Parts:
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 (see Notes)
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.)
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.
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.