555 Plant Moisture Tweeter

Stick two nails into your potted plant and it chirps when it's thirsty. No MCU, no app — just a 555, a soil probe, and a piezo.

Difficulty: Breadboard. Estimated build time: about 20 minutes. Estimated parts cost: about US$2.40. 9-part bill of materials with live supplier pricing. A passive plant-care alarm that audibly chirps when your soil drops below a moisture threshold.