I would like to make a threshold-temperature on-off switch (a.k.a. a thermostat). Short of buying a commercial sensor like an LM335Z (I know that it's super cheap) and figuring out how to use the output, I thought that it could be a fairly simple task to design something that would turn on an electrical device below a certain temperature.
Imagine a two-layer long narrow "wafer" made of two materials with dissimilar thermal expansion coefficients, perhaps rubber and very thin steel. This would be a physical action arm that engages and disengages a switch by its position. I would calibrate the switch to turn on at the right temperature with a separate temperature gauge, and have a working thermostat.
My current plan is to make a device that will help maintain an environment suitable for bread to rise (anywhere from 85 to 105 F). It could also be used to make yogurt. Or keep a lizard cage at the right temperature. The possibilities are many.
As a bonus, I'd like to teach my son some science and handicraft, so this has additional purpose beyond the practical.
It's just a vague idea at the moment, so I wonder if others with more experience in DIY projects of this type could offer advice. I have some wiring experience and am willing to purchase parts if needed (even if that raises the total price beyond what I could simply buy). I also have a fairly large store of random electronics parts and pieces that I may be able to use.