Cellular Phone Calling Detector Circuit Schematic


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 diagram:

Cellular Phone Calling Detector Circuit Schematic-Circuit diagram


  • R1 = 100K
  • R2 = 3K9
  • R3 = 1M
  • C1 = 100nF-63V
  • C2 = 100nF-63V
  • C3 = 220µF-25V
  • D1 = LED Red 10mm.Ultra-bright (see Notes)
  • D2 = 1N5819-40V 1A Schottky-barrier Diode (see Notes)
  • Q1 = BC547-45V 100mA NPN Transistor
  • L1 = Sensor coil (see Notes)
  • B1 = 1.5V Battery (AA or AAA cell etc.)
  • IC1 = 7555 or TS555CN CMos Timer IC

Circuit operation:

The signal detected by the sensor coil is amplified by transistor Q1 and drives the constable 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