This circuit came out in a publication called Electronics Handbook of 1993. The circuit activates a buzzer (Sonalert type) when the power of a circuit is cut off. The Rx value depends on the voltage of the monitored source, being typically 330 ohms to 470 ohms for voltages between 5 and 6 V, 1k for 9 or 12 V and 1k5 for 15 V.Q1 can be a BC548 and R1 is 3109k, R2 10k. The sensor is a common LDR.
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