Parts:
R1____________100K 1/4W Resistor
R2______________3K9 1/4W Resistor
R3______________1M 1/4W Resistor
C1,C2_________100nF 63V Polyester Capacitors
C3____________220΅F 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.)
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 200΅A, 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.
Source: Electronics Lab
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