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Driving LEDs with a parallel port

Discussion in 'Electronic Basics' started by dnrg, Aug 3, 2003.

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  1. dnrg

    dnrg Guest

    Please excuse my ignorance here, but if I use all available pins to
    drive 8 LEDs, would I still need resistors for protection? Adding an
    optocoupler for protection, whatever that is, is probably beyond my
    present abilities to put circuits together.

    I've seen some circuits driving 8 LEDs that use no resistors, and
    other circuits using seemingly arbitrary resistor values. I think the
    idea is to cluster and use 8 LEDs together rather than, say, two
    superbright LEDs due to the small amount of amperage available per
    pin. Apparently, one pin will only supply enough current to light one
    LED dimly.

    Also looking for an easy to understand tutortial on the web for
    controlling circuits from the parallel port. Can I buy a spare
    PCI-card parallel port so I don't fry my internal one? How much are
    these going for these days if they're even available?


    - Dana
  2. Lord Garth

    Lord Garth Guest

    It is always a good idea to use a buffer between the output and the LED.
    The buffer output should limit the LED current unless you want to
    damage the lamps.
  3. John Jardine

    John Jardine Guest

    You get about 4V for a logic high out of the port. Leds need about 3V so add
    a resistor that will drop a volt while limiting the current out of the port
    to say 5ma. (the port naturally can't supply more than a few ma's anyway).
    As you mention, the actual value is pretty arbitrary and something like
    220ohm would be OK. More for port protection than anything else.

    After a number of years use I managed to total my LPT port last month but
    fortunately was able to buy 6 ex equipment IDE PCB's for the princely equiv
    of 2$. These are for the older ISA bus and my old workshop PC. The PCI PCB's
    will be dearer but a spare is well worth having if you are serious about
  4. Si Ballenger

    Si Ballenger Guest

    You want to protect the parallel port on the MB. Getting an add
    on parallel card would be good protection. Not sure if the MB
    port could with stand a dead short on the pins with out damage.
    Have the parallel port pins operate transistors that switch
    on/off an external power supply for the LEDs.
    I've got a web page (below) that has some simple ways to control
    the parallel port pins just using notepad. Note that NT/2K/XP do
    not allow direct user level access to the parallel port hardware.
    A program called userport can fix this. A google search for words
    like parallel port and relay, LED and such will provided many
    sites with info. The bottom link has a decent sounding parallel
    port card for $8.50. SIIG makes a nice dual parallel port card
    for ~$50.
  5. Rich Grise

    Rich Grise Guest

    I'd say it's a crapshoot if you want to hang an LED right on a
    printer port pin - the schem. of the original shows an LS374
    which can source 2.8 mA, and sink 24 or something. So you could
    use them to drive the low side, but there's no 5V vcc coming
    out the port. So the temptation would be to high-side drive them
    and let the current fall where it may, but I wouldn't be
    comfortable depending on a limitation of the totem-pole output
    as my LED current limiter. What if somebody slapped in an HCT374?
    And how long can an LS374 source this 2.8 mA at +2.4v? Per buffer.

    If you can snag the PC's +5V somewhere, you could low-side drive
    the LEDs, otherwise you'll need external power, in which case you
    might as well build a buffer.

    Good Luck!
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