You could then run an opto isolator from that to tell you when the light is actually on .
An opto isolator was my first choice but from what I know you shouldn't put a high load (up to 5A) in series with it right? It wouldn't be able to handle the high current drawn through it right?
First off, here is your picture so people do not have to download it.
-Why do you want to control the lights?
Exactly as you pointed out, "people" never seem to understand that I want corridor&garden lights on and all other lights off. So I want to be able to monitor and switch them at specific hours according to rules that I will set up later.
-Did you realize that with this setup your light switches will operate in reverse? Eg. Up is on and down is off, then up is off and down is on.
Well with two way switches this is that case anyway (I don't mind).
-Do you realize that the relay will be on and drawing power for a considerable period of time?
For that purpose I will use a latching relay that draws power only when switching states.
-Do you want to know when the light is on or when the light is turned on. Eg. Detect the actual light or know when there is power going to the light?
I was considering just knowing when there is power going to the light since that way I could detect switching events in software anyway.
-Do the lights have to work as normal in the event of failure of the circuitboard?
Yes, the hole project should be "a fun little" addition to was already exist without causing any problems if it fails.
So, the goal is to alter the existing electric installation then. got it.
Well, yes, of course what I mean is I don't want to rewire the entire house
First off, you need to lay out how you can wire a DPDT switch to act like a 4-way switch. a dotted circle is not a schematic.
Sorry, was't at my PC at the time of writing.
This is what I had in mind. (I also have some schematics for the PCB I drew in Eagle but they are more of a visualization of my ideas that an actually useful schematic)
Second, as part of your altering the existing installation, you will need to install new hardware. The DPDT switch you mentioned, the Arduino, a separate power source will need to be run as what you show does not provide power.
Or you have to figure out how to use the power that is there, but that is switched.
Then, you need to make it safe, say, while you are altering the existing installation, you should install a large enough electrical box to house all of these bits AND in the US, by code, they have to be separated by a physical barrier.
All of that I have considered. I will need to run a neutral wire to the first switch of course.
Power Supply, Arduino, Relay etc.. This will all be installed next to the switch (or behind) that the Line goes to.
Once I am sure what the end project will look like (several prototypes later) this will be taken into consideration. I live in Greece so I'll have to research local regulations.
As for monitoring power, there are lots of ways. But since the place where you want to alter the existing installation is between switched loads, then you have to have two sensors -or- you have to use the other line.
I would replace one switch with a 4-way, manual switch and then put in your DPDT relay out side of the switched legs and your CT now only has to see one line.
That is actually a good idea, I'll have to look if I can find matching 4-way switches.
On the other hand, wouldn't it be possible to run both wires through the CT since by default only one of them will be carrying the current each time? Using the other line is also an option I have considered.
What you did not mention is if you have a 3-D printer and want to make it even more inconspicuous.
I have access to one, but as mentioned before, it all depends on how the final prototype is gonna look.