"James Sweet" wrote in message
news:Kn4Qj.6369$Mm1.4242@trndny07...



One thing to watch out for is the hysteresis in these devices. The
device I pointed out will turn the fan on at 30 degrees, but it won't go
off again until the temperature drops to 20. If your room is warmer than
that it will never turn off!


Gareth.



Could build something. I recall seeing some temperature controller
circuits that used a diode as the sensing element. Should be easy to
adjust the hysteresis by selecting component values.



Well this is the range of those devices:

http://uk.farnell.com/jsp/level5/mod...=en/212166.xml


Note that the opening and closing temperatures on some of them are the wrong
way around, but I reckon that the normally open switch that will switch on
the fan at 55 degrees C and turn it off at 35, part no. 1006853, would be
ideal. 55 degrees is not going to cook any electronics and it is unlikely
that room temperature is ever going to get to 35. Case solved, one
component, no design work required, and extremely reliable.



Gareth.