Photocells and the like are "electronic circuits" which need to be powered to function.
And if there are any "diodes" (electronic component) in these electronic circuits, these would need a certain "voltage" (7/10 volt) and a certain "polarity" of voltage to get a reading on a testing device. Some testing devices do not provide this high of a voltage through the test wires.
In English: Just follow directions, hook it up, and see if it works!





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