Soil moisture-sensing by hacking a solar light
The good news is that this IC works as is, even with our modification, so it requires no reprogramming whatsoever. That is also the bad news, since you cannot change the threshold at which the LED will illuminate. To change the threshold, you can move your probes closer together or further away. The closer they are, they better they will be at sensing small amounts of water, so the LED will only turn on when the soil is very very dry. The further apart the probes are, the more water that is needed to turn off the LED. Play around with your soil type, and what levels you would like your LED to turn on and off, and decide from there the spacing and possibly what type of probe you would like to use.
The only other thing I wanted this circuit to do was to always have the solar panel charging the battery. To accomplish that, wire the solar panel in parallel with the battery by soldering the solar panel wire that was attached to pin 3 to the ground side of the battery. This makes the final circuit look like this:
Usually for a more complex circuit, or a different battery such as a Lithum-ion battery, you would not want to hook up the power source directly to charge the battery like this, since overcharging is an issue. Here that issue is small, and even if the battery was overcharge it shouldn't explode or anything, just decrease the lifetime of the battery. So it's not very good practice to wire the solar panel to the battery like this, but for our purposes here it works.