This circuit is very useful to instantly reveal the lack of power in the local network. There are many practical applications in which such an alarm is important. We can mention the case of refrigerated counters, in which a lack of energy, even for a short time, can cause the deterioration of goods. We also have the case of medical equipment and industrial machinery, which can endanger human lives or production in the event of a power failure. The circuit shown in the figure uses three transistors of common types and is powered in the alarm part by 4 cells. In the presence of energy, the current consumption of the batteries is almost negligible.
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