CELL PHONE INCOMING CALL DETECTOR

Circuit diagram:

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.)

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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.

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.