Cellular Phone calling Detector

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


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.)


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.