John developed a clever circuit, designed to help with switching devices based on a temperature differential. He has put it to work to bring hot air from the roof cavity into colder areas of the house, to help reduce heating costs! It is compatible for use with thermistors and light dependent resistors, so can also be used for solar tracking with your solar panels, to control solar heating systems, and so much more. Setup is simple, and simple component value changes provide scope to change the duty cycle too.