Post by thread:[PIC]: Myke's Analog Clock: questions & variation

Greetings, I've been reading Myke's book (programming and customizing PICmicro microcontrollers). I found myself fascinated with the Analog clock project. It looks like a pretty nifty 1st project. But, of course, I couldn't copy it straight from the book! What would be the fun in that! So, with that in mind, I decided that the clock would be even cooler with LEDs for the seconds. Of course, I could add 60 more LEDs and 4 more driver chips and a PIC with more outputs (expensive and messy). How about using nifty bi-color LEDs. But how to drive them? Instead of using 4 x 1-of-16 demuxs like in the book, how about logically using an 8x8 matrix with 2 x 1-of-8 demuxs? This would provide room for 64 LEDs. Then I started wondering how to drive the bi-color. I figured out how to use a pair of tri-state demuxs (one with transistors) on each end of the matrix, but it seems messy. Then I realized that I could use 3 lead LEDs and simply consider it as an 8x16 matrix. I'll use program code to duty-cycle between the seconds LED and the minutes LED. So I'll only need one of the LEDs lit at any instant. (By the way, I think that this approach swaps 3 of the big 1-of-16 demuxes for 1 smaller 1-of-8 demux. Instead of 60 resistors, you would only need 8 [on the 1-of-8 demux]. You may need to add 8 transistors - see below.) But I'm a bit stumped on a few things. The last circuit design I did was college (and that was some time ago)! Can anyone help with... CONSIDER: Since each demux provides 1 LOW and the rest high, I'll need to invert the output of one of the chips (the 1-of-8 is smaller). If I do this, I'll be able to directly light one of the LEDs. However, most of the remaining LEDs will have reverse current applied (some will be HIGH-HIGH or LOW-LOW). QUESTION: Since these are diodes, is it OK to have reverse current applied to all of them? (With 120 LEDs [in 60 packages]: 1 will have forward current, 15 will be high-high, 7 will be low-low, and the rest [97] will be reverse biased.) QUESTION: Would it be better to add 8 transistors on the 1-of-8 allowing the selected output to be high or float? (This would result in 1 with forward current, 15 high-high, 7 low-float, and the rest [97] high-float.) QUESTION: If I need transistors, can someone recommend a part#? I need PNP because the active output from the demux is low, right? I also have a few questions about Myke's project info. QUESTION: Myke shows R1 and R2 being tied to "PIC". I assume that this should be "Vcc"? QUESTION: The other end of the battery should be tied to GND, right? QUESTION: What is the purpose of the 5 caps C4-C8? -- ----------- Jim Brunner _________________________________________________________ Do You Yahoo!? Get your free @yahoo.com address at http://mail.yahoo.com -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

This is okay as long as the reverse voltage isn't reached. Often this isn't stated explicitly but is 5v for most LEDs (accourding to the tech support bod I e-mailed when I was going to do exactly the same thing). Ben > QUESTION: Since these are diodes, is it OK to have reverse current > applied to all of them? (With 120 LEDs [in 60 packages]: 1 will > have forward current, 15 will be high-high, 7 will be low-low, and > the rest [97] will be reverse biased.) > -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spam_OUTlistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

At 06:13 PM 8/23/01 +0100, you wrote: >This is okay as long as the reverse voltage isn't reached. Often this isn't >stated explicitly but is 5v for most LEDs (accourding to the tech support >bod I e-mailed when I was going to do exactly the same thing). Any decent LED data sheet will have the reverse voltage specification. Usually spec'd at 5V, though I think I've seen 3.3 maybe once. Actual LEDs (by my tests, on the new known units of very recent manufacture) in the standard cheap colo[u]rs (super red/green/yellow) break down at considerably more than 5V, more like 12-30+ volts. I didn't look at your circuit, but if you have several in series, you can multiply the voltage rating (eg. 2 LEDs in series are good for at least 10V if the rating is 5V). Best regards, =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Spehro Pefhany --"it's the network..." "The Journey is the reward" .....speffKILLspam@spam@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body