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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