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This over/under voltage cut-outwill save your costly electricaland electronic appliances fromthe adverse effects of very high andvery low mains voltages.The circuit features auto reset andutilises easily available components. Itmakes use of the comparators availableinside 555 timer ICs. Supply is tappedfrom different points of the power supplycircuit for relay and control circuitoperation to achieve reliability. The circuit utilises comparator 2for control while comparator 1 output(connected to reset pin R) is kept lowby shorting pins 5 and 6 of 555 IC. Thepositive input pin of comparator 2 isat 1/3rd of Vcc voltage. Thus as long asnegative input pin 2 is less positivethan 1/3 Vcc, comparator 2 output ishigh and the internal flip-flop is set,i.e. its Q output (pin 3) is high. Atthe same time pin 7 is in high impedancestate and LED connected to pin 7 is thereforeoff. The output(at pin 3)reverses (goeslow) when pin2 is taken morepositive than1/3 Vcc. At thesame time pin7 goes low (asQ output of internalflip-flopis high) andthe ED connectedto pin 7is lit. Both timers(IC1 andIC2) are configuredto function in the same fashion.Preset VR1 is adjusted for undervoltage (say 160 volts) cut-out by observingthat LED1 just lights up whenmains voltage is slightly greater than160V AC. At this setting the output atpin 3 of IC1 is low and transistor T1 isin cut-off state. As a result RESET pin4 of IC2 is held high since it is connectedto Vcc via 100 kilo-ohm resistor R4.Preset VR2 is adjusted for overvoltage (say 270V AC) cut-out by observingthat LED2 just extinguisheswhen the mains voltage is slightlyless than 270V AC. With RESETpin 4 of IC2 high, the output pin 3 isalso high. As a result transistor T2conducts and energises relay RL1,connecting load to power supply viaits N/O contacts. This is the situationas long as mains voltage is greaterthan 160V AC but less than 270V AC.When mains voltage goes beyond270V AC, it causes output pin 3 ofIC2 to go low and cut-off transistor T2and de-energise relay RL1, in spite ofRESET pin 4 still being high. Whenmains voltage goes below 160V AC,IC1’s pin 3 goes high and LED1 isextinguished. The high output at pin 3results in conduction of transistor T1.As a result collector of transistor T1as also RESET pin 4 of IC2 are pulledlow. Thus output of IC2 goes lowand transistor T2 does not conduct.As a result relay RL1 is de-energised,which causes load to be disconnectedfrom the supply. When mains voltageagain goes beyond 160V AC (butless than 270V AC) the relay againenergises to connect the load to powersupply.

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