A low-cost Geiger counter built around the Soviet SI3BG (СИ-3БГ) Geiger-Müller tube. Powered by three AA batteries, it displays CPM and µSv/h readings while reproducing the classic radiation click through a piezo buzzer.

A low-power ATTiny1614-based capacitive soil moisture sensor that chirps like a cricket when your plants need water. Runs for up to two years on two AAA batteries and requires no app, Wi-Fi, or cloud service.