Senior project idea

[chessman]

Hey guys, I'm almost done with the design of my project.<p>Basically I'm creating an animated LED readerboard, so it's an array of 8 x 32 LEDS.<p>I'm using 74HC595 shift registers for the multiplexing. They have a max current of 35mA, and that's way too low for my brightness level of LED (I'm gonna split the columns of 8 into something like a 100 ohm), so I'm gonna run all the outputs through seperate BJTs.<p>The processing is done by a PIC 16F877 to give me the I/O ports I need, plus I already know the 16xxxx instruction set and architecture.<p>Anybody have any objections to this plan? Obviously I could use constant-current drivers for the LEDs so they all looks the same brightness, but I don't know anything about that area.<p>Thanks for your time,
Kyle

[greg123]

Kyle,<p>Are you planning on using the BJT's for switching and using a seperate supply or use them for a current amplifier?<p>You can make a simple 25 mA CC source for each led with a BJT and a diode.<p>G

[chessman]

I'm planning on using them as a current amplifier so I can run the LEDs off a single 100 ohm resistor.<p>Ack! Is that one BJT and diode for each LED?<p>I guess that's kinda what I was expecting but it really sucks...I don't know if I can handle that much wiring in the time I have <p>I'll just have to settle for the branch off the 100 ohm.

[L. Daniel Rosa]

If you're multiplexing for "column at once" (8 LEDs), then you only need 8 CCSs to regulate the current. If you try to activate only one LED at a time, it'll be too dim (I spoke to someone who did this). Eight NPNs and 32 PNPs (or vice versa)isn't so bad for matrix support.

[toejam]

as for all that wireing check out pcexpress.com You will be surprised at how fast you can make boards with your mouse. they give you a great deal on proto boards.

[chessman]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by L. Daniel Rosa:
If you're multiplexing for "column at once" (8 LEDs), then you only need 8 CCSs to regulate the current. If you try to activate only one LED at a time, it'll be too dim (I spoke to someone who did this). Eight NPNs and 32 PNPs (or vice versa)isn't so bad for matrix support.<hr></blockquote><p>How would I go about doing this?

[greg123]

Kyle,<p>Can you offer some more information on the project? A schematic maybe? <p>To clarify, you want to be able to activate one of 256 (8X32) led's at one time?<p>Also, is your connection somthing like the following?<p>LED Array<p>If so, you will only need the 8 ccs like dac mentioned.<p>[ April 24, 2003: Message edited by: Greg ]</p>

[chessman]

I would provide a schematic, but I don't have one made and I don't have the time to make it.<p>
I'll be activating anywhere from 1 to 8 leds at a time, in a column. This will be dependent on what the sign is displaying.<p>They're just set up in a matrix of 8 rows by 32 columns, and the columns will be multiplexed.<p>I would like to get all the LEDs to appear the same brightness even if some columns have 2 leds on and some have all 8 on.<p>Of course, it's multiplexed, so only a max of 8 LEDs are on at the same time

[chessman]

K guys, thanks for the help, I need to know how to do those CC supplies cause I really need to get the wiring going....<p>If I can't figure it out I'll stick with the CL NPN and resistor...

[Chris Foley]

Hi, Chess. It looks like "In for a penny, in for a pound". Since you've already done making dot-matrix displays from individual LEDs, it shouldn't be too much more trouble to do the drivers out of individual transistors. Check the multiplex ratio. If you do 32:1, any permissible amount of LED current will result in a display that's basically invisible. A ratio of 8:1 will be O.K. You may have to roll in the data on four chained '595s, and then use those outputs to drive transistors. It's problems like this that make you wish you had more time. Try putting a ribbon cable header on your display board, and just using another perfboard for the drivers (it can mount behind the display, and not take up too much room.) Remember power supply requirements, too. Use a scope, and watch for significant voltage drops at various points. Heavier (20 gauge at min) separate wiring for power to the driver board, and use a big low esr cap to ensure the '595s don't spit up (they do lose data on big Vcc dips). Good luck, and happy hunting.

[greg123]

Chess,<p>When is your project due? Are you still needing the CC schematic??<p>g

[chessman]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by Greg:
Chess,<p>When is your project due? Are you still needing the CC schematic??<p>g<hr></blockquote><p>The project's due at the end of June...<p>I'm not needing the schematic since L. Daniel Rosa helped me out a lot (thanks)<p>Thanks for the replies guys, but I've got it under control now <p>I'll post again if I need anything.