The only way you can use a relay or smart switch in a lighting circuit is to ensure that it can be wired directly to live and neutral as shown in figure 4. Simple switch in a 2 wire circuit.
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Why do smart switches need a neutral wire. It can be done but pulling wires through finished walls can be a real pain especially if you dont have the proper tools. Adding a neutral requires adding an additional conducting wire between the fixture and the switch. You could certainly hire an expert to get the job done but it will cost you. Theres a reason why almost every smart switch needs a neutral wire attached. This is why there arent any switches or relays that can work in a 2 way circuit without having a direct connection to neutral. It allows the circuit to be.
Here is a diagram of a standard switch with a neutral wire. Switch with a neutral wire. The switch still breaks the circuit but in this case the smart switch can tap into the neutral wire to keep power flowing through the switch the electronics in the switch still break the circuit to the light but the switch maintains a circuit so it is always receiving power. Thats what a neutral wire does in this case. A smart switch can simply tie into the white neutral wires to complete the circuit and get a source of power to keep the radios. In order so that the switch can be on to receive the rf signals it needs a return path for itself so the load isnt on all the time.
Smart switches need to stay powered up all the time which is why the neutral wire is critical. It needs to stay powered all the time. The neutral wire allows the completion of the circuit and the switch to have power even when its turned to the off position when you want the lights off.
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