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

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.

R1	100K	1/4W Resistor
R2	3K9	1/4W Resistor
R3	1M	1/4W Resistor
C1,C2	100nF	63V Polyester Capacitors
C3	220µF	63V Polyester Capacitors
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.)

Notes:

• 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.

Filed Under: hobby project, LED, Mobile