I'm an old analog guy but I'm tired of adjusting pots and changing resistors. It is so nice to hit a few keys and get exactly what you want when the first estimate is not what was expected. This circuit would be 2 photocells, 2 resistors, relay, and a FET using a UNO. My last UNO only cost $11 shipped to me.
That posted circuit should have some 220 ohm resistors between the op amp output and the transistors. Mostly because you are a neophyte and they could save you some grief. I don't see anything wrong with the output transistor arrangment Put a .47uf at the photocell input of the op amp to neg power. Your sensor leads will act as an antenna picking up radio stations.
Shortly will come the day when 20K and 50K pots will be rare. EIA standard values were fairly quick to be accepted for capacitors. Pots have been a holdout. Just something warm and fuzzy about the 20 and 50K. As they say, change only happens when enough people die. Ebay is a good mirror to the world. Listings for 20K pots still outnumber 22K pots by 10 to 1. If searching through an old parts box, you will likely never find one.
