Only One transistor and some other components are used in this Overvoltage protection Auto circuit.
Load is connected with NC (normally connected ) Pin of relay.
This circuit is very useful autocut circuit to protection of any electrical or electronics appliance from high voltage. If any device need 12v to working properly then this can be damaged on increasing the voltage. This circuit automatically cut the current supply to any appliance from input circuit . And when voltage decreased at normal level then automatic on the supply . So it help to protects the circuit damage by the high voltage. If you use this circuit for Autocut on over 6v then use 6 v relay and circuit need 6v .
At first when the voltage under or below the maximum limit ( at which it adjusted) the maximum voltage dropped through the Resistor R2 to ground and Base of Transistor does not get enough power to switch on the transistor. At this stage the transistor in OFF state and Relay not get supply and it is not activated.
when Input voltage of this circuit is increasing and cross a level then the input voltage through resistor R1 and preset also increased and even after dropping the voltage through R3 , enough amount of voltage is at Base of Transistor Turned On it and Supply passed Between Emitter and collector. Now Relay is activated and supply is Disconnected to appliance . And that is protected from High voltage supply.
Before using it should be adjusted by preset and tested the input voltage and set required the auto cut level according to suitable .