An alarm that sounds when AC power is lost.
Our basement sump pump sometimes trips the ground-fault interrupt, and if we don't notice, bad things happen. The goal was to be able to sound an alarm for at least 24 hours that would be audible on other floors of the house, and to last the shelf life of a 9V battery before needing maintenance.
This design seems to fit those criteria; if my assumptions are correct, it can sound its alarm for 2-3 weeks continuously, and while inactive, it should last the shelf life of the battery.
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