Circuit monitors blinking phone lights

Posted on Jul 30, 2012

A 2N5777 photo-Darlington cell picks up blinking light from the transparent plastic buttons. The power is switched ON and OFF by a hi-beta 2N3904 transistor. The circuit's 9 V battery can be left continuously connected. Less than a micro-ampere is drawn-even with normal, office ambient light and the phone lights not flashing.

Click here to download the full size of the above Circuit.

For noisy locations, the tone can be made louder with an output transformer (ratio of 250:8) or a 100 ohm speaker that replaces the 22 ohm resistor in the output.

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