Years ago I built a propane smoker out of an old refrigerator. I used the burner assembly from a small swimming pool heater, which had a millivolt / standing pilot gas valve (similar to a hot water tank). The pilot heated the thermocouple which created the power for the gas valve, so that it wouldn't fire unless there was a pilot flame burning. Also had a mechanical sensor for the thermostat control.
I had a couple of problems with the setup, it never did work very well. The first problem was that the standing pilot gave off a fair amount of heat, so you couldn't really use it unless it was cold outside. I used the main burner to heat the wood chips, but because the pilot threw off so much heat the main burner would hardly ever come on, so no smoke was being generated. The second problem was from the mechanical thermostat, which had about 20 degrees plus/minus from open to close. So if you set it at 200 it wouldn't fire until 190, then would stay on until 210 or more. Would be ok for somethings, but it wasn't a fine enough control for sausage.
Now this was 20 years ago, so I'm sure there are better solutions now (
AMNPS for smoke, for one thing).