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2N5777 2N3904 2N4250 2N4870
A 2N5777 photo-Darlington cell picks up blinking light from the transparent plastic buttons. The power is switched ON and OFF by a hibeta 2N3904 transistor. The circuit's 9 V battery can be left continuously connected. Less than a microampere is drawneven with normal, office ambient light and the phone lights not flashing. 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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