This is an automatic plant watering system. The frequency of watering can be adjusted to twice a week, weekly, biweekly, three times a week, or off. There is an LED to give user feedback while adjusting the settings.
The Arduino code is available here as plant.ino. Here is the circuit diagram:
The chip used was an ATtiny85 with an Arduino bootloader. The chip and other circuit components were soldered onto a Dot PCB (perfboard). Here is a picture of the final result, connected to a plant:
See harysdalvi.com/projects/wabisabi for more details.