Just saw your post regarding Photocells, and would like to add a little more info.
Simple models - such as the common directional types with the ½" threaded base, 3 wires (normally Black, Red and White), and rated 1.0 - 1.5 KW @ 120 VAC - work simply as a 1 pole contactor, with a light level sensor which turns on or off the contactor.
The Photo Electric Cell portion may be a Photo Resistor, or a Photo Diode. When Light falls on it (a significate level of Light), it - the Photo element - will either block the flow of current, allow a high flow of current, or create a flow of current from its self.
If the element blocks the flow of current, then the contactor will have N.O. contacts between Line input and Load output.
In this case, the contactor will remain open until ambient light levels fall enough to allow current to flow through the Photo element and energize the contactor - making the contacts close.
If the element allows a large current to flow under daylight conditions, then the Contactor will have N.C. contacts between Line and Load.
During times when daylight is abundant, the contactor is energized with the contacts held open - due to the N.C. arrangement.
Once ambient light levels drop low enough, the contactor is de-energized (due to no current flowing through the Photo element and through the coil circuit of the contactor), so the contacts close - resulting in a complete circuit between Line input and Load output.
With the case of an element "creating its own power", the element drives an amplifying device - either a small relay or a Transistor, then this amplifying circuit controls the large load carrying contactor.
For the common type PCs, here are the functions per wires:
[*] Black Wire: Line input for both the lighting controlled by the photocell and to the contactor within the device,
[*] Red Wire: Load output to whatever loads are controlled by the device,
[*] White Wire: End of the device's internal contactor's coil - carries very little current.
The Photocells used on 208/240VAC work the same way - only the "White Wire" is not normally connected to the system's grounded conductor - but instead is connected to one of the opposing circuit (the "Other" Ungrounded Conductor of the 208/240VAC circuit).
I thought we had schematics of Photo Cells posted here, but it looks like none have made it in!
Might be something to add.
Sure hope this message is helpful.
I'll double check it tomorrow after getting some sleep, and edit where needed!