Connect the GND of the battery to the GND of arduino – common GND. If you are using a 5V relay instead of 12V relay, you only have to connect the arduino GND. I am using a 12V relay. So the coil ...

It is a Clarke DCM10, a variant of Sieg’s X1, sometimes knows as a ‘micro mill’. Mine has a ~230V 150W brushed dc motor that needs driving from 240Vac mains power. Almost randomly, I settled on a ...