You can use this simple, three transistors schmitt trigger circuit to convert an anoloque data coming from any sensor like thermometer probe, photo rezistor, humidity etc. to digital signal. You can control a relay instead of Led. Also you can change trimpot with a sensor what you want.
Emitter Rezistor controls hysterysis level. This is 50 ohm as default as you see..
Try it between 50 ohm and 1 K ohm
For Maximum treshold level use 1 K ohm or minumum treshold level for 50 ohm.. Simulate it enjoy!
