Driving an LED board pulled from a network switch

Posted on
  • I recently picked up an older network switch, 26 port, from the town dump. Like most quality network switches, it had an array of LEDs to show the status of every port. 2 LEDs per port, plus a few other status LEDs - close to 64 LEDs.

    Well, that's cute, but what use is it... How do they control all those LEDs anyway?

    Oh - with 74HC595's, like anyone else would...

    I know how to use those! A bit of prodding around with an ohm meter got me the pinout, and I wired it up to an external power supply and an Espruino Pico

    ST_CP -> B8 (GPIO)
    SH_CP -> A5 (SCK)
    DS -> A7 (MOSI)
    MR -> A8 (GPIO)
    Gnd -> Espruino Ground, external supply ground
    Vcc -> Ext. supply +3.3v

    Powered it up off an external supply, and a little experimentation with SPI and:

    >function WriteLEDs(x){SPI1.send(x); digitalPulse(B8,1,5);}
    =function (x) { ... }

    Whole thing took only a few minutes once I had figured out the pinout

  • You are right, looks cute indeed!

  • Cool stuff @DrAzzy! Attach a motor and make some cool POV stuff ;)

  • Nice! It's great to see some old stuff repurposed. But what to use it for? :)

    It feels kind of strange to control each LED off a shift register though - I'd have thought they'd matrix them and use a dedicated LED driver like MAX7219. Maybe they didn't want any flicker for some reason.

    Do you think you could post up the switches name/product ID? It might help others find the info if they're googling for it?

  • Don't have the name/part number anymore. That was written on the box, which has been thrown out already.

    It would make sense for them to not want to have flicker, yeah - it was a nice switch, with the little plugin modules for higher speed connections and all, and you want to be able to clearly tell if there's any activity on the port. If there's any flicker, maybe they felt that could be confused with activity on a port, or something like that?

  • Possibly - the MAX7219 refreshes really quickly, but maybe it was something related to interference?

    Also I guess it's very possible that it's cheaper to buy 8 shift registers than one special purpose LED driver :)

  • If I were to place my uneducated guess, they used the LEDs as a rudimentary full systems check. Make a jig with some phototransistors and place it on top, give it power and network cables with known activity on them. If LED blinks in correct pattern everything is working.

  • Post a reply
    • Bold
    • Italics
    • Link
    • Image
    • List
    • Quote
    • code
    • Preview

Driving an LED board pulled from a network switch

Posted by Avatar for DrAzzy @DrAzzy