Low Voltage Circuit

High & Low voltage cut-off with delay & alarm Circuit

This High & Low voltage cut-off circuit serves as a protective measure for electrical appliances, shielding them from potential damage caused by both overvoltage and undervoltage. Additionally, it includes an alarm feature to alert users when the power supply is restored. This circuit proves invaluable in safeguarding valuable equipment from voltage fluctuations within a household setting. With minor adjustments, the same circuit can be adapted to function as an automatic voltage stabilizer.

Under normal mains voltage conditions, the voltage at the negative terminal of zener diode D4 remains below 5.6 Volts. In this state, transistor T1 remains in a non-conductive state. Simultaneously, the voltage at the negative terminal of zener diode D5 exceeds 5.6 Volts, causing transistor T2 to conduct. Consequently, the relay is activated, and the green LED indicator illuminates.

However, when the mains voltage surpasses the preset limit, causing the voltage at the negative terminal of D4 to rise above 5.6 Volts, transistor T1 conducts, while T2 becomes non-conductive. As a result, the relay disconnects the mains supply from the connected load.

Conversely, when the mains voltage drops below the preset threshold, T1 and T2 assume their non-conductive states, leading to the deactivation of the relay and the disconnection of the load from the mains.

The NE555 timer is configured as a monostable multivibrator with a pulse width of 10ms. When power is restored following an outage, a negative voltage is applied to the trigger pin of IC NE555, initiating its operation. The buzzer is activated through the forward-biased transistor T3, producing an audible beep as an indication of power restoration. Furthermore, transistor T1 is turned on during this time, resulting in the deactivation of T2. This 10ms delay ensures that the relay remains inactive, offering adequate protection against surge voltages and safeguarding the connected equipment.

High & Low voltage cut-off Circuit diagram with Parts list

High & Low voltage cut-off Circuit Diagram


  • An autotransformer is required to calibrate the circuit. Connect the autotransformer’s output to the transformer’s primary.
  • Set the voltage to 260V and deactivate the relay by adjusting VR1.
  • Now set the autotransformer to 160V and adjust VR2 to de-energize the relay.
  • The buzzer volume can be adjusted with VR3.

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