Temperature controller

This is a bit offtopic, but I'm posting this as a followup to my Sparkfun post since I'm too lazy to figure out how to post pictures there right now :-)

This is a differential temperature controller I designed a few years back. Honestly, I don't remember much about it other than I did it quickly at someone's request and tested it on my lab bench. Can't even remember who the guy was other than to say it was used to speed up a cooling fan on a battery pack when the battery got a certain amount of degrees hotter than the ambient air.

Normally the fan runs continuously with its speed controlled by a fixed resistor. When the temperature differential is exceeded, the transistor shorts out the resistor and the fan speed increases. Notice that it doesn't use hysteresis about the setpoint for example. Also, it isn't a true differential controller since the battery sensor output is made to be a fraction of the ambient sensor output instead of simply the difference between them. It's meant to be an example to start from, not the final item. So have fun with it.