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Dimming flickering LED candles PWM dimming and the like

#1 User is offline   cedd 

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Posted 06 November 2012 - 01:55 PM

Not sure if anybody's tried this, but it's worth an ask!

An upcoming show makes use of fake LED tealight candles. Having opened them up it's literally a button cell battery, a switch and an LED. The flickering effect therefore must be done within the LED.

Here's the complication. The director would also like some of these dotted around the set. Perhaps 30 or so of them. The idea being that the cast walk on holding tea lights for the last scene. We slowly fade up the fixed tea lights on the set, then the cast leave one by one, therefore transitioning to the set being lit by the fixed tea lights only. Then at the right moment we can dim the lot of them symbolically and all together.

Hacking the tea lights isn't a problem. I plan on buying a reel of thin 2 core speaker cable (car stereo stuff) and a load of telecom crimps. I'll solder to the terminals in the battery compartment of the tea light and leave them with a trailing lead that I can crimp in to the chain. They run on 3v so I'll be wiring them in series until I get somewhere near 12v, then I'll wire them in parallel with other 12v strings - all pretty straightforward.

But I then need to dim the things. So far I've been looking at this unit from Milford. It's a PWM LED dimmer. My concern is that as the LED's are flickering ones, the load isn't going to be constant and I don't know how they'll behave with a chopped supply. I don't want them all to suddenly synchronise their flickering, or for the varying load to be too much for the controller.
That controller is a 4 channel unit, though I only want to dim all the LED's at once. Looking at the spec from a similar flicker LED (no idea on the model of the tea light ones) here and some really simple guesswork I can reckon on about an amp if I use 30 of the things. Well within the capabilities of that controller on one channel.

What do people think? Anybody done it before?
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#2 User is offline   Richard P-W 

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Posted 06 November 2012 - 02:01 PM

In my younger days I remember wiring a flashing LED in series with several standard LEDs and they all flashed in unison...

#3 User is offline   timsabre 

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Posted 06 November 2012 - 02:15 PM

View PostRichard P-W, on 06 November 2012 - 02:01 PM, said:

In my younger days I remember wiring a flashing LED in series with several standard LEDs and they all flashed in unison...


Indeed wiring in series will not work as the forward voltage will change as the LED flickers, which will present a wildly varying voltage to them. And PWM dimming is unlikely to work with the flicker effect.

If you are dimming them anyway why not just get a few 8-way transistor output boards, fit standard LED's and flicker them yourself.
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#4 User is offline   adam2 

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Posted 06 November 2012 - 02:34 PM

I agree that connecting the flickering LED candles in series is unlikely to work well, or at all, as the current varies from moment to moment.

What MIGHT work, is to connect say 12 units in paralel, and then connect several such groups in series.
Having a relatively large number in parralel gives a fairly steady current, as the variations from each unit unit tend to cancel out.
I have done this with flickering LED Christmas lights with partial succes.

For dimming I think that you will need a variable voltage supply, and not PWM. These LED candles get steadily dimmer as the batteries fail, so an external supply that emulates the running down of the battery should be fine.

It might however be simpler to connect them all in paralel, from a DC supply of about 3 volts.
At an average current of perhaps 10ma each, dozens of lights would be less than one amp in total.

Buy or make a very basic 3 volt DC transformer/rectifier power supply to power the tea lights. This may be connected to a standard dimmer provided that a load lamp is used as well.
You may need to connect a resistor in series with each tea light if useing an external power supply, some types rely on the significant internal resistance of a a small coin cell in order to limit the current.

#5 User is offline   cedd 

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Posted 06 November 2012 - 03:26 PM

Starting to feel quite glad I haven't agreed to do this yet! We're currently at the "I'll see what I can come up with" stage. The show goes up next tuesday, and they're starting plotting tonight, so it's a tight timescale (find me a project I'm working on at the minute that isn't!).

I've been doing some reading up on the internal workings of flickering LED's since I made the first post and can now fully see the issue in wiring them in series. That complicates things slightly as the wiring suddenly goes from snipping in to one core of the 2 core, metering it to find +ve and crimping the candle in (pyro style), to parallel, so there are now 3 wires in play for each pole. Makes my very quick and easy crimp method suddenly be a bit more tricky (not much I know, but when you've got 30 of the things to do, it soon adds up!).

I can't really commit 50 of show budget to the PWM dimmer to play with without knowing it'll work.

I wonder instead if I could go for a relay board and maybe have 8 circuits of led's, possibly parallel wired and fed with 3v. That'd allow me to do a phased switch off, so insead of all the candles dimming together, they'd "blow out" over a second or two in 8 groups.
Lot more wiring involved, but I suspect it'll be an overall far simpler project.

Or I go back to him and say "should have come to me a month ago, sorry!".

Thanks for the replies folks.
C
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#6 User is offline   the kid 

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Posted 06 November 2012 - 03:58 PM

Ah, Maybe answering a question I am been looking at for a while. Do flicker LED's work on the current or the volts ? if I have a straight battery > led it will be fast but throw in a resistor of x value it will change ?
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#7 User is offline   bigclive 

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Posted 06 November 2012 - 08:17 PM

Can this be cheated? The cast have ordinary flickering candles and cup their hands around them as they turn them on, so they don't suddenly turn on, but are revealed as needed. When the cast place them on the set they actually turn them off, but a hidden light provides the effect that it's still lit. On cue all the hidden lights go out (along with any extra lights used to make the set glow) and the place plunges into darkness.

The flickering LEDs do indeed use a chip inside the LED. In the earliest ones it was a greetings card music chip! They are best operated with a resistor in series.

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