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]

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?

Yup,

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]

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?

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]

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?

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:

http://tinyurl.com/hvnt

 

[BigWallop]

on

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:

http://tinyurl.com/hvnt

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]

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 ?

Damn. You spotted the ruse

Andrew

Do you need a handyman service? Check out our
web site at http://www.handymac.co.uk

 

[Guest]

....

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 ?

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]

...

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

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

 

[John Rumm]

Thats quite neat. Not sure 4 diodes are good ebnough to light the LED
tho. Maybe a power Zenert?

That looks like it ought to put 13mA through the LED with a RMS forward
voltage of about 2.4V which should be plenty.

Must admit I did something similar for a "on charge" indicator in a
constant current battery charger I built a while ago. I found I could
actually get a decent light level out of the LED with only 3 series
silicon diodes (even though by "design" it should have needed more).

(although for this application I think I would just use a neon between
switched live and earth - or across the switch if you want the neon lit
when the switch is off)

 

[Dave Plowman]

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?

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.

 

[Dave Plowman]

(although for this application I think I would just use a neon between
switched live and earth - or across the switch if you want the neon lit
when the switch is off)

There are also mains LED indicators available for those who don't like
neons and can't be bothered with resistors and diodes.

 

[fred]

Thats quite neat. Not sure 4 diodes are good ebnough to light the LED
tho. Maybe a power Zenert?

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------------]<|-------------LAMP
ZENER
LED - red, low curr
ZENER - 3.3V 3W or greater
eg http://rswww.com code 447-2061 3.3V 5W zener 49p + vat but sadly min
order 5pcs.

Suitable for use up to 250W load.

 

[The Natural Philosopher]

That looks like it ought to put 13mA through the LED with a RMS forward
voltage of about 2.4V which should be plenty.

Mmm. If the LED is red, yes....

 

[BigWallop]

Mmm. If the LED is red, yes....

LOL !!!

 

[fred]

I doubt one neon will eat into your RCD leakage budget too much! ;-)

The only component I'll fit between live & earth is a Y-class cap :-!
</safety lectr>

 

[BigWallop]

The only component I'll fit between live & earth is a Y-class cap :-!
</safety lectr>

I Concur.

 

[Richard Savage]

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...,

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