Illuminated switch to show that cloakroom light and fan is still on

[NT]

Thats another downside of the circuit, you cant smooth the power with a little reservoir capacitor.

NT

[John Rumm]

Tis why I mentioned that one - its not as light as a light switch - but nowhere near as heavy and clunky as some of the DP switches.

The problem being the typical lack of neutral - hence nowhere to put it!

One solution you could try:

R1 LED1 -\/\/\--|>^--- | | | D1 D2 D3 | / Live in ------|>--|>--|>------o o----------> to lamp | | sw1 | D4 | -----

[PeterC]

um, how about a switch with very obvious on and off appearance and just look at it.

[Mike Barnes]

How about a pair of LEDs, reversed and paralleled, and mounted very close to each other (perhaps behind the same diffuser)?

And there must be a reason why you can't connect the diode pair in series with the load, otherwise it would have been suggested by now. What's the reason?

[dennis]

It will flicker wont it.

[Dave Liquorice]

[NT]

You can put a diode in series with the LED. But... you then need a leakage resistor across the LED, you get bad flicker, and you can only use an R dropper, not a CR.

NT

[NT]

filament lamps eat around 8x rated current at switch on, but diodes dont tolerate these overloads. So your real life lamp limit for a 5v

5w 1A zener will be 1/8A apx, or around 30w.

NT

[John Rumm]

I had thought of using a zenner but as you say there are difficulties finding suitable devices, and bog standard rectifier diodes are dirt cheap and robust. I used a version of this circuit on a battery charger I built for an "on charger" light. It was a wide ranging charger that might be dealing with anything from a single 1.2V cell up to about 30V and currents from 9mA through to over an amp. The circuit above (without D4 since this was DC) works flawlessly. Three diodes seemed to get ample voltage to light the LED even if it was a bit below the 2.5V spec.

[John Rumm]

That might solve the flicker issue - but TBH, 50Hz flicker is not going to be an issue for this application.

What a pair of LEDs? The have a max forward voltage of *significantly* less than mains, and a max power handling down in the mW range.

[Phil Addison]

In the end I opted for simplicity and used the Maplin panel mount red neon (BK52) which cost 99p, and took the return to earth; it works fine.

Here's the details...

1 Solder flex tail to neon, insulate with heat-shrink, and fit in 6.5mm hole I soldered the fine wires to the switched load and earth of the mains cable to guarantee a lasting connection Reconnect mains cable to switch and the earth contact of back-box The re-fitted switch with its new red neon Success!! for all the input and neat LED ideas, but its hard to beat the neon for simplicity, cost, very low current draw, no visible flicker (100Hz), all round visibility, and ready availability in panel mount.

Phil

[Mike Barnes]

People do complain about 50Hz flicker. Not me, but some people.

I don't understand the relevance of those parameters to the application. I see a single-LED series circuit being discussed elsewhere in this thread, which suggests that it's not impossible.

[Andrew Gabriel]

Replace the switch with an occupancy sensor (mains PIR) inside the room, or just leave the switch on all the time and add the occupancy sensor next to the light, or replace with a light fitting with a built-in occupancy sensor.

[Dave Plowman (News)]

I suppose it depends on the individual but I didn't notice it.

[Dave Liquorice]

Certainly so, got a bit of kit with a blindingly bright blue LED on the front. It lit the room up at night! Hacked into the wiring to add a series resistor, the thing would glow with the leakage through my dry fingers from the 3v supply. IIRC it now taking 60uA and is still too bright...

[John Rumm]

I am one of them - especially on TVs and monitors etc, but we are talking about a power on light here! ;-)

Only that if you don't take care in their use and treat them like ordinary diodes, they will sell destruct rather rapidly.

[NT]

,

Light bulbs and tv screens behave differently wrt flicker though. TV screens simply move the position of the light output at 50Hz, whereas with a bulb the whole lot goes on and off together at 50Hz, causing a much worse effect.

I just dont see any advantage in a 50Hz flickering circuit. It eats twice as much power for the same light output, it flickers.

NT

[Dave Plowman (News)]

For a warning light might be a good thing - draws your attention to it?

I use an LM 317 for this which flashes at 4 times a second.

[dennis]

I don't see why he doesn't just use a battery powered flashing LED like I suggested ages ago. All he needs is a double pole switch, one pole switches the light the other the LED. The batteries will last for months/years as the LED on flashes when the switch is on. No fancy components required. The 20A grid switches I posted a link to have more than enough insulation to do this.

[Dave Plowman (News)]

There wouldn't be room for even a PP3 inside most back boxes. Certainly not a plaster depth one.

But Phil has already solved his problem.