[jyl]

Thank you!

I looked up "voltage divider" and now I understand. I even sort of - dimly - recognize the formulas from college days.

Well, now it look like the components that I bought, i.e. the ones spec'd in the website, may not work. Which kind of makes sense, the website is for CPU cooling so they're want fans to turn on around 35C or so. I want fan and Peltier to turn on around 7C or so. I looked up the spec for the thermistor I was going to use and at 7C it is 24K ohm. The MOSFET I bought has 4.0v threshold gate voltage. So I'd need the potentiometer to be >12K ohm just to get the MOSFET to switch "on". And sure enough I bought a 10K potentiometer instead of a 50K one. (Edit: duh, I can just add a 10K resistor.)

Whatever, the stuff is cheap.

The most important thing is, with your tip I can now calculate which components I need instead of guessing. Thanks again.