I'm trying to put something together to control the heated seat in my car, and have come up with this:
The idea is to keep the seat at a reasonably constant temperature by switching the heating element off once a preset temperature is reached, and back on once the temp drops slightly.
R1 = NTC thermistor attached to the seat (5-15KΩ in the temperature range I'm interested in.)
R2 = Pot for setting the cutoff point
LOAD = The heating element - a resistive load of approx 2.6Ω @ 12V nominal (automotive, so really anything from 12-15V)
The circuit I based the design on used a relay to switch the load on/off, but I wanted to avoid this as a relay clicking every few seconds would be annoying, so I've replaced it with a MOSFET. This is cobbled together from a variety of sources, so the big question is: Will it work?
I know that the 741 isn't an ideal choice so I looked at using a comparator instead, as suggested by a few people elsewhere, but I don't know how to drive the MOSFET from an open-collector output.