Power an LED from a light switch?

[Pete]

Is it possible to power an LED from a light switch (live only connections, and possible earth) so an illuminated LED can indicate if the light is on or off?

[fred]

In article , Pete writes

fixed width font

LED ----RES180R---|>|---- | | SWITCHED-FEED----|>|--|>|--|>|--|>|------LAMP | | ---------||-- = DIODE

A bit heavy on component count but lights evenly independent of lamp load, choose diodes to match full lamp current. LED is low current (5mA).

All parts are live, including the LED. For safety don't poke the led thro the panel, use a plastic holder.

Extra components would be required to meet BS/VDE safety.

[The Natural Philosopher]

Yes. Its horrendously inefficinet tho. You need a diode in addition to the LED wired in series, and a series resistor. about 22Kohms will do and at LEAST 2W rated. It will get hot.

Alternatively a capacitor can be used, but tis late and I can't be bothered to calculate teh value...,

[BigWallop]

Why not use a neon lamp like the ones used in cooker units ? These don't have to be wired to anything fancy like diodes or heavy resistors. They are easily obtained and very cheap, and can be wired to live and earth if your not using an RCD breaker. It can be done remotely from one room to another, which I do quite regularly to indicate when water heater thermostats are switching on and off.

Like these ones:

[BigWallop]

connections,

Just thinking out loud here, but wouldn't the big GLS lamp in the middle of the ceiling, yes, the thing inside that light shade, tell you if the switch is on or off ?

[Andrew McKay]

Damn. You spotted the ruse ;)

Andrew

Do you need a handyman service? Check out our web site at

[<nightjar>]

Perhaps it is to tell him whether the switch is off before he takes the blown GLS lamp out to change it?

I've replaced all the filament lamps in the indicators on the machines in my factories with mains rated LED 'lamps', supplied with the same fittings. They are a lot dearer, but they are also far more reliable and the savings on maintenance (most take a disproportionate amount of time to change) covers the extra cost.

Colin Bignell

[RichardS]

Possibly an indicator for a loft, attic, cellar or cupboard light....

Or perhaps it's to stop the electricity usage when the (unobserved) light is both off and on simultaneously (junk science meets Schroedinger)

cheers Richard

-- Richard Sampson

email me at richard at olifant d-ot co do-t uk

[Dave Plowman]

Do you want the indicator to be on when the lamp is on, or would on when it's off be OK?

I think it's the former you want since you mention an earth. What the regs say about using the earth as a return for the tiny amount of current required, I'm not sure, as plenty of electronic equipment uses this method for RFI suppression.

[fred]

In article , The Natural Philosopher writes

I should have said that a low current red led was intended, which will only drop about 1.5V, should give about 5mA pk.

Power zener good & removes the need for the reverse diode, but I was thinking of cheap & easily available comps - none at maplin under 5.1V.

. . . but RS do a 5W at 3.3V, so here it is for that option: Fixed width font LED --RES330R---|>|---- | | SWITCHED-FEED------------]

[Richard Savage]

This sounds similar to a concept I used for a fermentation cupboard thermostat. IIRC it employed the reactance of a polystyrence capacitor placed in series with an electrolytic capacitor across live and neutral, followed by a diode from their junction with another electrolytic from diode to neutral. I think it supplied about 12 Vdc to the temperature controlling chip. Does any of that sound fesible

- and yes it is late here!

Richard