[masraum]

Originally Posted by jyl

I wouldn't use a spare PC, real waste of energy to run a PC 24/7 just to monitor a temperature sensor.

I can think of two approaches.

First one - buy a ready-made device. Google "differential thermostat" and you'll find products designed to close a relay if the difference in two sensed temperatures exceeds a user set level. These seem to be used in solar water heater systems.

Second one - build that device. Here is a DIY page
http://www.solaffect.com/

Thanks, I'll check that out. I'm actually going to be using the spare PC to act as the indoor unit/server for a home weather system. So the PC will be running and will already have the sensors logging to it. The only thing would be to do the programming which could probably be done with an alarm of some sort (if x>y+4 then alarm).

But, I'll also look into the thermostat that you mention. A reasonably priced commercial alternative would, I'm sure, be better than whatever I'd manage.