A simple pcb on which you can connect 4 sensors to the 4 gpio pins, also it has a voltage stabilizer which can take up to 30V (so you can safely power it up from 5,12,24V source)

A 40 kV 10 mA voltage source designed to power a Farnsworth-Hirsch fusor

Robocupの回路

A drinks dispensing flow meter control board for Beer, Wines and Spirits

Arduino R3 w/ MKR w/ ESP-12F

esp-01 extension board, with 4 mosfet transistors independently controlled, also has 1 power led and 4 status leds, and 3.3v stabilizations (can be powered up to 30V)