solar garage heater Answered

:) I have 5-10 arduinos at hand all the time... Thats why i often go with this option.

But those reptile-switches are cool! Didnt knew they existed and can be set at a free choice-temperature. Good to know! :)

But as far as i can see, the cheapest are in the ballpark of 60$ a pop. But the benefit is definitely the simplicity of the build: No building at all. Just install and use.

It seems to be a tradeoff: Cost vs. simplicity

Don't understand why these days everyone thinks an Arduino is necessary for the most basic tasks...

I simply assume you got mains power in your garage...

Use a thermo switch as used for reptile enclosures.

These units can be set for a specific temperature and provide the option to either turn a device on or off depending on the temp and with geat accuracy.

Simply connect your fan and place the temp sensor in a spot that gives you good readings, not under the roof or on a cooler wall for example.

Set the temp for the switch unit and specify if you need the on or off state as normal - in your case it off as you want to turn the fan on when the temp is reached.

If you are a bit into electronics you can also use a NTC or PTC resistor and some transistors to switch the fan.

Suitable circuits are available and I post some links if you need them.

If you are a bit capable of programming, the simplest solution would be a little Arduino with a Temperature-Sensor like the cheap TMP35 or TMP36.

The skeleton of the code would look something like this:

- Delay
- Read both analog-voltages of the sensors
- Decide if air from the Outside is hotter than the inside-Air. If Yes:
- Check if inside-Air is below 70°F. If Yes:
- Switch Fan on which draws warm Air in from the hotbox.
- Loop up to delay

It would be a pretty straight forward code. I could help you with it if you want...

To your question about the parts: Any Arduino would do. I would suggest a TMP36 as sensor. Connect them like shown here:

https://learn.adafruit.com/tmp36-temperature-senso...

The TMP36 is around 4$ at your local electronics-shop or on Amazon.
An Arduino is also cheap. If you want a bigger UNO R3, you can go with something like 3$ (shipped) http://www.aliexpress.com/item/Free-shipping-high... or if you want a smaller one (simpler do conceal in a project-box) a Nano V3.0 for around 4$ (shipped) http://www.aliexpress.com/item/For-arduino-Nano-3-... would fit the bill.