Current Sensor setup with existing home wiring

So I have a personal project idea, my kids and us adults tend to leave lights on from time to time, we already have motion sensor lights in key areas around the house(hall, kitchen, bathroom) but there are other areas where motion is not the solution as in the bedrooms and so on.

I picked up a few current sensors last week for something else, and was thinking of the following:
I wanted to setup some way to detect if the lights were left on in a room and (maybe be able to turn them off remotely with app or other device)

But for the most part(phase 1) I would just like a way to determine if someone left a light on somewhere on the other side of the house, as all room doors are kept close when no one is in them, unless someone goes to that side of the house or into the rooms, the light could potentially be left on for hours or days before it goes noticed.

Since this is an existing home (not new construction) would a current sensor be the way to go? Can something else be used? If the current sensor can be used, how can it be done safely in the existing wall fixture?

One of my coworkers argued that introducing another element inside the electrical box for the switch is a potential fire hazard, which I can see and understand, but if not placed inside the box, what are my options, I would like to incorporate the solution into some form of centralized fashion and be able to control from my phone or external device.

How much is lighting really costing you? If you are running CFL or LED lights its probably only pennies a day, hardly worth the trouble of rewiring.

If you want to do it in a standard way via fixed wring then X10 is the standard- search for "X10 smart switch".

Increasingly there are light bulbs (Phillips Hue and others) that have wifi controllers built in- no need to touch the house wiring at all. This may well be the easiest solution.

First off. Is the US other country (I know US parts)?

Assuming US, I would not put a current sensor in the switch box. Don't think you have enough room in there, and it is not a good idea mixing low voltage with line voltage in the same box. Besides, you would have to find a way to bring your signal to your monitor.

The best way of finding out if an single room light is on may be to use a photo sensor in the room. This way you stay clear of the line voltage as well. If you have attic access, you could mount it in the ceiling near the light fixture. That way you would get a higher reading for the lamp output and not worry about sunlight skewing the sensor.

I have a few photo sensors available, so measuring the light within the room would work, but lets say this scenario, during the day a few rooms tend to get alot of sunlight (due to no curtains on the windows) just blinds.. so light does come and go how can i account for sunlight vs actual room light?

I know i can build in some ranges for the sensor data provided by the photocell. But i dont want to kill the power to the room if its really the sun, seems like that would be a waste to do..

I recently posted in another area about monitoring my HVAC system, and this seems to tie into it in the way that there has to be a way to determine if current/power is being provided to a device/appliance.

Using a current transformer (CT) you can safely monitor the AC current to anything, anywhere you can access the wires leading to its location. You could, for example, monitor power to all rooms (circuits, at least) from the central breaker location. In many U.S. home installations, lighting to one or more rooms is on a separate circuit to wall receptacles.

Example of using a CT:

I just bought some of those clamp style sensors, will be in next week, but i currently have the AC712 current sensor which is what i was trying to use. Could i still use the same calculations and possibly the same example sketch in that link with that sensor i have?

The problem I have with the AC712 is the exposed wiring. These sensors require to cut the wire at some point. I suppose you could attach them to the back of a switch, where you have exposed screws. But then you are mixing line voltage with low voltage in the same box. And you have the problem of getting the signal wiring out.

Where do you plan on using these? In some junction box, or at the breaker panel? Don't be a bonehead and try cutting the wire at some point and splicing in the connector. I see plenty of that stuff in the houses I work on.

Anything i do will be at end points, no cutting into existing wires, just adapting my sensors to the existing wiring. All self contained with no room for anything making contact with each other..

Everything i have built with the arduino are self contained in their own project boxes and only key wiring are run out of the boxes to the destinations.