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This circuit use to detect a phone call in the cell phone. With 1.5V operating power supply, it using a coil sensor (L1) that can detect the field emitted by the phone receiver during an incoming call and will drive the flashing LED. The device must be placed a few centimeters from the cellular phone depend on the coil sensitivity.

Parts:

R1—————————————————100K 1/4W resistor

R2—————————————————3K9 1/4W resistor

R3—————————————————1M 1/4W resistor

C1,C2———————————————–100nF 63V polyester capacitor

C3—————————————————220uF 25V electrolytic capacitor

D1—————————————————LED 10mm ultra bright

D2—————————————————1N5819 40V 1A schottky barrier diode

Q1—————————————————BC547 45V 100mA NPN transistor

IC1—————————————————7555 CMOS timer IC

L1—————————————————-Coil sensor

B1—————————————————-1.5V battery

When a cell phone ringing, the signal detected by the sensor coil is amplified by transistor Q1 and drives the monostable input pin of 555 timer IC. The IC’s output is doubled by C2 and D2 in order to drive the ultra high LED at a suitable peak voltage. The current consumption on standby mode is less than 200uA. L1 can be made by winding 150 to 150 turn of 0.2mm, enameled wire on a 5 cm diameter former. Remove it from the former and wind with insulting tape. It can be also replace with a commercial 10mH miniature inductor, but with lower sensitivity. Use CMOS IC timer, only these device can safely operate at 1.5V supply or less.

source : www.redcircuit.net

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