I'm hoping someone may have some ideas that I haven't seen yet.
I'm trying to run a little USB-powered (5v) heater to keep a small area outside from freezing or getting too cold. I'm aiming for around 15 degrees Celcius.
It would ideally also be somewhat smallish.
Right now I have a couple of KSD9700 temperature switches. It's rated at 15C, but the hysteresis seems to give it +/- 3 degrees. So it fluctuates all the way from 12 to 18 degrees. And the two I have vary widely in their behaviour, which is probably a symptom of a cheap eBay purchase. But it does the trick for the most part.
I've also seen KSD301, but they look to be much bigger. Not sure if their behaviour is similar.
These Texas Instruments ones seem to be ideal: TMP709, so I'm planning to pick up a few of those. They appear to have a much smaller hysteresis (2 degrees), which means much less temperature fluctuation.
Maxim also has some: MAX6501 that look to be the same kind of thing.
It looks like I can combine those with a transistor to power the 5v heater.
Both are smd, so a small PCB could hold all that's needed in a tiny footprint.
I'm trying to keep things really simple, so I'm trying to avoid using a microcontroller + separate sensor etc. Although I recall the very first of the TI Launchpads came with a chip with a built-in temperature sensor that was part of their demo. But that does require each chip to be programmed, which adds complexity.
Am I missing anything really easy and obvious? Or maybe something less obvious but much more ideal?
ps, does 2 degrees hysteresis mean it would be 15 +/- 2 = 13 to 17 degrees, or is the 2 the sweep from high to low, ie 14 to 16 degrees?