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Eddy Current:
First, you need to understand how devices work that you use.
That means you should know electrical circuit theory, how basic components work, take complex devices apart and study what is inside, and run experiments.
Consider a simple electromagnetic relay. It is a binary device. This might have a NO and a NC contact, and a coil. It takes no input power to be in one state, and a certain amount of power maintained to be in the other state. There are some relays that are bistable latching, and only require transition power to change state. The GE RR-7 is bi-stable latching, and has a set coil and a reset coil.
There are solid-state relays that require much less control power to change state than an electromechanical relay of the same current rating.
If you have a two terminal device how are you going to get power and maintain the power to operate the relay?
Take your photocell apart and trace the circuit. Run bench test experiments on the control, and see how it works.
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