Tips for your first real Arduino project, from someone who messed up plenty

I went down a similar path with an automatic watering setup for my greenhouse. A moisture sensor stuck in a tomato pot, a cheap 12v solenoid valve off a garden hose line, and a Nano doing the timing. Ran it off an old phone charger at first and it browned out constantly untill I realized the valve was pulling more current than the board could give. Fed the valve from its own adapter after that and it's been quiet ever since, wich lines up with your motor advice completly.

The notebook thing hit home too. I lost a whole weekend last spring because I'd swapped two sensor wires months back and written nothing down. Now I tape a page inside the enclosure lid with a hand-drawn wiring map, ugly as it is.

One thing I'd add from my own pain: seal anything going outdoors. My first sensor probe died in six weeks from condensation creeping into the connector. A dab of silicone around the gland and the replacement has lasted two years. I also learned to check the soil readings at diferent depths, because the surface dries out long before the roots do and my early code was watering way too often. Still cheapest and most satisfying build I've done, and the tomatoes don't care that it cost less than a takeout dinner.

That bit about buying two of everythign saved me this winter. My coop door servo died during a cold snap in January, and having a spare in the drawer meant the birds were shut in by dark instead of me rigging up string and a pulley at ten at night. Now the spare lives in the same box as the notebook so I'm never hunting for one without the other.

Also started dating every note I write, stuff like 'Jan 12, moved the wire to pin 9, works, don't ask.' Turns out the 'don't ask' part mattered a year later when I had to undo it.

The light sensor route has one trap that took me a while to figure out, which is that it only takes one bright thing to fool it. My first version would open the door at two in the morning whenever a neighbor's floodlight hit the panel, and then close again an hour later once it went off, which the hens did not appreciate. I fixed it by requiring the reading to stay above the trigger level for a solid few minutes before the code commits to anything, and I moved the sensor so it faces the open sky rather than the yard. A small clock module would prolly be even more reliable, but I liked watching the seasons shift the schedule on their own.