@bjornhallberg I'm new to the forum and just got my Arduino today (still waiting for the other parts to arrive for my project), so I can't yet explain how to write any code, however I understand the following:
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1 - The design of your circuit has little to do with the sensor you use to operate them, thus you can decide on which LEDs to buy based on power consumption only (and maybe the capability to dim with burning).
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2.- Then you can build your circuitry and plug it in normally to the power source.
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3.- After you know the circuit works properly, THEN you bring the sensors.
Basically you would use these sensors to control when the lights turn on/off, and that can be done in a variety of ways and with an array of different sensors.
In my case, for example, I'm trying to open/close a circuit based on proximity, pretty much what you want, but I plan on doing so using a bluetooth module for the Arduino that pings my phone to see if its around, given the phone is found, the Arduino would emit voltage in a pin and that should (I still have no idea, but somehow) open the circuit and stop it from functioning, given the phone is out of reach, the voltage is stopped, and the circuit goes back to operating normally.
In your case I don't think this would be a good approach because you want it to work whenever someone comes close, so maybe PIR sensors, or pressure plates, or IR sensor in the walkway or any other sensor you can use to identify someone's around.
I would recommend you go and complete steps 1 and 2 and then come back with an idea of how you want to interact with your new circuit and the folks here will help you out on which sensors are better suited for you.