One evening late in the summer, I decided to take a little dip in the hot tub before bed. I went out to the patio, swung back the cover and slipped in. As usual, there was the sting of hot water that always goes away after a moment or two. However, this time it just got worse! I quickly got out and discovered that the temperature was 112 degrees and climbing! The temperature controller had gone bad.

In this DIY Jake will build a new controller to replace the broken one in his hot tub. I guess this curcuit could also be used for a ton of other applications, use your own imagination.

Link: Op-Amp Hot Tub Temperature Controller