Alarm. - Here is Circuit Schematic AC Power Loss Alarm using LM555 and FET. The circuit is powered by a 9v battery and is packaged in small plastic box with a 9v battery compartment. A very popular opto-isolator is used to monitor the power line. As long as there is AC power to the input, the n-channel FET Q1 is kept turned off. As soon as power is lost, +9v is switched to the beeper circuit. I used a CMOS version of the 555 timer to produce a 50ms pulse every 3 seconds. The output of the timer drives an n-channel FET which in turn drives a very loud siren type beeper, available from Radio Shack. The result is a sound maker which is very loud but only produces short chirps. The sound is loud enough to be easily heard without drawing a lot of average current from the 9v battery. Using the short 50ms pulses every 3 seconds, the average current when beeping is reduced to about 2 milliamps. In standby mode the current is about 2 microamps.
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