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Controlling hundreds of LEDs

Discussion in 'Electronic Basics' started by Danny T, Dec 21, 2004.

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  1. Danny T

    Danny T Guest

    Hi All,

    I'm trying to control hundreds of LEDs from my parallel port (8 data
    pins - will soon be replaced with the output of a programmed PIC, also 8

    I'm not an electronics guy (I'm a software developer), so trying to
    control x00's of LEDs from 8 pins looks a bit tricky...

    There's no way to change them all at the same time, so I reckon I'll
    need some sort of "addressing" system. Using some of the pins to select
    which LED I'm controlling, and one for the data.

    Trouble is, even using 7 bits as the "address", I've only got 128
    channels, but I imagine I'd need a bit for "send" too, so I can set the
    address and the data, then have it read in one go.

    What are my options? And what components would I need to achieve
    something like this (I imagine right down at the end, I'll need a number
    of registers for the LEDs. If these come in chips with 8 output legs, I
    guess I'll need 38 for 300 LEDs. The bit between my 8 data pins and the
    computer isn't my area though, so I'm stuck!

    Oh, and this is just a hobby thing, so the cheaper the better. I'm sure
    I can already buy scrolling screens, but I don't have the money, and I
    want the fun of building one! :)
  2. John Fields

    John Fields Guest


  3. May I ask if you can take advantage of any of the status bits?

    Is there any pattern to how the LEDs are turned on and off? Are there
    groups that can bit always be lit simultaneously or must they all be
    controlled independently?

    Maybe some can be lit up sequentially? In this case, maybe a 555 timer
    connected to a counter series could reduce the number of permutations
    required of the parallel port.

    The parallel port data bits can be made to go high and low at different
    frequencies. I believe it operates at 8 Mhz, but at any given moment,
    it should be possible to dictate which pins go high and low. By cycling
    back and forth, you create on/off states with frequencies you can choose
    by software. If you are willing/able to build a frequency detecting
    circuit, this could buy you a lot of additional permutations.

    Consider the following set of bits I used to operate 4 stepper motors
    that needed to turn at different speeds (I'll show 2 of the motors).
    First bit indicates forward or reverse, while 2nd bit indicates step
    or no step.

    Motor 1 (data bits 0 and 1)

    Motor 2 (data bits 2 and 3)

    What my software sent out for the first bits:


    Note that data bits 0 and 1 are cycling at twice the frequency as
    data bit 3. During all this cycling (8 parallel instructions to the
    parallel port), data bit 2 has yet to go high. Circuitry can be designed
    to sense and respond differently to these frequency differences. I merely
    offer a means by which to control a large number of LEDs from a single
    8 bit parallel port.

    I'd like to stop here and see how you answer my questions and how
    you and others in here respond to my frequency and 555 timer ideas.

  4. Of course, you could have some kind of 16 bit register 8 bits
    at a time. I need to head home and would have to think on this.
    I think there are a lot of people in here that know how to do
    this better than me.

  5. John Fields

    John Fields Guest

    I'd like to stop here and see how you answer my questions and how
  6. Si Ballenger

    Si Ballenger Guest

    The below link has some info on individually controlling 320
    christmas light strings via the parallel port. These could just
    as well be LEDs. To turn on/off an LED will probably take an NPN
    transistor and two resistors per LED instead of using a relay.
    You've actually got 12 pins on the parallel port to work with.
    The bottom links are some pages I've made with some simple
    parallel port setups. I like tinkering with the 74HCT259 chips,
    as they demultiplex and also latch, and cost only $.40 each. You
    could use the 4 control pins to select the 259 chip pin and set
    high/low, and the 8 data pins to control other 259 chips that
    would individually clock the selected 259 chips. Get a couple
    chips working as a test, then expand from there.
  7. Danny T

    Danny T Guest

    I want to created an led text display (like in the doctors waiting
    room!). There's no real reason, I'm just toying around with things (got
    a PIC Programmer coming this week) to see how different things can be
    achieved (such as sending data to a large number of objects, without a
    connection to everyone), for my master plan to take over the world :)
  8. Danny T

    Danny T Guest

    This is entirely possible. I've never played with the parallel port
    before, so my first step today was to connect the 8 data pins up to
    LEDs, all connected back to port 25 (ground) and see if I could send
    data. (this worked - I have tacky dancing lights!)

    I didn't touch the status ports, because I wasn't entirely sure if I
    could use them in the same way as the data pins, or if they're
    "special". Currently I'm using a horrible interop DLL (I'm writing
    managed .NET code in C#) and writing to the dataports is simply a call like:

    Output(888, 0); // All off
    Output(888, 255); // All on

    888 being the address of pin1. If I change this number, I don't know
    what will happen - maybe the status ports are the same, maybe not. I'll
    look it up tomorrow when I'm a little more awake :)

    All independant. I want a text scroller, but my point of the exercise is
    to see how it's possible to control lots of things from less pins.
    Eventually, I want to be controlling a little robot or something, which
    will consist of motors, steering etc.. The more things I learn before
    then, the easier it'll be!

    This wouldn't work for scrolling text, where I'd need to control all
    LEDs to show a "frame", wait for a sec, then rewrite them all (possibly
    shifted horizontally by one) for the next frame. It might be possible to
    do something clever if they're all just shifted in rows, but I'd rather
    have the flexibility of a full animated display, not just scrolling.

    By frequency, do you mean the rate at which I'm turning the data pins
    "on and off" (+5v / 0v)?

    I'm a little confused by your example. What your software sent - is this
    8 x 4bit signals, or are you sending 4 signals across all 8 pins?
    If the first, then you're using 2 bits per motor, but I don't have the
    luxury of enough bits for each LED. If the second, I don't udnerstand
    how your motor would know which singals sent are for it, and not the other?


  9. Danny T

    Danny T Guest

    Was just looking at the datasheet for this - I think it'll do exactly
    what I want! Will need a lot of them hooked together though, and with
    the 3 input pins of some being attached to output pins of others, it's
    going to be a bit fiddly on the software side!

    Can I just make sure I understand this correct...

    Using the 3 input pins, I choose which output I'm modifying (a binary
    value of 0 to 8), and send the input on the input pin. Then I clock(?)
    the "latch" pin, which copies the input state to the output pin I
    addressed, and then holds it at that until I change again?

    So to set the output bits alternating, with 1 being on, 2 off, 3 on
    etc., I'd do something like:

    1-3 = address
    4 = input
    1 2 3 4
    0 0 0 1
    0 0 1 0
    0 1 0 1
    0 1 1 0
    1 0 0 1
    etc. But with flashing the "latch" pin on and off in between each?

    It'd vertainly work, but with so much addressing, isn't it going to take
    seconds to do each frame? :-\


  10. One easy way to extend the number of ports on a PIC is to use serial-in
    parallel-out shift registers. The 4094 is a good example. You set up a
    port pin to a value, then 'clock it in' to the 4094 using another port.
    Once you have all 8 bits clocked in, you 'strobe' the data out to the
    outputs. Thus, 3 port pins controls 8 outputs.

    You can also 'daisy chain' these parts, since they have a serial-out pin
    as well. Thus, you can string together a few of them.

    Doing 2d arrays of LEDs can also be done by flashing them on a line at a
    time. You divide the lights up into rows and columns, set up the row
    values for a particular column, then flash it on for a while. By doing
    this faster than the eye can follow (say 1/100 of a second for each
    column) it looks like they are all being controlled independently.
    However, it takes far fewer bits (40 for a 32x8 array of 256 LEDs).
    That's doable with 5 of the shift registers above, and a few milliSMOPS.

    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
  11. Dmitri

    Dmitri Guest

    Danny T wrote:

    Speaking of the scrolling screens, you may want to use the same approach:
    use specialized LED controller ICs. Maxim makes a whole line of such
    devices (so do other people).
    Check out this page at maxim's website:
    This is only one device of the product line. It controls up to 64
    individual LEDs, and can be daisy-chained. You may want to browse through
    links on that page to other ICs to see if there is one that suits you
    better. Make sure you read through the application notes, they are a
    tremendous source of information that pertains to what you are trying to
    do, regardless of whether or not you will end up using Maxim's IC.

    This IC runs for $10.81 at Digi-Key, but you can also request a sample
    from Maxim to play with it and see if it works for you.

    Dmitri Abaimov, RCDD
    Cabling Forum, color codes, pinouts and other useful resources for
    premises cabling users and pros
    Residential Cabling Guide

    Article posted with Newsgroup Archive
    no-spam read and post WWW interface to your favorite newsgroup -
    sci.electronics.basics - 8162 messages and counting!
  12. John Fields

    John Fields Guest

    Good plan, it worked for me!^)

    Probably a good place to start is by defining the size of the
    characters you want to display, and a 5X7 matrix works nicely unless
    you want to get fancy, so what I'd do if it was me would be to set up
    my display with seven rows of LEDs, and if you want to use about 300
    LEDs that comes out to 300/7 = 42.857 columns. If each character is
    five LEDs wide and you have a couple of LEDs separation between
    characters that's a total of seven columns of LEDs per character, so
    if you had six characters that would be 42 columns times seven rows
    of LEDs = 294 LEDs. Not bad.

    Now, to drive the LEDs I'd use garden variety 8 bit serial-in
    parallel-out shift registers like XX164's configured as seven rows of
    six shift registers per row, like this:








    And I'd hook them up like this:

    6 shift registers--->

    7 shift registers

    | | | | | | | | | | | | | | | |
    | | | | | | | | | | | | | | | |
    [R][R][R][R][R][R][R][R] [R][R][R][R][R][R][R][R]
    | | | | | | | | | | | | | | | |
    | | | | | | | +-----+ | | | | | | | +--->
    | | | | | | | | | | | | | | | | |
    +-+--+--+--+--+--+--+--+--+ | +-+--+--+--+--+--+--+--+--+
    | Q0 Q1 Q2 Q3 Q4 Q5 Q6 Q7 | | | Q0 Q1 Q2 Q3 Q4 Q5 Q6 Q7 |
    D7>---+-|Da,Db | +--|Da,Db |
    | | | |
    | | | |
    D0>-+---|CLK | +---|CLK |
    | |___ | | |___ |
    Dn>---+-|CLR | | +-|CLR |
    | | +-------------------------+ | | +-------------------------+
    | +-----------------------------|-+----------------------------->
    | |
    | | Vcc
    | | |
    | | +--+--+--+--+--+--+--+----------+--+--+--+--+--+--+--+--->
    | | | | | | | | | | | | | | | | | |
    | | [1][2][3][4][5][6][7][8]<-LEDs->[1][2][3][4][5][6][7][8]
    | | | | | | | | | | | | | | | | | |
    | | [R][R][R][R][R][R][R][R] [R][R][R][R][R][R][R][R]
    | | | | | | | | | | | | | | | | | |
    | | | | | | | | | +-----+ | | | | | | | +--->
    | | | | | | | | | | | | | | | | | | |
    | | +-+--+--+--+--+--+--+--+--+ | +-+--+--+--+--+--+--+--+--+
    | | | Q0 Q1 Q2 Q3 Q4 Q5 Q6 Q7 | | | Q0 Q1 Q2 Q3 Q4 Q5 Q6 Q7 |
    D6>-|-|-|Da,Db | +--|Da,Db |
    | | | | | |
    | | | | | |
    +-|-|CLK | +---|CLK |
    | | |___ | | |___ |
    | +-|CLR | | +-|CLR |
    | | +-------------------------+ | | +-------------------------+
    | +-----------------------------|-+----------------------------->

    If you used high efficiency 2mA LEDs you could use HC164s, but if you
    used regular 20mA LEDs you'd be better off running them at 10ma and
    using LS164s.

    The way the thing works is that you'd present a parallel 7 bit data
    word to the serial data inputs of the first shift registers on each
    row, then hit all the shift registers with the same clock. That would
    move the seven data bits into column 1. Then you'd get the next 7 bit
    word and clock it into the serial inputs. That would move the new
    data into column 1 and move what was in column 1 to column 2. As you
    kept on loading and clocking data into the array the columns would
    scroll horizontally and you'd see your characters scrolling by. I've
    shown the CLEAR activated by logic, but if you don't have a line to do
    that with, just load ones into the serial inputs and clock them
    through. The logic for the characters is negative true; that is, if
    you want an LED to light up send a zero. If you want to do it the
    other way, just reverse the polarity of the LEDs and connect them to
    ground, but only if you use high efficiency LEDs driven by HC. LS
    won't source enough current to make either high efficiency or regular
    20mA LEDs light up nicely.
  13. CFoley1064

    CFoley1064 Guest

    Subject: Controlling hundreds of LEDs
    <continued from s.e.d.>

    Hi, Danny. The 'HC595 is a serial input, parallel output IC. You output the
    data one bit at a time (Data In). After every bit, you toggle the clock active
    (CLK), then inactive to shift the data bit one further down the line. You then
    present the next bit of data, clock again, and so on. After all the bits are
    shifted in, you toggle the Latch pins active, then inactive to present all the
    data that has been shifted into the ICs, one bit at a time, to the output pins
    of the ICs.

    Your hookup will go something like this (view in fixed font or M$ Notepad):

    D2 o----o--------------------------o-------------------------o-------
    | | |
    D1 o--o--------------------------o-------------------------o---------
    | | VCC | | VCC | |
    | | + | | + | |
    | | | | | | | |
    | | .--------o----------. | | .---------o---------. | | .-----
    | | | RST | | | | RST | | | |
    | '-oLatch | | '-oLatch | | '-oLatch
    | | | | | | | |
    '---oCLK HC595 | '---oCLK HC595 | '---oCLK
    | | | | |
    | | | | |
    D0 o------oData In SQ8o------oData In SQ8o-----oData
    | 1 2 3 4 5 6 7 8 | | 1 2 3 4 5 6 7 8 | | 1
    '--o-o-o-o-o-6-o-o--' '--o-o-o-o-o-o-o-o--' '----o
    | | | | | | | | | | | | | | | | |
    | | | | | | | | | | | | | | | | |
    | | | | | | | | | | | | | | | | |
    ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    No matter how many of the '595s you have chained together, you'll only need the
    3 printer/PIC port output pins, D0, D1, and D2 (with the caveat about buffering
    mentioned above).

    If you're still not clear on the concept, google "serial shift register".

    By the way, you need a 5V power supply that you have to connect to each IC pin
    16 (+) and 8 (-), like it says on the data sheet.

    Another quick (possibly dumb) question...

    If you go to (the URL has a session ID, so prob won't
    work!) and search for the 74HC595, and click through to the product
    page, it lists several:


    but I don't understand the differences between them (there's a hefty
    price difference!). What am I missing?

    Go with the HC595N. The "D" is a SMT package. I believe the "AN" is an
    extended temp range IC.

    A scrolling LED alphanumeric display is a tall order. Each character will
    require a minimum of 35 LEDs (5X7), plus you'll need a line below and a line
    between, which means 48 LEDs per character. And if you want scrolling and
    animation, serial shift registers may be too slow (although not for 6

    You need to decide what you can do and what you can afford. Here's a good rule
    of thumb: at 300 LEDs (about 6 to 8 characters), you'll spend more than twice
    as much on the LEDs as the ICs, If you have more like 3000 LEDs, serial won't
    cut it. For that, the LEDs will be a large majority of the price. Decide what
    you want to do and what you can afford, and go from there (hobbyist LEDs, and
    5X7 LED displays, are available at hobbyist prices at

  14. Si Ballenger

    Si Ballenger Guest

    In the latching mode (used to control the individual LEDs), you
    use the A0, A1, and A2 input pins to select the chip output pin
    you want to set high or low. You set the D data pin on the chip
    at 0v for the chosen output pin to output 0v, or set the D pin to
    +5v for the output pin to output +5v. Then you clock the LE pin
    with +5v to latch the output pin in the desired state (0v or
    +5v). The same A0, A1, A2, and D control lines can be connected
    to all the 259 chips. You clock the LE input on the individual
    chip you want to control. The As and the D can be done using four
    of the output pins on the parallel port. The other parallel port
    pins can be used for clocking theindividual chips You can use two
    of the 259 chips in the demultiplex mode to clock the individual
    259 chips used in the latching mode. Using the eight parallel
    port data pins and 17 259 chips (2 in demultiplex and 15 in latch
    mode), you could individually control 120 LEDs. The output page
    below shows the latching mode schematic, and the ppswitcher3 page
    shows the chips in the demultiplex mode. If you used the parallel
    port control and data pins, this number could be multiplied many

    Using parallel port data pins 0, 1, and 2 to control the As, pin
    3 for D, and pin 4 for the LE clock on a chip, here are the byte
    sequences to turn Q7 on and off:

    00001111 sets all As high (Q7) and the D high for +5v output
    00011111 clocks the chip LE pin high
    00001111 drops the LE pin low to latch Q7 at +5v output

    00000111 sets all As high (Q7) and the D low for 0v output
    00010111 clocks the chip LE pin high
    00000111 drops the LE pin low to latch Q7 at 0v output
  15. Danny T

    Danny T Guest

    Sounds easy... But - milliSMOPS? Are you having a laugh? Google doesn't
    know what they are either! :p
  16. Danny T

    Danny T Guest

    Right, I think I understand - except for what you've labelled as SQ8 on
    your diagram. Does this hold the "last" bit that's about to be dropped?
    (and does this work based on the clock pin, and not the latch?) So if I
    sent 16 bits (clocking active in between each), and *then* latch, would
    I get 16 unique outputs, which I can then redraw before the next latch?

    Think this will be the easiest way, though I don't know how fast I can
    shift the data (300 LEDs is more than 600 outputs)!


  17. Danny T

    Danny T Guest

    The AN is half the price! I was just checking the "Technical Info" PDFs
    on rswww, and I can't find any difference (in temp, or anything else) :-\

  18. I am busy at work and may need to put off most of my answers until later.
    If you are going with a scrolling display, then a serial to parallel type
    of IC is a good start, but it depends very much on how many LEDs you want
    to control. I could suggest considering exactly what the dimensions of the
    display will be and letting this dictate design requirements. I think this
    reflects real world engineering.

    I could for instance consider using the serial to parallel IC only to
    control the vertical axis of the very last LEDs at the right end. You should
    be able to control many of these with a serial to parallel IC. Since you have
    a scrolling text, all of the status of the LEDs left of the very first
    LEDs are dictated by the status of the LEDs on the right end. All
    subsequent LEDs can be lit by activating a 555 timer IC whose output is
    connected to a counter IC. Each step of the counter activates the next
    LED to the left of the preceding LED. Each preceding LED turns off. This
    in principle can very cheaply and efficiently control many hundreds (even
    thousands) of LEDs.

    My example showing the data bits shows only the first 4 of the 8 data
    pins as this is all that is actually required to drive two stepper motors,
    one pin dictates forward or reverse, and one pin dictates step or no step.
    4 pins therefore can control two motors.

  19. CFoley1064

    CFoley1064 Guest

    Subject: Re: Controlling hundreds of LEDs
    If you've got an 8-bit shift register, what happens when you shift in the 9th
    bit? The joke is, "It falls into the Bit Bucket, of course", meaning it
    disappears out the end. Seriously, if you've got a line of serial data, you
    can chain 8-bit shift registers so the first bit is shifted into the second IC
    when the 9th bit is shifted in. It's a simple shift register concept, and the
    SQ8 pin on the '595 is where you either put the bit bucket (;-) or you put
    another shift register IC.

    If you're doing 5X7 matrix displays, you don't have access to all 70 LED pins
    anyway. You have to address them as row/column. If you do it that way, you
    will minimize the shift register outputs. But, like the professional displays,
    you then have to be concerned about a multiplexing setup where you use
    persistence of vision to go across the columns fast enough that they don't
    appear to flicker.

    I've been following the flurry of posts here, and it seems like you've got a
    ways to go on your understanding of the hardware end of things. If you want to
    do a project over the holidays, possibly you might want to consider a smaller
    concept (like one alphanumeric character display) until your hardware
    capabilities catch up to your software skills.

    Good luck
  20. Danny T

    Danny T Guest

    Thought so. Looks like a nice (*cough* easy) way to do what I want!

    First thing is to build my PIC Programmer - if I don't get past that... ;-)

    I wasn't planning on jumping right into anything, just wanted to make
    sure any smaller "test cases" are headed in the right direction, and not
    that I learn to control 15 LEDs and then someone says "that'll never
    work for more than 20"!

    Thanks for your help (and everyone else). I've learnt a lot before I've
    even started fiddling with hardware :)

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