Please suggest dead simple "freeze-prevention" circuit.

I have two ways to solve this problem:

1: if autopilot reads temperature below +2C from the airspeed sensor; switch on pitot heating. - this does not really regulate power, and needs to use more than required power, because the measured point is not the tube itself.

2:(I would prefer) - a simple circuit (few components, that can reliably be mounted to the pitot itself); if temperature or pitot tube itself <2C , then adjust heating current to keep 3degC

voltage ~14v , maximum needed effect ~4W - if you say op-amp +4 resistors, NTC, + a power transitor/FET, then I can do that already.
What I am looking for , is small design, if all can be epoxied to pitot - then the heat loss in a transistor, if in linear mode, can be used for something good - and component count should be kept at a minimum

If all you need is 4w, why not just leave it turned on permanently? Very simple, and very light.

4W is in case only the short outer/front metal part is to be heated, with other planes with highter cruise speeds, or demands for lower operating temperature, higher wattage would be needed.

When not in flight in cold air, it would be too hot on the ground, at least to touch.

Finally, 4W(or more) for 1,5hour is a lot of wasted power - especially with waiting & pre-flight checks on ground.

OK This should work. You’ll have to find the value for R1 yourself.

scruffy.JPG

I am assuming this is not on a full scale aircraft but FWIW most airplanes I have flown call for pitot heat on below 10C in visible moisture, 2C seems low to me.

KenF - Thanks, without a schmitt trigger, the PNP would get hot as well - the principle itself is fine. steve108 - yes - this is for UAV, much slower cruise speed, so the effect is not the same, but I may be wrong - testing remain. I posted 2C just to give people an idea. I decided to go all the way, a nichrome wire around tube, ~10W max, a foil-mounted NTC and Atmega8L with a PWM output to a switching FET. The controller will also report the actual pitot temperature and raise alarm if it gets too cold. After all, it's not that much more to make a tiny PCB, than make everything fit around tube.