Phone calling Detector


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.


  • R1   - 100K       1/4W Resistor
  • R2   - 3K9        1/4W Resistor
  • R3   - 1M   1/4W Resistor
  • C1,C2 - 100nF   63V Polyester Capacitors 
  • C3  - 220µF   25V Electrolytic Capacitor
  • D1  - LED  Red 10mm. Ultra-bright  
  • D2 - 1N5819  40V 1A Schottky-barrier Diode (see Notes)  
  • Q1 - BC547   45V 100mA NPN Transistor  
  • IC1 - 7555 or TS555CN CMos Timer IC  
  • L1  - Sensor coil (see Notes)  
  • B1  - 1.5V Battery (AA or AAA cell etc.)