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LED mark space

This topic has 16 replies, 9 voices, and was last updated 29 October 2015 at 12:38 by duncan webster 1.

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28 October 2015 at 17:14 #31807
duncan webster 1
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@duncanwebster1
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28 October 2015 at 17:14 #209616
duncan webster 1
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@duncanwebster1
A little bit off metal bashing, but of relevance to our signalling system. I've got all the gremilns out (famous last words), the usual cause of failure is now flat battery (no-one seems to remember to charge them up). I've read somewhere many years ago that turning an LED on and off very rapidly reduces the power cosumption, but hardly reduces its apparent brightness. Tried google to no avail. Anyone got any info on best mark space and best frequency?
28 October 2015 at 17:44 #209620
Bazyle
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@bazyle
I think you might try 1/20th of a second which is a figure I remember for persistence of vision,
28 October 2015 at 17:51 #209621
Neil Wyatt
Moderator
@neilwyatt
I always believed this but some googling suggests the effect is debatable and marginal, needing an extreme brightness for the flashes. a 5% duty cycle seems to be indicated and would still need at least half the normal power (roughly. 10x the current for 1/20 the time) to attain similar brightness, so it hardly seems worth the effort.

60Hz to avoid flicker when seen by the rods of the eye, but 20-20 should be OK.

What about a small wind/solar generator instead?

Neil

Edited By Neil Wyatt on 28/10/2015 17:52:54
28 October 2015 at 18:12 #209623
john swift 1
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@johnswift1
I expect the information you want will be on the data sheet for the LED's your using

for example , one white LED I looked at –

Average Forward Current = 30 mA

Peak Forward Current = 100 mA （Duty＝0.1，1kHz）

the Joule thief may give you ideas –

**LINK**

**LINK**

John
28 October 2015 at 18:34 #209625
SillyOldDuffer
Moderator
@sillyoldduffer
This is a link to an application note that explains how better efficiency can be got from a LED.

But I think your real problem is not how to make the battery last longer, it is how to remind people that it needs recharging. How about fitting a low battery alarm ? They aren't difficult to make if you can't buy a ready made module.

Cheers,

Dave
28 October 2015 at 18:51 #209631
duncan webster 1
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@duncanwebster1
I'm on with a low battery alarm, but I confidently expect the drivers to ignore it, after all they ignore the fact that the green LED has died without mentioning it (the green LED array seems to give up the ghost first). We're also looking into a solar panel. SillyOldDuffer's link looks just the ticket, thanks. I'll post the rsukts, but don't hold your breath, need to wait till running season is over before undertaking any major rebuild of the unit.
28 October 2015 at 20:06 #209638
jason udall
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@jasonudall57142
I believe that Sir Clive Sinclair..made much of his startup cash and reputation from the inovation of doing this very thing to led watches and pocket calculators (’70’s)……
The reasoning is something like 5x current for 1/20th time…if rate high enough the eye sees this as infact brighter…
One gotcha. ….if moving then flashes will appear as morse….
28 October 2015 at 20:07 #209639
jason udall
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@jasonudall57142
Boring morse… but morse…
28 October 2015 at 20:16 #209643
jason udall
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@jasonudall57142
Btw…green led need more volts per diode than red…thus given same number of diodes.(series) you will need more charge in battery…
28 October 2015 at 20:23 #209647
Neil Wyatt
Moderator
@neilwyatt
The other route is just to make them dimmer. The human eye follows a log law for sensitivity, so that for a given drop in current the drop in brightness will be perceived as rather smaller. Or use smaller lights to increase the 'surface brightness' and therefore the contrast with the background.

Neil
28 October 2015 at 21:00 #209650
Michael Gilligan
Participant
@michaelgilligan61133
Posted by Neil Wyatt on 28/10/2015 20:23:34:

The other route is just to make them dimmer.

.

Sorry, Neil … I think that is almost exactly the wrong approach.

The OnSemi Application Note, linked by Dave [SOD], explains, very concisely, why pulsing the LED is so efficient:

[quote]

A pulsating LED drive circuit can enhance the light output of an LED by using a peak current of a much higher level than sustainable under direct drive conditions [1][2]. A high peak current pulse of short duration with a “OFF” period between pulses allows time for the LED’s junction to cool down. High drive currents can result in a degradation of the light output and the life expectancy (time to half light output) of an LED. However, the reduction in the life of a pulsed LED is minimal if the peak current is below the maximum current limit specified for the device.

[/quote]

The beauty is that, the 'slight drop in perceived brightness' can be achieved with much lower energy consumption by pulsing than by simple voltage dimming.

MichaelG.
28 October 2015 at 21:59 #209654
Neil Wyatt
Moderator
@neilwyatt
I didn't specify the means by which the dimming is achieved.

Neil
28 October 2015 at 22:19 #209658
Michael Gilligan
Participant
@michaelgilligan61133
Posted by Neil Wyatt on 28/10/2015 21:59:05:

I didn't specify the means by which the dimming is achieved.

.

No you didn't … but you did mention 'the other route'

MichaelG.
29 October 2015 at 09:05 #209679
Martin Kyte
Participant
@martinkyte99762
I make a lot of opto-genetics Led systems for the Lab and I use PWM for dimming control. I tend to go for base frequencies of around 5 to 10kHz. I generally use National Semiconductor switching regulator chips of which the LM3404 is probably the simplest and use a frequency of around 500kHz for the switching frequency of the regulator.

These little switching regulators have a good efficiency figure of 80-90%.

What is more to the point with your signals – You should also have a look at the Led emitters you are using so you get a high light ouput to current ratio and if this is a signalling system ensuring that directional emitters are employed. There is no point in genearating lots of photons most of which end up no where near the eye. Lots of emitter beam shapes to choose from and plenty of lenses too. I presume you only need the signal to be in the approach direction of the track so you only need a narrow beam angle. This makes the LEDs brighter and so you can reduce the current.

regards Martin
29 October 2015 at 09:40 #209690
Ian S C
Participant
@iansc
One thing to watch for in switched LEDs is radio frequency interference (RFI), the LED spot light I have in my workshop has a major problem which I cured by running it through an isolating transformer, RFI on the power cable, it's a mains powered unit.

Ian S C
29 October 2015 at 12:38 #209717
duncan webster 1
Participant
@duncanwebster1
I'm always amazed by the knowledge of people using this site, and how helpful they are. I've found an article (ME 2009) about using a lens to make the apparent diameter of the light bigger, and to make it more directional, so I'm going to experiment with this and pulsing. As the whole thing is driven by a microprocessor pulsing is software mods, so quick and easy. The built in routine for pwm runs at <1kHz so I'm not anticipating RFI. As an amusing aside I once worked on a plant which had a high power RF heating system. We were asked by 'the management' to prove that we weren't interfering with everyone's radio, so I consulted a chap who had worked for the BBC for advice. He advised me to tune a transistor radio to Radio Blackpool (the nearest station to our operating frequency) and wander round our site and the local area to see if there was a problem. No fancy tests or meters, I like solutions like that. I'll let you all know how it goes, but once again thanks to all who have replied. At least I now know why the green gives up first.

By the way Neil, I can't make the present red LEDs dimmer as they have a bult in switch mode power supply, if you reduce the input volts the current goes up (steeply) and the brightness stays the same until you reach a very low voltage, when it just gives up the ghost. I'm not planning on using these in te new manifestation!

Edited By duncan webster on 29/10/2015 12:38:56
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