This simple high voltage detector alarm circuit will indicate when the mains voltage rises suddenly or on high voltage spikes. Most of the time we cannot notice it because it happens very fast. But the circuit will do this job very effeciantely by producing an audible alarm in that situation so you can
secure your important electronic appliances by switching them off until the voltage gets stable.
The transformer steps down the mains voltage from 230V to 12V and the diode bridge D1 to D4 rectify this voltage. The 1000uF 50V capacitor is used to filter the voltage coming from the diode bridge. After filtration the voltage goes in the high voltage alarm circuit that is built around two transistors. The voltage detector circuit is build around transistor Q1, and Q2 is working as a switch. The transistor can be replaced by any other general purpose values like 2N3904, 2N2222 etc.