifier IC is used as a current to voltage converter.

This cellular phone detector electronic circuit diagram can be used to verify the presence of an active cellular phone in the tested area

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