Flex sensor is indeed a problem with the heat.Something to measure the distance one end of the bimetal (that's what you're using, right?) may be easier.For the heating control, you don't normally control the voltage, it's much easier to control the current. Use a MOSFET and PWM to control it.
A 100k series resistor should do fine for that sensor.Hook it up, see what readings you get.Wire up the heating wire with MOSFET switch, make sure you can control this properly.Combine them together. The PID is the hard part.For all the separate parts you'll be able to find lots of tutorials.
i buy one MOSFET IRFZ44N
You buy the wrong sort of FET, you need one like this:-https://www.adafruit.com/product/355
Tnx. Does it have a similar? i can not find it in stores.