Arduino Forum

Using Arduino => LEDs and Multiplexing => Topic started by: John_Smith on Dec 05, 2012, 10:02 pm

Title: Full brightness multiplexing.
Post by: John_Smith on Dec 05, 2012, 10:02 pm
All my projects till now have used latched segments of 7 segment displays.  ( or occaisionally 5 x 7 text character latched displays )

I run the LEDs at 18mA DC for maximum brightness when in direct sunlight, and maximum LED reliability.

I now need to make a display with 16 letters and 8 numbers, so I have decided I might as well go for 8x5 dot characters, but if I latch them my usually way, I will need 120  TPIC6B595s  !

So now I am thinking multiplexing ( which I have avoided to avoid RF interference, which doesnt apply now as I dim the displays with pwm anyway )

I have done some searches and found there are some conflicting ideas out there about duty cycle and brightness.

I know that if I use 50% duty cycle drive with my latched system, the brightness appears half as bright.

Some say pulsing the LED at 50mA for 10% will give the same apparent output as a steady 25mA, the only way I can see this happening is if the light output is 5 times brighter at 50 mA, but the graphs dont show this.

Can anyone suggest the best way to get maximum brightness from the LEDs in a matrix format ?
Title: Re: Full brightness multiplexing.
Post by: Hippynerd on Dec 08, 2012, 06:09 pm
I dont expect you will get any answers (any useful ones at least), have you done  any testing? Maybe you could make a test rig, with several LEDs, and run them each with different duty cycles and visually compare them.

Even if you had a bunch of fancy equipment to measure everything, what will/wont work will depend on how it looks, so setting stuff up and testing it seems the most appropriate.
Title: Re: Full brightness multiplexing.
Post by: dhenry on Dec 08, 2012, 06:21 pm
Two things and they contradict each other:

1) after a certain point, high current barely increases brightness: most small leds light up at 5ma, almost as much as they at at 20ma.
2) the only other thing you can do to increase brightness in a multiplexing is to increase current. And led rated at 20ma will typically take multiples of that current, happily.
Title: Re: Full brightness multiplexing.
Post by: John_Smith on Dec 08, 2012, 06:56 pm
I have experimented quite a bit with pwm dimming, and with current through the LEDs.

I generally limit the current to 18 mA, even though the LEDs I use are rated at 25mA, I find theres not much difference in brightness, and  I have had less than 10 LEDs fail in the last 10,000 LEDs I have used.

I have decided to use latched TPIC6B595s to drive th LEDs, as I want them as bright as possible in full sunlight.  I have designed a single sided board today, which was not as complicated as I feared.
Title: Re: Full brightness multiplexing.
Post by: cmagagna on Dec 08, 2012, 07:02 pm
Quote
Can anyone suggest the best way to get maximum brightness from the LEDs in a matrix format ?


I think what you're looking for is "peak forward current". Most of the time you can pump a lot more current through an LED if your pulse is short enough (typically 0.1ms), this can counteract the dimming you see.

You'll have to look at the LED spec sheet to find the maximum peak forward current because it varies; for example you can drive these at up to 175 mA:

http://www.us.kingbright.com/images/catalog/SPEC/APTR3216SYCK.pdf

but these you can only push up to 60 mA:

http://www.ebay.com/itm/3000-yellow-green-super-bright-1206-smd-led-light-bulbs-car-SMT-Brightness-High-/370691404486?pt=US_Car_Lighting&hash=item564eedf2c6

In any event, as dhenry states you should be able to get close to what you want by giving the LEDs more current.

Good luck!
Title: Re: Full brightness multiplexing.
Post by: John_Smith on Dec 08, 2012, 07:16 pm
I use quite a lot of peak current with IR LEDs, you can pulse some at 1 amp, in some TV remotes they dont even have a resistor in series, but use the internal resiastance of the battery to limit the current.

But if I can only drive my LEDs at 150Ma, and its only for 1/8th of the time, it would be about the same " power" being supplied to the LED as driving it at 18mA DC.   

So what I was trying to find out ( as I havn't actually looked into multiplexing before ) is that if I have all 800 LEDs on at once, and I want them the same brightness as if they were all on at 18mA,   would I need 8 rows and 100 columns with 100 LEDs on at once ?
Title: Re: Full brightness multiplexing.
Post by: dhenry on Dec 08, 2012, 07:25 pm
It depends on your duty cycle (how you multiplex them) and your frequency.

Assuming sufficiently high frequency (100hz for example), you are then at the mercy of your multiplexing arrangement. If you scan 1 led at a time, your dc for each led is 1/800; and your need 800*18ma for each led to reach an average brightness of a 18ma constantly-on led;

If you scan 1 column at a time, your dc for each column is 1/100 and you need 100*18ma on current;

If you scan 1 row at a time, your dc is 1/8 and you need 8*18ma on current.

etc.
Title: Re: Full brightness multiplexing.
Post by: John_Smith on Dec 08, 2012, 07:32 pm
I think I will stick to my latched 18mA DC system, I don't really want to be pulsing 1.8 amps around the display screen . ( I have a wireless link controlling it and dont want to compromise reception )

I will play around with multiplexing for my next indoor displays, when I dont need the grunt to fight the sun.

Thanks all for comments.
Title: Re: Full brightness multiplexing.
Post by: cmagagna on Dec 08, 2012, 07:32 pm
Quote
But if I can only drive my LEDs at 150Ma, and its only for 1/8th of the time, it would be about the same " power" being supplied to the LED as driving it at 18mA DC.  


Sort of yes, sort of no.

Human eyes respond to light logarithmically and not linearly...pumping twice as much power through an LED may or may not make it emit twice as many photons, but your eye definitely won't see twice as much brightness.

Quote
So what I was trying to find out ( as I havn't actually looked into multiplexing before ) is that if I have all 800 LEDs on at once, and I want them the same brightness as if they were all on at 18mA,   would I need 8 rows and 100 columns with 100 LEDs on at once


If you multiplex 8 rows you'll have a duty cycle of 12.5% which is higher than most LED's limits for peak forward current (10%). My guess is you could drive them to around 75% of peak forward current and be fine; worst case scenario you'd be shortening their useful life from 15 years to 12 years.

If you'd like a real-world scenario I'm currently working with a 24x16 matrix (24 LEDs controlled by 3 TLC5916s X 16 rows controlled by MOSFETs + a 74hc154) and at 10 Khz (0.1 ms strobe cycle) running them in the matrix at about 75 mA each seems to be about as bright as driving them at 20 mA directly.
Title: Re: Full brightness multiplexing.
Post by: retrolefty on Dec 08, 2012, 07:37 pm
Quote
I have done some searches and found there are some conflicting ideas out there about duty cycle and brightness.

I know that if I use 50% duty cycle drive with my latched system, the brightness appears half as bright.

Some say pulsing the LED at 50mA for 10% will give the same apparent output as a steady 25mA, the only way I can see this happening is if the light output is 5 times brighter at 50 mA, but the graphs dont show this.

Can anyone suggest the best way to get maximum brightness from the LEDs in a matrix format ?


I wished to avoid the whole problem of figuring out led brightness Vs scanning rate Vs duty cycle when I began to build a 5x5x5 led cube. So doing research on different designs I came a across a method that takes all the thinking out of it and also eliminated needing 25 current limiting resistors at the same time. There are shift registers that have constant current output pins (mine are active low sinking) where you add one 'programming resistor' for the chip that sets the constant current value amount to your desired wish for all the output bits. Mine used two 16 bit shift registers wired in series to drive the 25 vertical columns of the led cube, and then one of 5 pnp switching transistor were used to enabled to supply the source current for a whole level. Sized the two programming resistors for 20ma and the things runs perfect with nice even brightness no matter how many leds are commanded on for any specific level. Such registers cost a little more but the saving in series current limiting resistors and the board space they took up is well worth it in my opinion, plus it let me get on with developing the sketch software without having to consider if I would have to deal with 'variable or low brightness' led by having to play with scanning speed and duty cycle.

Lefty
Title: Re: Full brightness multiplexing.
Post by: dhenry on Dec 08, 2012, 07:41 pm
Quote
I don't really want to be pulsing 1.8 amps around the display screen .


That's just for individual led. Multiply that figure by the numbers you have.

This is why large commercial displays do not utilize multiplexing.
Title: Re: Full brightness multiplexing.
Post by: dhenry on Dec 08, 2012, 07:43 pm
Having said that, let me just say that it's nuts to wanting to run your led at 18ma average current.

You are creating a hurdle so high for yourself that you practically have to fail.
Title: Re: Full brightness multiplexing.
Post by: cmagagna on Dec 08, 2012, 08:11 pm
Quote
This is why large commercial displays do not utilize multiplexing


I was under the impression that all large displays, commercial or otherwise, use multiplexing like this one:

https://www.adafruit.com/products/607

Using one driver per pixel seems like it would cost infinity money.
Title: Re: Full brightness multiplexing.
Post by: John_Smith on Dec 08, 2012, 10:00 pm
Quote
Having said that, let me just say that it's nuts to wanting to run your led at 18ma average current.


Quote
Using one driver per pixel seems like it would cost infinity money.


The display needs all of that and more to compete with the direct sunlight.  Theres a pic of the display facing into the sun on the homepage

www.scorebauds.co.za

This particular display I am on now is for team names , for the actual scores and countdown timer I usually use  16 to 20    3,000 candela LEDs per segment, or up to 140 LEDs per digit,  so a 7x5 array of similar LEDs will be a lot dimmer anyway.

Title: Re: Full brightness multiplexing.
Post by: dc42 on Dec 08, 2012, 10:08 pm

So what I was trying to find out ( as I havn't actually looked into multiplexing before ) is that if I have all 800 LEDs on at once, and I want them the same brightness as if they were all on at 18mA,   would I need 8 rows and 100 columns with 100 LEDs on at once ?


Yes, and you would need to run each LED at about 144mA when it is on. So that's up to 14.4A per column. Possible using mosfets and good grounding.
Title: Re: Full brightness multiplexing.
Post by: John_Smith on Dec 08, 2012, 10:34 pm
thanks dc42,

I am sure its possible, but I have to have decide this weekend, so I have no time to experiment .

I think I would have needed a double sided board for the normal MAX chip type multiplexing ?  and the MAX chips here are over 10 times the price of the normal TPIC6B595N

I managed to design a single sided board with 5 TPICs for a 5x7 module today, and I know it works and I can get the chips. ( double sided boards are a lot more expensive here, especially the setup charges for a prototype board like this )

When I get a chance I will try some multiplex boards
Title: Re: Full brightness multiplexing.
Post by: dc42 on Dec 08, 2012, 10:46 pm
Why not use thirteen TPIC6B95s as the column drivers and eight P-channel mosfets as the row drivers?
Title: Re: Full brightness multiplexing.
Post by: John_Smith on Dec 08, 2012, 11:06 pm
That could be good, trouble is I cant find the max allowable current pulse for the chips I have, and I have designed a single 5x7 board that I can just stack side by side for any number of digits, or build them up into bigger panels later on ( there is no border around the pcb )

I will look at that idea when I get a break though, and also see if I can do it on a single sided board, thanks
Title: Re: Full brightness multiplexing.
Post by: John_Smith on Dec 09, 2012, 09:31 pm
The good news is I have an extended period to design this project ( the pcb manufacturer closes down this week til mid Jan and is not taking any new jobs before then )

So I think I will try the Mosfet and TPIC idea, or possibly the MAX 7219,    I dont know how Tadya can sell them for $1.25 when Mouser is $11 or more, and I see Tadya have an online discount of 15% until the 12th!

Which library would be best to experiment with, ( I only want single colour )

I am now going to RTFM about multiplexing, thanks for the leads.

Title: Re: Full brightness multiplexing.
Post by: dhenry on Dec 09, 2012, 10:14 pm
Quote
how Tadya can sell them for $1.25


Trust me, you do not want to know.
Title: Re: Full brightness multiplexing.
Post by: John_Smith on Dec 11, 2012, 11:19 am
Hmmm,   I asked my manufacturer about pulsing the LEDs, and they replied :-

Quote
Hi John,
I see . It's ok with leds . We did pulse testing on such leds.
But it will increase the decay of light a little bit .
You can try an samples .


So I asked what pulse width and frequency they tested at  ?  they said :-


Quote
Frequency is 1.0hz .
1 time per second. We tested 3 monthes . And if leds ok . then we use the chips.


which I work out to being on all the time  :-)

so I asked if I can pulse them 150mA at 10% M/S ratio, and they replied :-

Quote
Hi John,

No . 30mA is the max current .

150mA will burn the leds.

If need 150mA . You should use 0.5W


So it looks like I am abandoning multiplexing for now as I have a couple of thousand of these LEDs left, even though I can't get specs on them :-)
Title: Re: Full brightness multiplexing.
Post by: Headroom on Dec 11, 2012, 11:11 pm
I is somewhat off topic, however the statement that an LED appears to be half as bright at 50% duty cycle then at 100% is incorrect for several reasons.
The relation between drive current and luminance is not linear.

It is pretty well explained here:

https://ledshield.wordpress.com/2012/11/13/led-brightness-to-your-eye-gamma-correction-no/ (https://ledshield.wordpress.com/2012/11/13/led-brightness-to-your-eye-gamma-correction-no/)
Title: Re: Full brightness multiplexing.
Post by: John_Smith on Dec 12, 2012, 03:19 am
Thanks Headroom,   thats an interesting link.

So running the LED for half the time only loses a quarter of the perceived brightness,  and you could go down to 1:7 ratio before losing half the perceived brightness.

I just googled LED graph current luminance, to see what increase in current would restore full brightness, and again found pages of conflicting stories about PWM,  i.e. from electronics-tutorial.ws  .
Quote
So pulses at a frequency of 100Hz or more actually appear brighter to the eye than a continuous light of the same average intensity.


A theory from an old HP document according to another page http://electronics.stackexchange.com/questions/17528/does-pulsing-an-led-at-higher-current-yield-greater-apparent-brightness

I think I will have to knock up 2 samples of my 7x5 display and actually test it in the sun while varying the duty cycle of the one.
Title: Re: Full brightness multiplexing.
Post by: cmagagna on Dec 12, 2012, 05:29 am

So running the LED for half the time only loses a quarter of the perceived brightness,  and you could go down to 1:7 ratio before losing half the perceived brightness.


That makes sense, your eyes perceive light intensity as a logarithmic function. Here's the short table I use:

Step | Duty
===========
  0  | 0
  1  | 1
  2  | 3
  3  | 7
  4  | 15
  5  | 31
  6  | 63
  7  | 127
  8  | 255
Title: Re: Full brightness multiplexing.
Post by: John_Smith on Dec 12, 2012, 07:22 am
Another point I have been ignoring,  while trying to maintain maximum brightness,  is that each time I buy another batch of LEDs they are much brighter than the previous ones anyway !

It was only a couple of years back that normal ( 25mA ) cheap LEDs broke through the 1000 mcd mark, now 11,000 are common.

Title: Re: Full brightness multiplexing.
Post by: John_Smith on Dec 12, 2012, 06:09 pm
  DC42 suggested 
Quote
Why not use thirteen TPIC6B95s as the column drivers and eight P-channel mosfets as the row drivers?


Sounds a good idea , and  I have built a test panel, but only 7 x 5 LEDs.   (  I only have enough LEDs for the project in this format !  I am going to leave a physical gap between each character display - later on I might move to 8x8 and do scrolling )

I want to get this one panel going to test in the sun.

I have 5 rows driven by mosfets, and 7 columns with 7 resistors to DRAIN1 to DRAIN7  of the TPIC6B595. ( DRAIN0 not used )

One problem is that I have migrated to the RFM22B wireless modules ( and RF22 library )  that uses SPI and the pin 2 interrupt.

I have been using  Shiftout to run my 7 seg displays ( latching ),  but I now have have 24 characters that I wish to update , and it sounds like SPI is the way to go speed wise to multiplex each row sequencially ?  Crossroads uses SPI for everything ( sorry this is new ground for me )

I have used SPI and each driven chip needs a chip select input ?  So if I need 24 chip selects I could do that with 3 CD4017s and one clock pin of the arduino ?   

My old Shiftout version just pumps out the data to the first data in pin.

If the receiver detects an interrupt ( I am hoping it is only when a message arrives )  it will presumably disrupt the display refresh?

I could possibly blank the display with the notG while it is dealing with the incoming data..

I am also trying to get my head round the array to store the data in.     It used to be a binary byte for the 7 seg , which I just called up for  shifting out the data for each digit.

Now  I must send out the right byte pattern for the top row of the number for each digit, latch it in,  then load the  second row, and so on ?

I have searched for a library to do this, but they seem to be devoted to particular chips  ( which are expensive here )

I am going to write a sketch to run this one display to show a couple of letters, using shiftout for now, if anyone has some pointers I will appreciate it .
Title: Re: Full brightness multiplexing.
Post by: dc42 on Dec 12, 2012, 06:31 pm

I have used SPI and each driven chip needs a chip select input ?  So if I need 24 chip selects I could do that with 3 CD4017s and one clock pin of the arduino ? 


You don't need 24 chip selects, you can daisy=chain the TPICs from the SPI pins just like you do with ShiftOut. So one CS pin for all the TPICs.  


If the receiver detects an interrupt ( I am hoping it is only when a message arrives )  it will presumably disrupt the display refresh?


If the ISR uses SPI to communicate with the radio, then possibly, but not necessarily. It depends on how the ISR is written, and on whether you can use the same SPI clock fate for both devices. Or, if you can afford to wait a few microseconds before servicing the interrupt (which depends on whether the receiver can buffer incoming data), then you can disable interrupts while you send data to the TPICs.



I am also trying to get my head round the array to store the data in.     It used to be a binary byte for the 7 seg , which I just called up for  shifting out the data for each digit.

Now  I must send out the right byte pattern for the top row of the number for each digit, latch it in,  then load the  second row, and so on ?


Use an array with 8 bytes per 8x8 display, one byte for each row. Shift out the bytes for the rows you want to update.


I am going to write a sketch to run this one display to show a couple of letters, using shiftout for now, if anyone has some pointers I will appreciate it .


The standard ShiftOut is extremely slow. It is not difficult to write a much faster version. See the code starting at line 446 of https://github.com/dc42/arduino/blob/master/Libraries/Lcd7920/lcd7920.cpp (https://github.com/dc42/arduino/blob/master/Libraries/Lcd7920/lcd7920.cpp). It could be made even faster by calculating and storing the port address and but mask just once during initialization instead of once per byte sent.
Title: Re: Full brightness multiplexing.
Post by: John_Smith on Dec 12, 2012, 07:57 pm
Thanks thats pretty encouraging.

Quote
You don't need 24 chip selects, you can daisy=chain the TPICs from the SPI pins just like you do with ShiftOut. So one CS pin for all the TPICs. 


OK so I will use MOSI for the data, clock for clock, and CS for latch .

The RF22 presumably sets the polarity, speed,  etc for the SPI, so I will just fit in with that with a different CS pin?

Quote

Or, if you can afford to wait a few microseconds before servicing the interrupt (which depends on whether the receiver can buffer incoming data), then you can disable interrupts while you send data to the TPICs.


I usually send the transmission several times anyway to be sure ( its just updates of scores now and again ) so I can disable interupts while refreshing each rows data.  ( if the SPI is fast enough there should be time before the next refresh using GMikes timing check  )

I also have a countdown clock that I use "blink without delay" routine for, using millis(), will this be OK ?
Quote

Use an array with 8 bytes per 8x8 display, one byte for each row. Shift out the bytes for the rows you want to update.


Thats basically what I am putting together now for the test which I was going to try with shiftout, but I will try SPI instead.
Can I send out 24 bytes in one go while the CS is low, and then raise it to latch the data?

I'm afraid the example with the LCD display is confusing for me at the moment, I will try my baby steps method to get it running with one panel, and see what happens with 35 LEDs up against the African sun in the morning.

Title: Re: Full brightness multiplexing.
Post by: dc42 on Dec 12, 2012, 08:02 pm

Can I send out 24 bytes in one go while the CS is low, and then raise it to latch the data?


Yes, you can.
Title: Re: Full brightness multiplexing.
Post by: John_Smith on Dec 12, 2012, 08:10 pm
Great,  thanks ,  long night ahead ...
Title: Re: Full brightness multiplexing.
Post by: Hippynerd on Dec 12, 2012, 11:32 pm

I think I will have to knock up 2 samples of my 7x5 display and actually test it in the sun while varying the duty cycle of the one.

Crazy.
Title: Re: Full brightness multiplexing.
Post by: John_Smith on Dec 13, 2012, 06:23 pm
Well, crazy or not Hippynerd, I did the test, ( as you suggested in the second post of this thread ? ) and the difference is like chalk and cheese.

As I said, there are so many contradictory theories out there when I Googled about multiplex / brightness /pwm, that I had to check for myself.

Have a look at the photos, they are as they appear to the eye, the second 12% multiplexed row of LEDs are supposed to be only half the perceived brightness of the middle 100% row.

(http://i.imgur.com/MJrfA.jpg)  (http://i.imgur.com/CNL31.jpg)

I do not have the time to measure the results unfortunately, but while I might get away with the 100% row at 50 meters into the sun, with a bit of filtering , there is no way the 12% row will work. ( I don't have the budget to use the power LEDs that the big video screens use )

I have no idea how much extra current at 12% would be needed to bring the " brightness "  up to the level of the 100% LEDs, but the manufacturer of my LEDs does not recommend pulsing them at more than 25mA anyway !

Sorry to say,  multiplexing will not work for my project, it would have saved me 80% of the cost of driver chips , ( which I had budgeted for anyway ) but I would have had to go for a double sided pcb and some Mosfets.
And then there might have been problems running the high speed SPI data to each of the 24 displays over 2 meters.
With Shiftout  the fastest response I need is to change the timer display every second , and I dont have to worry about the receivers interrupt.

I see a lot of videos people put on you-tube of their LED multiplex projects, but many record them with the room lights off.
Take the board outdoors on a clear day, with the LEDs facing into the sun, and let me see the results, this is the worst case scenario I have to cater for with my outdoor projects.

Thanks everyone for your inputs, it was worth a try, and very interesting.

Title: Re: Full brightness multiplexing.
Post by: Hippynerd on Dec 13, 2012, 09:56 pm
That is a good experiment. Im unclear about the 12% number. If you are running one row at a time thats 1:7, or about 14%, if you run them 1:5 you should be at 20%.
My guess is 20% will end up being only about 1/2 as bright as full on.

I wonder if results would be different, if they were done indoors?

I will probably have to do some testing too, Im curious about how brightness will vary depending on duty cycle.

The best way to take pictures of LEDs seem to be in dim light, with the camera set for outdoor (natural light).
Title: Re: Full brightness multiplexing.
Post by: John_Smith on Dec 13, 2012, 11:27 pm
Yes, I got that confused with the 8 outputs of the TPIC,   it should read 20%, I am switching 5 mosfets to feed the rows, and 7 outputs of the TPIC are sinking the respective LED columns.
(http://i.imgur.com/Bs1oK.jpg)(http://i.imgur.com/hhK82.jpg)

The difference does show up indoors, but I had to take the shots from far away and crop the target, or the camera changes its exposure for the different levels.  At a distance it measures the average room light.

The brightness at 20% would be fine indoors, it is still very bright to look straight into, but for example in my lit room, when the 100% row is lit, there is a big blurred red light on the ceiling, but when the 20% one is on you can hardly see it on the ceiling.

I took the earlier pics at sundown in the only spot of sun available.   I will try it in the morning in the full sun, and try walking down the road, and take some more pics, but I am not too hopeful.

I will definately use multiplexing for my indoor projects, but as I said, the normal LEDs get stronger every few months, so it will reach a point where the 20% is bright enough  -   soon I hope !



Title: Re: Full brightness multiplexing.
Post by: Hippynerd on Dec 14, 2012, 03:55 am
I would like to use multiplexing with some tail lights, and my guess is that brightness will be an issue. I have no idea how im ever going to get around to it, but its still in the head scratching phase.

I think there are really too many factors to calculate things, the electronics stuff can be measured and calculated, but perception and biology are weird enough that I think you really have to test stuff to see how well it will actually work.

Tail lights that you can play pong or tetris on are just too fun to not try.
Title: Re: Full brightness multiplexing.
Post by: John_Smith on Dec 14, 2012, 06:22 am
I see some cars have pwm for the brake / tail lights, my wife thought I had lost it when I kept shaking my eyes / head at cars in front  ( to see if the tail lights were PWM  brake lights - the line of dots )

I dont know how they measure the perceived brightness,  ( I have never studied light ) the difference in brightness of my two rows doesn't look too bad when you look at the LEDs directly.
Why the dimmer one fails to even show on the ceiling, or can hardly be seen in sunlight, doesn't seem to tie in.

The LEDs I am using are only 3,000 mcd ,  I have asked my supplier to quote on their 18,000 mcd version,  presumable PWM at 20% would give me more light than my current one on 25 mA DC.   

But I am not too sure anymore :-)

Title: Re: Full brightness multiplexing.
Post by: Hippynerd on Dec 14, 2012, 05:26 pm
I just had a thought. Do your signs have to be red LEDs? It seems to me the red are the lowest of the brightness, with a tail light, you need red, white ones with a red lens wont produce as much light as a red LED (another thing to test...), so for tail lights, you need red, but for sign board, you might be able to use a color that produces more light (white? blue?).

I have some RGB LEDs in front of me, and it says the red are 2000-2500 mcd, the blue are 2500-3000, and the green are 6000-7000 MCD. maybe you should be using green LEDs?
Title: Re: Full brightness multiplexing.
Post by: John_Smith on Dec 14, 2012, 06:05 pm
Its true that the green are VERY bright, ( OK they run at a higher power than the red - the voltage being higher for the same 25 mA )
My supplier offered me 20 odd candela cheaper than the weaker red ones, but for some reason people want red !

(http://i.imgur.com/oxAEw.jpg)

OK I can't use blue on outdoor displays or the emergency services  would complain, but both green and blue are much brighter.

The white ones 30 degrees are now 26,000 mcd.

The other consideration is that I need 1680 LEDs for this project, and I still have 1720 in stock !
Title: Re: Full brightness multiplexing.
Post by: John_Smith on Mar 01, 2013, 09:44 pm
The final outcome of my experiments for an outdoor text display work fine,  I am using SAA1064 chips multiplexing at 50%,

(http://i.imgur.com/UVHm0sR.jpg)

I left the board facing the sun for 3 hours, and the front was too hot to touch ( the customer wanted black ! ) but I fitted a fan inside and the air coming out the ventilation vents was less than 40C.

My LED supplier can now sell me LEDs 5 times brighter than these, so I will go normal for multiplexing for the subsequent boards !
Title: Re: Full brightness multiplexing.
Post by: Headroom on Mar 02, 2013, 12:01 am
The human eye is more sensitive to green light, so green LED usually appear brighter than the red, or the blue.
This article may also of interest if brightness perception is an issue:

https://ledshield.wordpress.com/2012/11/13/led-brightness-to-your-eye-gamma-correction-no/ (https://ledshield.wordpress.com/2012/11/13/led-brightness-to-your-eye-gamma-correction-no/)
Title: Re: Full brightness multiplexing.
Post by: Grumpy_Mike on Mar 02, 2013, 12:06 am
There is a big error with that board.
During the whole of the first half Man United have not scored!!!

By the way, did you know I used to teach Garry Bailey, I supervised his final year project as well.
Title: Re: Full brightness multiplexing.
Post by: John_Smith on Mar 02, 2013, 06:45 am
@  headroom,     thats another interesting article.  As it says there are misconceptions and I just used my simple tests - trying different M/S ratios on the actual batch of LEDs I was going to use, and taking them out in the sun to see at what point they became difficult to discern.

Even 25% was not good enough, but It might be that I have chosen the 50% ratio to suit my strange eyesight ! 

Yes the green is definately brighter,  which always made me wonder why we chose red as a danger signal, or traffic light.  Perhaps it was the easiest colour to generate using a fire with a filter ?

@ Mike      One of the bugs I have to sort out today is that after I had boxed this one up for a photoshoot, I saw that if I change the scores, some of the LED columns go out  !    But being both a ManU supporter, and an Edgemead resident, perhaps this was more PC.

Gary Bailey does a lot of TV commentary on soccer games here, what a great guy ( must be your influence Mike )  Wonderful sense of humour, and not Grumpy at all.
Title: Re: Full brightness multiplexing.
Post by: Grumpy_Mike on Mar 02, 2013, 07:52 am
Quote
Gary Bailey does a lot of TV commentary on soccer games here,

Yes that is why I mentioned it. The last time I saw him I was driving through Manchester and we pulled up at the same set of traffic lights. He wound down his window and said Hi Mike, I said isn't it supposed to be the other way round? I also know Les Kershaw the chief scout, he used to lecturer in chemistry at my University. Mind you in Alex Ferguson's autobiography he called him a Physics lecturer. My grandad first took me to old Trafford when I was nine. It was so long ago Bobby Charlton had hair!
When I was in New York two years ago I was sold a "New York Reds" badge in the United Bar, they had a banner saying "New York Reds, like Urmston only bigger"