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7 segment led displays?

Discussion in 'Electronic Basics' started by kash, Jul 14, 2003.

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  1. kash

    kash Guest

    Hello to all,
    Question about 7 segment displays:
    Do I need to light up the segments individually, or is there
    some sort of encoding. What else do I need if I want three
    7 segment displays to display a voltage ?
  2. Well, you have to decide which segments you want lit up for each
    digit to be displayed. It's pretty standard, but most people do
    either use the term "encoded" or "decoded" for the process of
    converting a binary value into the right segments to use. For
    example, a binary value for '8' would want to be decoded so as
    to light up all 7 segments, A-G.

    But in the case of using three of these digits, it's common
    practice to first light up all the right segments for the first
    digit, while the other two digits aren't powered and are blank,
    then do the same for the second digit, then the same for the
    third, and then back again to the first in a continual loop.
    Each digit gets 1/3rd "duty cycle," which if the current is high
    enough and the loop speed fast enough is just fine and the
    blanking of two out of three digits all the time won't be

    In doing it that way, you can wire-or all the A-G lines and just
    enable each digit through it's common anode or common cathode
    line. Saves I/O pins, if that's desired. There are other ways
    of doing it, but that one blends some simplicity with reducing
    the number of distinct I/O lines needed.

    Of course, you'll still need to work out how to measure the
    voltage range you are looking for, with the precision you want,
    and figure out what value to display.

  3. A seven segment display has individual terminals for each of the segments.
    One is a common, one is typically a decimal point, and the rest access the
    segments, one for one.
    Different types of seven segment display will sometimes have more that one
    common, and they are available as both common cathode or common anode. You
    should have a slash sheet for the display you decide to use, if you build your
    display from scratch.
    To display something like a voltage, you must first convert it from analog
    to digital format, then you must have a circuit that drives each of the segments
    in each of the displays. There are two common methods of doing this.
    The first is to write the bits for turning each segment on or off into a
    latch of some sort. This means that for each digit you display, you must have a
    separate latch device. Also, it means that your circuit must convert each digit
    into seven segment format.
    The second is to multiplex the display, meaning that you wire all the A
    segments together, all the B segments together, etc. The common terminals
    remain separate. Then, you will turn on the proper segments for the first digit
    and apply power (or ground, whichever your display requires) to the first
    digit's common terminal.
    After a brief interval, typically milliseconds, you will then set the
    segments for the second digit and apply power to its common terminal... be
    repeating this for each digit, you can sequentially light them and make it
    appear that the whole display is lit all the time. The display is updated
    hundreds or thousands of times per second, creating the illusion of having all
    the different digits lit at once.
    The reason why multiplexing is so common is that it greatly reduces the
    number of wires needed to light up the display. Consider a three digit display,
    hard wired to latches- you would have to have 24 wires for any arbitrary
    display. But with three digits multiplexed, you only need ten wires. Decimal
    points are often set with a jumper wire in this case.
    If you have your data in BCD format (unlikely, considering your question),
    then you need four bits of data for each digit. A decoder would convert the 4
    digit code into the seven segment output patterns.
    This is not a project for somebody to try without a little expertise. You
    will find that ready made chips with analog to digital converters and internal
    multiplexing are your best bet. The ICL71xx family is a good place to start.
    Here is a link to the PDF slash sheet for these devices.


    Chip Shults
    My robotics, space and CGI web page -
  4. A basic 7 segment display is just 7 LEDs with their cathodes (or
    anodes, depending on type) connected together. It is up to you to
    decide which segments to light.

    Some 7 segment displays do include a decoder and possibly a data
    latch, so you can just supply binary or BCD data, and they will
    display the right pattern.

    If you want several displays to display a voltage (as for a
    voltmeter), you need to design a circuit including an
    analog-to-digital converter, binary-to-BCD decoder, and BCD-to-seven
    segment decoder. (It is probably cheaper and easier to buy a
    ready-made panel-mount DVM)
  5. Mo Morgan

    Mo Morgan Guest


    I have a problem regarding supplying DC to a variety of different pieces
    of equipment. Fairly basic, I know, but it's not the kind of thing you
    want to bungle into and get wrong.

    I'm a musician, and I have lots of equipment that I take on the road in
    rack cases. Some of this equipment has internal transformers and feeds
    straight off the mains; others require external PSUs. This causes two
    problems - the first being the obvious one of having to plug in and
    accommodate loads of PSUs and cable, the other is that these units
    frequently overheat and burn out.

    So, my idea is to build a power distribution unit in a 1U 19" box for
    each rack case. On the back of the box will be a single power connection
    taking AC from the mains, and a series of fans to provide adequate
    ventilation. On the front will be various connections: eight standard AC
    mains outputs and eight adjustable DC outputs.

    Obviously, wiring up the AC outputs is straight-forward, but when it
    comes to the DC outputs I'm afraid my knowledge of electronics falls
    short. The requirement for each of these outputs is as follows: a switch
    to isolate it from the mains, a switch to control the voltage output
    and, ideally, a switch to control the level of resistance. Some of the
    equipment I want to power runs at 9V/600mA, some is at 12V/300mA etc -
    so I'd like each of the DC outputs to be configurable depending on what
    gets plugged into them. Basically, it's eight PSUs in a box.

    Put precisely, I need to know the following: how to convert AC to DC,
    how to step voltage down to a user-defined level (never more than 12V)
    and how to adjust resistance to a user-defined level (between 200mA and

    I'm in the UK, so I'm starting with 230-240V AC. Any help with this
    project would be much appreciated.

    Thanks in advance,
  6. Bill Bowden

    Bill Bowden Guest

    There is drawing on my site for a simple regulated power supply.
    The usual parts are:

    1. Mains transformer to reduce 240VAC to 12VAC.
    2. Bridge rectifier (4 diodes) to change AC to DC.
    3. Filter capacitor to maintain the DC fairly constant.
    Note: the 22,000uF shown is a little large and can
    be around 4700uF for light loads of 600mA.
    4. Adjustable voltage regulator to maintain a constant
    output voltage for varying loads.
    5. You just need 2 resistors to set the output voltage to
    what you want. There is a table showing values to use.

    You will not have to adjust the resistance for different
    loads. The regulator will do that for you. The output
    voltage will always be whatever is set by the resistors
    and the load current will be controlled by the regulator.
    But the regulator (LM317T) may get a little warm under load,
    so you might have to mount it on a metal surface to keep
    it cool.

  7. Terry King

    Terry King Guest

    MM, Bill has given you the right idea.

    (Almost) all these devices use a positive supply, and the
    negative supply connection is connected to their 'common
    earth ground'. This means you can power a number of them
    with their grounds (American for Earth) connected to
    a common ground. This means you can build one 'raw' supply
    of, say, +18 volts and then have several simple regulators
    for several outputs.

    This all assumes you have 'common grounding' and all the
    audio ground loop issues are under control, or minimized with
    balanced lines etc.

    The LM317T is a versatile regulator. If you have trouble finding
    them, drop me a note and your mailing address. Right now I'd die
    with a couple hundred 317's...
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