For a few experiments such as solar air heaters, I needed a automatic turning on and off of a fan. Probably the simplest way to do this is with a bimetal snap disc thermal sensor.

A bimetal snap disc thermal sensor is basically a switch that closes when a specific high temperature is reached and opens when a specific lower temperature is reached. That way, by connecting in a fan in series with the switch, the fan would turn on when the high temperature is reached and off then the lower temperature is reached. The following is the simplest circuit that can be used. WARNING: Do not do this unless you are familiar with electronics.

A bimetal snap disc thermal sensor

Simplest fan (or other device) control circuit

Unfortunately the only snap disc sensor I could find in town was a cheap one for around $5.00CDN that worked the opposite way. It opens when the higher temperature is reached and closes when the lower temperature is reached. So a relay was added to reverse this, and power had to be supplied to the relay. The resulting circuit was still simple, but not as simple as it could have been. WARNING: The following are some photos containing electronics arrangments that are extremely ugly. :-)

The following is from my mini can solar air heater.

Fan and electronics.

The following is from my outdoor to indoor airflow solar air heater.

The following is from my indoor to indoor airflow solar air heater.