am really impressed people thank for your respond. it's actually a low temperature sensor. the fan is going to be a small fan too like the one on a laptop processor. the point is i have a some fanless point of sales terminal that print tickets wirelessly and most times, the printer and the modem get burn and i have to replace them often. so i thought if there can be a circuit with a thermal sensor that detect the rise in tempratue on either the printer and or the modem and start the corresponding fan that cools it down, then i won't have to replace the printer or the modem that often.
thank u in advance every one.
regards.