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555 Power Out Alarm | 6,728 view(s)

>555 Power> Out Alarm – techmag2003.tripod.com

ElectroSuite.com – 555 Power Out Alarm. This circuit can be used as a audible ‘Power-out Alarm‘.

555 Power Out Alarm - ElectroSuite.com

555 Power Out Alarm

It uses the 555 timer as an oscillator biased off by the presence of line-based DC voltage. When the line voltage fails, the bias is removed, and the tone will be heard in the speaker. R1 and C1 provide the DC bias that charges capacitor Ct to over 2/3 voltage, thereby holding the timer output low (as you learned previously). Diode D1 provides DC bias to the timer-supply pin and, optionally, charges a rechargeable 9-volt battery across D2. And when the line power fails, DC is furnished to the timer through D2.

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