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Propeller Professional Development Board 16 Segment display current draw — Parallax Forums

Propeller Professional Development Board 16 Segment display current draw

I'm re-purposing my PPDB to work with a DE0-Nano and I was wondering why the common cathode pins are not driven by NPN transistors. I calculated that if all 16 segments are on there would be a 277ma current draw. That would certainly damage the DE0-Nano and probably the Propeller so either my calculations are wrong or I'm not understanding how the Propeller pins handle currents.

Comments

  • JonnyMacJonnyMac Posts: 7,419

    It's a development board, allowing the developer the choice of how to drive those LED.

  • Thank JonnyMac. I'll have to dig up some transistors then :)

  • kwinnkwinn Posts: 8,684

    @blittled2 said:
    Thank JonnyMac. I'll have to dig up some transistors then :)

    You could use a couple of ULN2803's, but that would eat up 16 I/O pins (as would transistors). A better choice would be a pair of TPIC6595's or similar chips that combine an 8 bit shift register and I/O buffering. Three I/Os to drive the 16 segments.

  • Thanks for the ideas kwinn. I was thinking the transistors would only be needed at the common cathode pin so you would only need 6, one to drive each digit unless each led segment draws to much current for a DE0-Nano pin.

  • Checkout the this forum topic concerning tonyp12's creation of a 17seg LED driver for the PPDB.
    I have one of his boards and it works very well.
    I don't have a schematic of his design, but I do have a diptrace PCB (*.dip) file for one of his versions.
    I don't know how active Tony is on this forum these days.

  • kwinnkwinn Posts: 8,684
    edited 2021-02-06 16:30

    @blittled2 said:
    Thanks for the ideas kwinn. I was thinking the transistors would only be needed at the common cathode pin so you would only need 6, one to drive each digit unless each led segment draws to much current for a DE0-Nano pin.

    You need to control the individual segments to display the desired characters and the individual common cathodes to select the desired character that will be lit. The DE0-Nano may be able to to drive the segments directly, but they might not be bright enough when multiplexed, in which case a driver may be needed. The common cathode of the leds will definitely need a driver IC or transistor.

    Edit - replaced "Prop" with "DE0-Nano"

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