DESIGN AND IMPLEMENTATION OF AN AUTO CUT-OFF CIRCUIT WITH ON-TIME DELAY AND GSM-BASED VOLTAGE MONITORING

Abstract

This paper presents the design and implementation of an inexpensive auto cut-off circuit with on-time delay, intended to protect household appliances such as refrigerators, televisions, and VCRs from undesirable over-voltage, under-voltage, and transient surges. The circuit is fabricated using discrete components including transistors, relays, and supporting passive elements, and is capable of operating as a standalone protection unit or in conjunction with existing stabilizers. The on-time delay feature ensures that loads are safeguarded against frequent switching surges and rapid power resumption following mains failure, thereby enhancing equipment longevity. The system provides four layers of protection: over-voltage cut-off, under-voltage cut-off, transient suppression, and prevention of frequent ON/OFF cycling. To enhance user awareness, the design integrates a GSM module that transmits voltage status updates via SMS, enabling remote monitoring of line conditions. An 8051 microcontroller serves as the core controller, powered by a regulated 5V DC supply, and communicates with the GSM module and Bluetooth modem using UART protocol. The microcontroller also interfaces with relays through transistor switches to execute protective actions. Experimental validation demonstrates reliable cut-off operation under abnormal voltage conditions and effective delay management during power restoration. The proposed system offers a cost-effective, user-friendly, and scalable solution for safeguarding household and industrial loads against voltage fluctuations and surges, contributing to improved reliability and safety in power distribution networks.