Low Voltage Circuit

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

This High & Low voltage cut-off circuit will safeguard electrical appliances from both overvoltage and undervoltage. When the power source is restored, the circuit also sounds an alarm. This circuit is great for protecting your valuable equipment from voltage fluctuations at home. With a few tweaks, the same circuit may be utilized to create an automatic voltage stabilizer.

The voltage at the negative terminal of zener diode D4 will be less than 5.6 Volts when the mains voltage is normal. Transistor T1 will not conduct in this state. The voltage at the negative terminal of the zener diode D5 will be larger than 5.6 at the same moment, causing the transistor T2 to conduct. The relay will turn on and the green LED will light up.

Because the voltage at the negative terminal of D4 is larger than 5.6 V, the transistor T1 conducts when the mains voltage exceeds the set limit. At the same time, transistor T2 will be non-conducting, causing the relay to be deactivated, cutting the mains supply from the load. T1 and T2 become non-conducting when the mains voltage falls below the set limit, causing the relay to de-activate and disconnect the load from the mains.

The NE555 timer is set up as a monostable multivibrator with a 10ms pulse width. When power is restored after a power outage, a negative voltage is obtained at the trigger pin, triggering the IC NE555. The buzzer is driven by the forward biassed transistor T3, which produces a beep as an indication of power resumption. In addition, the transistor T1 is turned on, which turns off T2. As a result, the relay will remain de-activated for 10ms, providing enough delay and protecting the equipment from surge voltages.

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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