ESP32 Deep-Sleep Wake-on-Touch

ESP32 sleeps at ~10 µA, wakes when you touch a wire, blinks an LED, sleeps again. Battery lasts months.

Difficulty: Breadboard. Estimated build time: about 30 minutes. Estimated parts cost: about US$9.50. 4-part bill of materials with live supplier pricing. A coin-cell-friendly proximity touch sensor.