For that reason, [Hulk] designed a simple relay module in order to cut the amount ... [Hulk] was able to design a PCB with 12 modules on it, presumably saving fabrication costs.

To support loads of widely varying sizes, the PCB supports an optoisolator that switches up to 1 A, a MOSFET that switches up to 2 A, and an on-board relay or SSR that can switch up to 3 A.

The digital signal output to be detected (usually marked as DO in the module) is applied to D2 input of the Arduino. Analog output (marked as AO) from the module is not used here. Sensor-head of the ...