"solar cell"  

•  A solar cell made from a mono crystalline silicon wafer is shown above. 
•  A solar cell (also called a photo voltaic cell) is an electrical device that converts the energy of light directly into electricity by the photo voltaic effect.
• It is a form of photoelectric cell (in that its electrical characteristics—e.g. current, voltage, or resistance—vary when light is incident upon it) which, when exposed to light, can generate and support an electric current without being attached to any external voltage source, but do require an external load for power consumption.
•  Photo voltaic’s  is the field of technology and research related to the practical application of photo voltaic cells in producing electricity from light, though it is often used specifically to refer to the generation of electricity from sunlight.
•  Cells can be described as photo voltaic even when the light source is not necessarily sunlight (lamplight, artificial light, etc.). In such cases the cell is sometimes used as a photo detector (for example infrared detectors), detecting light or other electromagnetic radiation near the visible range, or measuring light intensity.
• The operation of a photo voltaic (PV) cell requires 3 basic attributes:
• 1.  The absorption of light,  generating  electron-hole pair.
• 2.  The separation of charge carriers of opposite types.
• 3.  The separate extraction of those carriers to an external circuit.

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Cellular Phone calling Detector

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Flashes a LED when detecting an incoming call

Powered by one 1.5V cell

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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.