I start a lot of projects and never finish them. So in the spirit of documenting where I've been, here's my Poorman's Nest.  Here I'm using

• one of my Microview's I had laying around
• A potentiometer (on pin A0)
• A small digital temperature sensor (on pin 3)
• A 4.7k pullup resistor
• A 220v relay (not shown, but follow the  the yellow wire to pin 5)

Here's my code:

I abandoned this one because:

• We have baseboard heat, which draws close to 20 amps at 110V. I got a little nervous messing with this kind electricity.
• The relay draws about 200 mA when held closed. That's significant, and means the thermostat would need a fatty battery or a dedicated power source.

I might resume this project if:

• we move somewhere with a more modern 24V heating system
• I can find a latch-type relay that doesn't need power to keep it open or closed.