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Driving large 7-segment LEDs — Parallax Forums

Driving large 7-segment LEDs

dtalvacchiodtalvacchio Posts: 36
edited 2010-10-08 21:21 in BASIC Stamp
Hello--

I'm using a BS2 to build a digital clock whose digits are driven by the MAX7219. I have the clock working with 1-inch 7-segment LEDs, but I would like to use much larger (3-inch) digits. Does anyone out there have any experience with this sort of circuit? I read the Maxim AppNote for driving digits at higher than normal voltage, but as I'm very new to electronics, I'm having a hard time following how the suggested techniques work. I'd be grateful to anyone who could make this clear to me.

Thank you--
Dominick

Comments

  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-03-01 04:39
    Dominick,

    ·· That App Note is exactly what you need.· What don't you understand about it?

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  • BeanBean Posts: 8,129
    edited 2006-03-01 12:19
    Dominick,

    Large LED digits are usually wired to have several LEDs in series. This means that the voltage needed to light them is often greater than 5volts. If the digits are NOT muliplexed you can simple switch the cathode of the string of LED to ground to light them (with the anode of the string connected to +12 (or whatever)).

    It gets complicated when the digits ARE multiplexed because then you need to switch a voltage greater than 5V and that requires either a special IC (looks like the MAX7219 is limited to 5v) or two transistors.

    I was working on a driver circuit for some 4" LED displays I have, but the special chip didn't work properly. I think I'm going to go back to just using transistors or FETs.

    Bean.

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  • dtalvacchiodtalvacchio Posts: 36
    edited 2006-03-01 17:51
    Thanks for the responses. I'm looking now at the data sheets for two large LED displays, one at 2.3 inches and the other at 3 inches. It seems that the voltage requirement for the 2.3 inches is only 1.7 V (typ.)/ 2.4 V (max), and so these ought to be driven fine by the MAX7219, right? The voltage for the 3-inch display is much higher, at 8 V (typ.)/ 11.2 (max), so this is where I'd have to use the transistors? It seems strange to me that there is such a large leap in voltage requirement for digits which are only 0.7 inches larger. Am I reading this correctly?
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-03-01 18:47
    dtalvacchio said...
    Am I reading this correctly?
    Can you scan the page or provide a link to a PDF?

    Sometimes large displays use a bunch of LEDs in series, ergo the higher VF (VLED).· Sometimes large displays depend on brighter/more powerful elements and a lot of diffusion·to spread the light around inside, requiring fewer LEDs (and have, therefore, lower VF.)
  • dtalvacchiodtalvacchio Posts: 36
    edited 2006-03-01 18:59
    2.3 inches: jameco.com/wcsstore/Jameco/Products/ProdDS/97201.PDF

    3 inches: http://forums.parallaxinc.com/jameco.com/wcsstore/Jameco/Products/ProdDS/202542.pdf
  • BeanBean Posts: 8,129
    edited 2006-03-01 19:04
    Dominick,
    The specs for the 2.3" are for EACH diode. There are four diodes in series so you must multiply by 4. So that gives 6.8V typ and 9.6V max.

    Bean.

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    http://www.parallax.com/detail.asp?product_id=30012

    "SX-Video OSD module" Now available from Parallax for only·$49.95
    http://www.parallax.com/detail.asp?product_id=30015

    Product web site: www.sxvm.com

    "Never under estimate the importance of being nice when you don't have to be."
    ·
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-03-01 19:54
    Yeah, once you go over about 4.5V/segment you need to use the AppNote.· The ones I am using are at around 3.7/segment (IIRC for dual-blue LEDs) on 1" displays.· I will need to recheck that.· In any event the 5" displays I have are 17V/segment but I haven't connected those to a MAX7219.

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  • dtalvacchiodtalvacchio Posts: 36
    edited 2006-03-03 18:04
    Thanks for the clarification. I'll keep studying the AppNote. At this point I don't know enough about transistors etc. to be able to figure out how this circuit works. If I have more specific questions, I'll post them here. Thanks again.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-03-04 02:59
    Dominick,

    ·· That App Note has a lot of different configurations.· Depending on how many digits you're trying to drive, and what configuration they are (CA or CC) we might be able to pick the correct circuit and explain it a little better.· Otherwise an alternative which requires more chips but is simpler is to use shift registers and ULN chips on CA Displays.

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  • youngbillyoungbill Posts: 54
    edited 2010-10-02 13:20
    Chris,I tryed the app note also,On a 7219 max ,so I could drive a large display. 7 mosfets were required and 3 transistors,,,,And that was just for one digit,,,I had 6 digits,,,Is there something a little less complicated than the app notes. Ie do you have any links to a shift register project ,thanks
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2010-10-08 21:21
    I'm afraid I don't have any specific AppNotes since the exact configuration will vary slightly depending on the type of display used (CA or CC), how much current and voltage you need to drive and whether or not the display is multiplexed or direct segment driven.
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