Mains Failure Indicator

063 GENERAL INTEREST MAINS FAILURE INDICATOR When the mains voltage is present at the input terminals, the transistor in the optocoupler is on, T1 is off and silicon-controlled rectifier Thl is in the conducting state. Since both terminals of the piezo electric buzzer are then at the same potential, the buzzer is inactive. If the mains voltage drops out, transistor T1 conducts and causes one of the terminals of the buzzer to be connected to earth; the thyristor remains in the conducting state. In this situa- tion there is a large enough potential difference across both the buzzer and D5 to cause these elements indicating the mains fail- ure both audibly and visibly. When the mains is restored, the circuit returns to its original state. A touch on the reset button then interrupts the current through the SCR so that the thyristor goes into the blocking state, and the other terminal of the buzzer is connected to ground. ELEKTOR ELECTRONICS JULY 1989 SUPPLEMENT D1...D4 = 1N4004 8940...