Light switch warning indicator

This can't be done unless you have a neutral at the switch position. When the switch is on there is no potential difference between the terminals therefore no power source.

Dave, I was thinking just that after submitting the post! Could the earth be used instead of neutral for low currents? Is that a good idea?

Otherwise, I was considering a coin battery solution where the current flowing through a wire is sensed.

It could be battery or supercap powered.

Or, if the box is grounded, there will be line voltage between the load side and the box in a correctly wired normal (not 3-way) circuit.

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Might be difficult to fit all this inside a switch backing box?

I hope you're not recommending this? In the UK it would be forbidden - not least because it unbalances the line and neutral.

There's almost always a neutral in the switch box, occasionally I see an arrangement with power coming into the light box and a separate switch drop but it's fairly rare. Also it'd be possible to power the reminder with a battery in that case, it wouldn't have to draw much power. If the lamp is incandescent you could even use a rechargeable battery or memory backup capacitor to power it and "steal" power to charge the battery when the switch is off.

UK electrical boxes are incredibly tiny compared to those in the US/Canada. Our standard boxes are 2.5" deep but you can get them up to

Connecting between line and ground would not be legal by code, but if the circuit is designed carefully it would not be a danger either. I'd lean towards powering it with a battery personally, much safer for the novice.

Don't try to measure current through the wire, it's far easier to measure voltage across the switch. Zero voltage means the switch is on.

Right. In the UK the latter is the norm. Lighting circuits are radials and the feed goes from one ceiling lamp to another, with the switch drop from that lamp. Of course if you have floor outlets for table lamps etc you're more likely to find a neutral at the switch.

Yup - although it might be tight to fit all that within the switch back box. In the UK, some are so shallow there's just enough room for the cable.

I did not address the regulatory issues. But, I assumed he was talking about a few mA of current just to operate a flashing high brightness LED, the the line balance wouldn't be a practical issue unless it tripped the RCD/GFCI/whatever.

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Dave, Could you expand a bit more on what you mean by "unbalances the line and neutral." please?

Thanks.

In the UK, neutral is connected to earth at some point - the last possible one being at the incoming main. After that, they are separate. Use the earth cable in the house as a return and you are now not drawing equal current on line and neutral. And RCDs work by looking for a difference in the current flow between line and neutral, so you don't want to impose a load which does just that. Of course, if it's only a couple of milliamps it's unlikely to matter, but in principle it's a bad idea.

We currently do not have RCDs installed at the fuse box - they are plain wire fuses.

James Sweet mentioned in an earlier post, using a rechargeable battery or memory backup capacitor to power it and "steal" power to charge the battery

when the switch is off.

How would this work with no neutral? I am assuming a large wattage resistor would be required to drop the voltage to a suitable level and using the earth as a neutral? However, I don't think there will be enough space behind the switch plate for the resistor?

If you just need some kind of indicator to show whether a remote light is on or off, you can wire a neon lamp (with resistor) across the switch. In the UK you can buy this ready made as a backplate for a standard single light switch plate. It`s so you can find the switch in the dark. When the neon is on, the light is off and vice versa. It`s not going to beep of course.

Ron

Hi,

minutes when a light switch is turned on.

transformer cannot be used.

On Thu, 29 Dec 2005 09:02:49 +0000 (GMT), "Dave Plowman (News)" put finger to keyboard and composed:

What about using a dimmer switch set to about 90% duty cycle? The circuit could grab some power during the 10% off time.

- Franc Zabkar

This is not being built for a blind person - This light switch often gets left switched on all the time as it controls a light outside.

It already has a neon light on it which seems to dim when the light is switched on?

If I build the circuitry into a separate box next to the light switch, is there an easy way to sense when the switch is on/off?

In the UK lighting switches are usually single pole, so if that's also the case in the US change to a double pole one and use one pole for the sounder.

why not use one of the movement sensor type lights

On Tue, 3 Jan 2006 12:27:55 +0000, M.Joshi put finger to keyboard and composed:

The neon is probably wired across the switch contacts. When the switch is off the neon sees the full mains potential. When the switch is closed the potential is zero.

Here's my suggestion:

- Franc Zabkar