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555 city communication electric acousto-optical alarming cir

2015-01-31 09:29  
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555 city communication electric acousto-optical alarming circuit
The multivibrator is composed of 555 time base circuit and R4, R5, C2. It always has electricity, 555's 4 foot is clampped to low level(about 0.3V) by 4N25's output terminal, 555 is forced in reset state, 555 oscillator does not vibrate. After power failure, the photoelectric coupler 4N25 does not work, the photosensitive tube is in off condition, so 555's 4 foot is high level, 555 starts to vibrate, the oscillation frequency fc=1.44(R4+2R5)C2. The figured parameter's oscillation frequency is 2028Hz. Its oscillation square wave which is outputed by 3 foot sends out 2kHz sonorous sound by VT1 driving small B, at the same, it drives LED2 to flash, it warns the user that it is power failure.