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Cellular Phone calling Detector

  • Flashes a LED when detecting an incoming call
  • Powered by one 1.5V cell

Circuit diagram:




100K 1/4W Resistor


3K9 1/4W Resistor


1M 1/4W Resistor

C1, C2

100nF 63V Polyester Capacitors


220µF 25V Electrolytic Capacitor


LED Red 10mm. Ultra-bright (see Notes)


1N5819 40V 1A Schottky-barrier Diode (see Notes)


BC547 45V 100mA NPN Transistor


7555 or TS555CN CMOS Timer IC


Sensor coil (see Notes)


1.5V Battery (AA or AAA cell etc.)

Device purpose:

This circuit was designed to detect when a call is incoming in a cellular phone (even when the calling tone of the device is switched-off) by means of a flashing LED.
The device must be placed a few centimeters from the cellular phone, so its sensor coil L1 can detect the field emitted by the phone receiver during an incoming call.

Circuit operation:

The signal detected by the sensor coil is amplified by transistor Q1 and drives the monostable input pin of IC1. The IC's output voltage is doubled by C2 & D2 in order to drive the high-efficiency ultra-bright LED at a suitable peak-voltage.


  • Stand-by current drawing is less than 200µA, therefore a power on/off switch is unnecessary.
  • Sensitivity of this circuit depends on the sensor coil type.
  • L1 can be made by winding 130 to 150 turns of 0.2 mm. enameled wire on a 5 cm. diameter former (e.g. a can). Remove the coil from the former and wind it with insulating tape, thus obtaining a stand-alone coil.
  • A commercial 10mH miniature inductor, usually sold in the form of a tiny rectangular plastic box, can be used satisfactorily but with lower sensitivity.
  • IC1 must be a CMOS type: only these devices can safely operate at 1.5V supply or less.
  • Any Schottky-barrier type diode can be used in place of the 1N5819: the BAT46 type is a very good choice.

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