Post by thread:LED Moving Message Display

> Thanks, that's a start, but not quite what I had in mind...looking for a PIC > or similar microcontroller-driven set of multiplexed LEDs to allow roughly a > 7 x 80 pixel field to be controlled. Perhaps use a Hitachi LCD display and > a few buttons for operator interface. Any other suggestions will be greatly > appreciated. How big do you want it? The HCMS 2975 has a 7*80 field :-) I asked about this earlier on the list, with no response so far. But seriously, you might want to build your display the way they did. A 320 bit shift register, with some latches. Serially clock your data into the shift register, then latch the display. Shifting 5*7=35 bits and then latching will cause the display to scroll. The problem I'm running into with my displays (and I believe all my problems are now related) is that current surges from the LEDs are causing my PIC to brown out. <sigh> HP recommends splitting the supplies, but when you have pins like: Vled Vlogic GND on the smaller displays, I have no idea how to wire them up without causing ground loops. Surely HP wasn't asking me to connect the grounds of two different supplies together. Perhaps they were... Returning to our original subject, consider using separate supplies. Jon Doran

Hi Bill, You'll need 80 outputs. There are all-in-one shift registers with powerful outputs, you'll probably need to get those from the manufacturer as samples or order a tube of them. Or you could use 10 ser-par shift registers and 80 transistors to accomplish the same thing(or a bunch of darlington arrays). Then you'll also need a 1 of 7 sequenced output, maybe out of a 4017 type counter with 7 very very strong transistors. The idea is to present 80 outputs, wait, then blank, sequence to the next horizontal line, and present the next 80 outputs, etc. A lot of work, but perhaps you have lots of time on your hands. Remember that your displays and your drivers must match, polarity-wise(unless you use 560 discrete LEDs). That's sort of the key to a project like this. Cheerful regards, Bob At 06:43 PM 8/1/99 -0400, you wrote: >Thanks, that's a start, but not quite what I had in mind...looking for a PIC >or similar microcontroller-driven set of multiplexed LEDs to allow roughly a >7 x 80 pixel field to be controlled. Perhaps use a Hitachi LCD display and >a few buttons for operator interface. Any other suggestions will be greatly >appreciated.

> The problem I'm running into with my displays (and I believe all my > problems are now related) is that current surges from the LEDs are causing my PIC > to brown out. <sigh> HP recommends splitting the supplies, but when you > have > pins like: > Vled > Vlogic > GND > on the smaller displays, I have no idea how to wire them up without > causing ground loops. Surely HP wasn't asking me to connect the grounds of tw o > different supplies together. Perhaps they were... > > Returning to our original subject, consider using separate supplies. In this case yes the two supplies would have ground common. One V+ to Vled and the others to Vlogic. We had a product that we tapped the Ni-Cad pack(not recommended) one cell down for the logic supply to keep the LED swiching noise out.

Ok, now, don't forget about the LED intensity. Even if you apply twice the LED nominal current (the max peak recommended by the manufacturer), you can not make it bright enough to see at the day light if you apply current for less than 10% of the time. If you are planning to time share current, in a scanning technique, remember this. My personal experiments with scanning display, ended in a maximum of 1/8, (1 on, 7 off), using 40% more than the nominal current, at a rate bigger than 200Hz. If you will build a display like that, I think you could go a little further, and do it not only horizontal shifting message, why not do it scroll vertical, or zooming, dropping, pouring, and sometihng like that? The only trick here is to have a memory bit mapped space, and your processor can set or reset any bit into that memory. That block of memory is then sent (10 or 20 times a second) to the LED driver system, that will copy that to the LEDs, each bit corresponding to a specific LED. If you think about it, it is not that difficult. But remember about the scanning, a 80 x 8 led matrix would require one whole horizontal line (row) of LEDS turned ON all the time, so you would need a 80 drivers to constantly feed the columns. Considering a 50% increase in the LED current, lets say from 20 to 30mA, you would need a power supply around 30mA x 80 = 2.4 Ampers, just for the LEDS consume. Wagner

Wagner Lipnharski wrote: > > Ok, now, don't forget about the LED intensity. > > Even if you apply twice the LED nominal current (the max peak > recommended by the manufacturer), you can not make it bright enough to > see at the day light if you apply current for less than 10% of the time. Agreed > If you are planning to time share current, in a scanning technique, > remember this. > > My personal experiments with scanning display, ended in a maximum of > 1/8, (1 on, 7 off), using 40% more than the nominal current, at a rate > bigger than 200Hz. Yep.. had to do the same.. > If you will build a display like that, I think you could go a little > further, and do it not only horizontal shifting message, why not do it > scroll vertical, or zooming, dropping, pouring, and sometihng like that? Not difficult to do. {Quote hidden}> The only trick here is to have a memory bit mapped space, and your > processor can set or reset any bit into that memory. That block of > memory is then sent (10 or 20 times a second) to the LED driver system, > that will copy that to the LEDs, each bit corresponding to a specific > LED. > > If you think about it, it is not that difficult. > > But remember about the scanning, a 80 x 8 led matrix would require one > whole horizontal line (row) of LEDS turned ON all the time, so you would > need a 80 drivers to constantly feed the columns. Considering a 50% > increase in the LED current, lets say from 20 to 30mA, you would need a > power supply around 30mA x 80 = 2.4 Ampers, just for the LEDS consume. I am doing that currently with a 16c66 running at 8Mhz.. Driving a 96x7 led matrix. Using non latched shift registers (74164) from memory.. driving uln2003 darlingtion arrays thru 300Ohm resistors for the columns and the rows driven from 2N2955 Transistors.. Because the shift registers are not latched, you must have the rows off while shifing data in.. A pig of a design really, but it's someone elses board and just my controller. Using 96 bytes of contiguous ram for the display buffer, and most of the remaining ram for comms buffers.. effects buffers and the like. Everything is updated real-time and is commanded thru an rs485 link at 9600 baud. The display is refreshed thru a real time interrupt, and the serial data to the display is bit bashed as the SPI and UART are mutually exclusive. Also, given that each bit has to be masked from the relevant byte, it's not that much quicker. The characters are stored in lookup tables styled along the lines of Don McKenzies old Z80 code. (Hi Don!) tho had to put them across 2 banks so as not to cross a bank boundary... Bloody banks.. The Character generation routine is very very very tight hand coded assembler, because when it is triggered after a display line refresh it only JUST has time to finish before the next refresh is due. So tight is the timing. The display ram is locked by the char gen routines using a semaphore to prevent the isr from touching it, should the char gen take to long. Otherwise the display glitches and presents a tearing type of picture. Anyway.. writing the code only took me about 2 days on and off, and it was Hugely educational when it came to shaving 80 or so cycles from routines to stop jitter and glitching.. The effects are the fun part.. Only completed the slide/wipe and roll so far.. I'm looking at external ram for some more spiffy effects.. It's fun to see how far you can push a $13 chip, when your competition is doing it with a full blown HC11 with 32K Ram & Rom at 16Mhz.. But then they will write in C won't they :p) Cheers... Brad... PS. If you want to look at CHEAP, and very good led signs.. take a look at what out South African buddies are up to. I have gotta say, those years of trade embargoes have turned South Africa into a very clever continent.

On Mon, 2 Aug 1999 13:40:39 +1000 Gennette Bruce <spam_OUTbruce.gennetteTakeThisOuTTAFE.NSW.EDU.AU> writes: >Here is a simple idea that works really well - it uses just 3 diodes >and 1 >extra capacitor. [...] >The really cool thing is that the outputs from the 2 diodes are >completely >isolated from each other It doesn't quite work that way. A diode with DC current flowing through it acts like a small resistor to AC signals. Also if one of the loads changes greatly, the regulator's limited load regulation will appear at the other output. You'd be just as well off to forget the diodes and have seperate circuits for the analog and digital sections that join only at the regulator's output pin. The ground circuits are likewise seperated. This is a standard good design practice. In extreme cases, a RC or LC filter in one of the circuits may be needed, but going to two regulators would be better. ___________________________________________________________________ Get the Internet just the way you want it. Free software, free e-mail, and free Internet access for a month! Try Juno Web: dl.http://www.juno.com/dynoget/tagj.

Bill Kichman wrote: > > Does anybody have any good leads on really cheap LEDs?? I want to build > this 7 x 80 led moving text sign, but don't want to go broke buying the 560 > leds. Are modules cheaper? Are they available in 4" height? The modules are definitely cheaper. I don't have a price on hand though. Even considering time savings on construction they must be a better alternative. I've used upto 50mm. I don't know if they come larger. You can get matrix displays with zero display current with mechanical switching modules with good daylight visibillity. These (that I've seen) come in sizes to 16" in height. I'd hate to see the price though :-( -- Best regards Tony "COMING SOON" A Parallax to PIC source code converter. http://www.picnpoke.com Email ???

Hey thanks I 'll check it out. Bill K. Cornwall, PA USA -----Original Message----- From: Bob Blick <EraseMEbblickspam_OUTTakeThisOuTSABER.NET> To: PICLISTspam_OUTMITVMA.MIT.EDU <@spam@PICLISTKILLspamMITVMA.MIT.EDU> Date: Friday, August 06, 1999 12:36 AM Subject: Re: LED Moving Message Display >At 09:57 PM 8/5/99 -0400, you wrote: >>Does anybody have any good leads on really cheap LEDs?? I want to build >>this 7 x 80 led moving text sign, but don't want to go broke buying the 560 {Quote hidden}>>leds. Are modules cheaper? Are they available in 4" height? > >HSC Electronics in Santa Rosa, CA has 4 or 5 inch modules for about 5 >dollars if I recall. They don't do mail order, but if they have them at >that price you ought to be able to find them elsewhere for a similar price. >Perhaps http://www.allcorp.com or Marlin P. Jones? > >Cheers, >Bob >