[Alan Anderson]

The photosensor does not output a voltage. It uses its NPN transistor driver to either pull the output wire to ground or to leave it alone. You need to connect it to a pullup resistor in order to be able to measure anything with a voltmeter.

Fortunately, the DIO signal pins on the Digital Sidecar provide exactly the pullup resistor you need. If the photosensor output is connected to a DIO signal pin, you will read 5 volts when the output is inactive, and 0 volts when it is active. (The white wire will be active when an object is detected.)