to multiplex or not to multiplex..

Interactives

Hello again smart people, I have another question about a project of mine for work. I have a map that requires 100 leds.. 50 blue, 50 green. Upon pushing a button, 25 of the blue lights blink for 3sec, go out, trigger an incandescent bulb to go on, then 5 green lights blink, go out, then trigger another incandescent bulb- all stays the same for five seconds or so, then resets. From a previous project I ordered some LN2803s that I was gonna use to run some relays with- so I suppose I could use one of those to control several different segments, though I'm not sure I could power all 100 leds. It seem like I would need a huge voltage source, since the blues and greens drop like 2V each. I've read some articles about multplexing led displays with stamp, but that seems like it might be overkill, since really only 30 out of the 100 LEDs have to do anything. Any suggestions?

KenM

First of all, I am not one of the "smart people" so you will need to wait for more responses.

If I correctly understood your post, there will be a maximum of 30 leds on at one time, is that correct?

Assuming so, with each led running at ~15 mA, that would require just less than 1/2 amp. Not such a big power supply.

Regarding the control of the leds to multiplex or not, for me one factor to determine the answer would depend on available board space. A very simple approach would be to cascade several shift registers, like the '595. The parallax sight has example code on using the '595s.

A single chip approach multiplexing is to use a MAX7219, which can control up to 64 individual LEDs.

Hope this info helps more than it confuses.

K

Interactives

yes that is helpful, I went on to read that someone else had already asked a very similar question today, the answer to which was the max7219! Only 30 leds will have to be turned on and off- the other 70 stay on all the time. Its a museum display that's supposed to show that 70% of marine animals, and 5% of land animals went extinct in the devonian period. (Yes, the content is just as geek friendly as the medium for dislpaying it.) All of the material I've found on multiplexing seems to be centered around 7 segment displays, and seeing as I dont need anything as complex as alphanumeric symbols, I was hoping to avoid jumping head first into what seems to be complex programming. But I supose I'll just have to hold my breath and jump in! Thanks for the suggestion

Jon Williams

A 7-segment LED actually has eight discrete LEDs (don't forget the decimal point) so a MAX7219 can in fact control 64 discrete LEDs. You just have to setup the registers to be in bit mode (not decoded). It's not that tough.

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Jon Williams
Applications Engineer, Parallax
Dallas, TX· USA