Here is an adjustable overvoltage under voltage sensing switch schematic utilizing only two variable resistors, two transistors and one relay. This may be used as circuit protection against power surges or any unwanted over voltage or under voltage conditions. Simply vary the voltage input at the power source to see it in action. As is, it is set up to apply power to the relay only between about 11 volts and 13 and a half volts. Enjoy
