Power-line outage flashes red alert
Edited by Bill Travis
Vasiliy Borodai, Zaporozhje, Ukraine -- EDN, June 13, 2002
This Design Idea expands on a circuit in a previous one to configure a power-outage detector with a flashing alarm (Figure 1, Reference 1). The circuit plugs into a mains outlet and uses trickle-charged nickel-cadmium batteries. The green-LED monitors the presence of line voltage. The BZV55-C4V3 zener diode, D1 protects the batteries against overvoltage. Voltage bias from R6 and the green LED keeps Q1 off, so the Q1-Q2 RC oscillator remains off. When a power outage occurs, capacitor C2 discharges in approximately 2 to 3 seconds, and the dc bias disappears. Now supplied by the batteries, the RC oscillator starts, and the red LED flashes at a rate of approximately 2 Hz. R3 and C3 set the flashing frequency. You can use an infrared LED instead of the visible LED. The infrared LED couples with an infrared receiver to provide a convenient remote alarm.
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