Monitoring house light status

[wildyoungdirgy]

How can one monitor the output of a house light switch (apart from looking at the lightbulb)?

I need to electrically monitor whether lights have been switched on.
I also need to switch them on remotely.
The remote bit is easy ...pair of isolated relay contacts across the house lightswitch.
The monitoring bit is more of a problem....

There's no Neutral in a house light switch box so I can't get a path from the switch output to Neutral for say a relay coil or optoisolator.
If I use the mains power neutral in my control box then I could get the return path to neutral here but them I am (borrowing) sharing a Neutral.
i.e. have a path between the lighting Live and a power circuit Neutral which I believe is illegal?

Any ideas.....Thanks
Dirgy

 

[paulh53]

Why do you want to do this

 

[mapj1]

Do you want to know if the light is actually lit, or just that power is reaching the holder (the bulb may have failed.)
in the first case a photocell seems to be the answer,
In the second, yes if you monitor by conection you would need a 'borrowed neutral' type of circuit but you can limit the current drawn to a low enough level that there is no safety implication (by monitoring through a series 100K resistor and a neon lamp for example.)
However, by using a monitoring circuit across the switch (such as an opto-coupler with a high value series resistance) you would find the opto coupler was 'off' when the switch was 'on' but you would have the total isolation you might desire. (although you would have a trickle current of a few mA through the lamps at all times, not enough to light them, but if the bulb was removed, you should treat the holder as live - though I'm sure most people dont stick their fingers in when taking the bulb out anyway.)
If this makes no sense, come back for more.
I too am puzzled - what is the application?
M.

 

[breezer]

light dependant resistor or photo cell would be better since it would not be connectected to the lamp, and it could be used to say if the lamp is Actually on by the light given from the lamp, as oposed to saying yes there is a supply at the lamp

 

[plugwash]

you can always get a double pole switch and use one pole to switch the light and the other for your control signal.

 

[felix]

Have you considered current transformers. To make one, all you need is a ferrite ring and some coil winding wire from an electronics store. Come to think of it, you might even get a ready wound coil on a ferrite ring. You put either one of the wires in your switch through the ring (not both) and that's it.

This will give you total electrical isolation and will even tell you that you have current, not just voltage. Of course you'll need a bit of electronics at the other end to detect the induced voltage in the coil but, since you're into relays and things, you probably know how to do that.

Actually, your electronics should be looking for an induced CURRENT in the secondary because a current transformer should feed into a low impedance. This minimizes the voltage loss at the primary.