LCD to LED

ion

Hi,
My mother has a commercial call display. She is 76 and complains that she can not read it all the time. As I did not found a unit with LED display, I would like to convert it from LCD to LED. I can get the specs of the LCD, which is a standard 4X20, but I do not know how to drive a 5X7 LED matrix to simulate each character.
I need some suggestions or help
Thank you
Ion

Chris Savage

Would it help if the display (LCD) was backlit?· Seems like that would be a much easier and more cost-effective way to make it more readable.· Or you could user an LCD with larger characters & backlight.· Plenty of those and high-contrast displays around if it is, in fact, a standard 4X20.



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Chris Savage

Knight Designs
324 West Main Street
P.O. Box 97
Montour Falls, NY 14865
(607) 535-6777

Business Page:·· http://www.knightdesigns.com
Personal Page:··· http://www.lightlink.com/dream/chris
Designs Page:··· http://www.lightlink.com/dream/designs
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Jon Williams

Multiplexing that many 5x7 LEDs will be very tricky. I found a neat display device that uses an eight-bit buss and a few control lines, and has an ASCII decoder and the mutliplexer built right in. The part number is DLO 7135; I believe it is made (or was made) by·Siemens -- but may be out of production. I think, though, that you can get these through surplus dealers like BGMicro.

I've attached a bit of SX/B test code that writes a string to a single display unit.

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Jon Williams
Applications Engineer, Parallax
Dallas Office

Post Edited (Jon Williams) : 11/29/2004 2:27:22 PM GMT

ion

You are always so kind. Thank you.
Ion

Jon Williams

Just beware of the cost -- I found my modules for about $2 each at a local surplus store -- the only place I found them on the web was Alltronic and they were $20! Wow ... maybe writing your own display controller (using an SX28 perhaps) isn't such a bad idea.

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Jon Williams
Applications Engineer, Parallax
Dallas Office

Paul Baker

There is also the HDLX-2416 which is a 5x7 display which accepts ascii and some control, they are availible from digikey the catalog page is http://dkc3.digikey.com/PDF/T043/1449.pdf , they are fairly expensive and the character height is about 1/2" which maybe abit small for older eyes to clearly read them. Also consider alphanumeric displays, especially if your looking to build a homebrew controller (since there are fewer elements per character to drive) here is a link to a 2" alphanumeric display data sheet: http://rocky.digikey.com/WebLib/Stanley/AAR121.pdf

ion

Thank you very much Paul,
I will look into it. In fact , I was wrong. When I loocked in details at the display, I saw that it drives Alphanumeric characters 16 segments. Mai big question , is, if it is a way to steal the signal from LCD ( disconect the conector) , and use that signal to drive a 2803 Darlington. From there I can attach a regular display LED. What I do not know if the signal going to one of the LCD segments it is a steady DC voltage or is a pulsing voltage. If it is a DC, will be TTL or Cmos compatibile or not. This are my big questions.

Jon Williams

If you're not in a huge hurry we will be doing an AppNote for the SX/B compiler that shows how to make a display from raw 5x7 LED modules -- these are much cheaper than the "smart" modules that have built-in decoders and even allow bit-by-bit message scrolling.

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Jon Williams
Applications Engineer, Parallax
Dallas Office

Paul Baker

Unfortunately, your question is somewhat outside of my expertise, I have a few LCD glass that I have purchased from allelectronics ( http://www.allelectronics.com/cgi-bin/category.cgi?category=365&item=LCD-99&type=store·), I did what Harley Shenko recomended in his customer comment to test them out but I have not used them in its intended project yet (turning an infrared toaster oven into a reflow oven).

But if the LCD display is a glass (no chip is located in the LCD package) the signal is a pulse train since LCDs must not have a DC component or the segments will burn in (permanently be on). I am assuming you do not have access to an oscilloscope, since it would provide many of the answers to your questions.

First off, try to find the spec sheet for the chip which is driving the LCD display. If its only a driver, bypass it and pull your signals from the decoder (the·chip feeding the signals to the driver) and ignore the rest of this post. If it is a decoder/driver continue reading.

One potential option is to turn the segment pulses into a DC signal. This can be accomplished by using retriggerable monostable multivibrators (MSMV) the spec sheet for an example part is www-s.ti.com/sc/psheets/sdls043/sdls043.pdf , where you set the pulse duration for a period which is longer than one clock period of the LCD so that when a particular segment is being driven with a pulse train the MSMV will continue to be retriggered effectively turning the pulse train into a DC signal. The benifit of this is that the decoding will be done for you, but youll need to decipher which wire corresponds to which segment there are several different ways this can be done (which method to use depends on what tools you have access to).

Another option·is to bypass the LCD decoder/driver (the chip that the LCD segments are connected to) this will require you to build your own decoder, and assuming you can find the specs to the LCD driver circuit you can design the system without having to do in system mesurements.

Now that I think of it you may be able to just do a direct stealing of the LCD signals, the only thing about this is that the display will not be as bright as it would be by a steady DC signal. How bright the LED display will be depends on the duty cycle of the signal.·If you can figure out what the pulse-high voltage is, taking a multimeter reading of a segment which is on can indicate what is the approximate duty cycle, the lower the RMS DC voltage of the driven segment, the lower the duty cycle, and the dimmer your LED will be.

I do not have much experience in using Darlingtons, but I'm pretty sure that they will only be needed if you do the third option since the LCD is a capacitive load and the driver was most likey designed with this in mind, so the sourcing capability of the decoder/driver would likely not be able to drive a resistive load (the LED). But the MSMV and your own decoder/driver should be able to drive the LED directly (dont forget to include current limiting resistors since the forward voltage for the 2" alphanumeric displays is only 2 Volts).

Paul


Post Edited (Paul Baker) : 12/3/2004 4:48:14 PM GMT