This project came from the need to turn a window fan on during summer evenings when the outside temperature was a few degrees cooler than the inside temperature. It also needed to turn it off the next morning as the outside warmed up above the inside temperature, so it wouldn't start heating the house.
A regular thermostat wouldn't work, because we were interested in the DIFFERENCE between the two temperatures. If the inside was 75F and the outside was 70F or lower, we wanted it on, but if the outside was about 72F or higher, we wanted it off. But if the outside was 80F and the inside was 85F, then we still wanted it to come on. Thus "differential".
This can also be used for other control situations where the difference in termperature, not the actual temperature, is important.
The original schematic was found on solaffect.com's website. I made minor changes to it, but thank you to solaffect for the original.
I added a power supply circuit and enclosed everything in a 120V outlet box. It plugs into the wall. The duplex outlet serves two purposes. One side is hot all the time, and power the AC adapter "wall wart". The bridge on the hot side of the outlet is cut, so the 2nd outlet is switched by the project circuit and that's where the fan plugs in.
I used a small strip circuit board to build it on, and there was plenty of room in the outlet box to mount it on the bottom. I added a divider above the circuit board to be sure the low voltage section was completely isolated from the 120V wiring.