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.
They are be found on places such as ebay. SupplyHouse.com and Grainger.com are two frequently used online sources. In the UK they're often referred to as thermostats or bimetalic thermostats and can be found at Sinolec Components - www.sinolec.co.uk or RS Components - uk.rs-online.com.
Make sure you get one where the temperature that turns on the fan (the switch closes) is higher than the temperature that turns off the fan (the switch opens.) Most people use ones that turn on at 110F and off at 90F.
WARNING: Many of the following photos and videos show Romex wire being used for the length of wire extending into the solar air heater to the snap disc sensor. These were used in experimental situations. If you're making a solar air heater or something else where the wires will be in a heated location then consider using teflon insulated wire instead.
Why? If the insulation melts it may cause a short circuit and result in a fire. For example, if it's a sunny day and no air is being circulated through the solar air heater because the house is already hot enough then the temperature where the wire is inside the solar air heater may get over 200F (93C). This may be over the wire's insulation's rating.
The following are the simplest circuits that can be used, one using AC and one using DC. WARNING: The following circuit connects to household power. Do not do this unless you are familiar with electronics. If you're using batteries as the power source instead then it's safer.
The above box was then inserted into the top of my outdoor to indoor airflow solar air heater as shown in the following photos.
The following videos give step-by-step details for making the AC circuit. The first video introduces the snap disc sensor, tests a prototype circuit and shows how to determine the size of fuse needed for your fan or blower. It's especially useful for anyone not familiar with electronics.
This next video wires up the actual circuit in a safe manner (safety is imporant here since it's being plugged into the household wiring), assembles it all in a nice box, puts it in the actual solar air heater and does a quick test.
Adding a thermostat for added control based on room temperature
If your room is already hot enough then you don't want your solar air heater to provide more hot air. This can be controlled by adding a thermostat to the circuit. The thermostat is basically another switch that opens when the room is hot enough and closes when the room needs more heat. The following diagram has the themostat added.
Safer use of household power
The snap disc sensor is located in the solar air heater. Using the above schematic, the sensor has 120VAC across it. It may be preferable to have lower voltage wiring in this area and for that the following schematic can be used. All other circuitry can be housed in a box that is mounted to the blower/fan enclosure and so all wiring there can have the higher 120 volts AC while only lower voltage 24 volts AC wires would extend out to the location of the snap disc sensor.
For performance measurement purposes it's often handy to add an hour meter to the circuit so you can see how long your solar air heater has run for. An example is shown below on the left of an hour meter from Grainger, part no. 2PAW9, for $23.77. In the photo on the right the hour meter is mounted on the solar air heater input vent for convenience. Click here for details of this particular solar air heater.
The hour meter would go on the AC side of the circuit anywhere where these would be no power while the snap disc has turned off the fan. See the circuit on the right below. (This particular one also has a manual switch thrown in.)
Examples in solar air heaters
At the time the projects below were done, the only snap disc sensor I could find in town was a cheap one for around $5.00CDN that worked the opposite way desired. 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.
The following is from my outdoor to indoor airflow solar air heater.
The following is from my indoor to indoor airflow solar air heater.