Any simple circuits out there for controlling the speed of a small fan according to the temperature? I'll be mounting two SATA hard drives along with a small switched mode power supply in a 19" 1U metal cabinet, and in order to keep noise levels down I would prefer to have the fan only run when needed, and then maybe even at a low speed unless the temperature is very high. I assume a regular 12V PC type fan will do.
I searched eBay for something ready made in order to save me the trouble, but couldn't find anything suitable.