The easiest way to determine how your photocell works is to connect a multimeter in resistance-measurement mode to the two leads and see how the resistance changes when shading the sensor with your hand, turning off lights, etc. Because the resistance changes a lot, an auto-ranging meter works well here. Otherwise, just make sure you try different ranges, between 1MΩ and 1KΩ before 'giving up'.This guide was first published on Jul 29, 2012. It was last updated on Oct 15, 2018. This page (Testing a Photocell) was last updated on May 04, 2015.