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Strange LED display

Discussion in 'General Electronics' started by electronicjo, May 12, 2004.

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  1. electronicjo

    electronicjo Guest

    I've managed to salvage a LED board(4 seperate 8-seg displays on one
    board with a simple 12 pin connector) from an old microwave hoping to
    use it in my own application. Apparently, this board has a strange
    design that is difficult to understand:
    The two components between the pair of LED displays are normal LEDs.

    Some things to note:

    1.) 5V - Pins 1 through 4(from left to right on PCB connector) are
    "Display select". When any of these pins are connected to 5V, the
    corresponding display will light up when the pins from Note 2 are

    2.) GND - Pins 5 through 11 control exactly which LED segment bars
    will light up. Also note in the diagram that each segment is
    connected to the next displays' corresponding segment.

    3.) Pin 12 of the connector is the common cathode of the two regular
    LEDs and will only light when Pin 3 is high.

    Now obviously the problem is this: How can each display have different
    numeral values if each segment from each display is interconnected?
    The only solution I can think of is quickly switching each display
    with a number accordingly so that is appears to have different
    values. Unfortunately, the main board of the microwave with all the
    circuitry is damaged, so attempting to analyze that won't work(and
    also the ICs are antique).

    Any comments appreciated.
  2. Rich Grise

    Rich Grise Guest

    Yes, that's precisely correct. (I presume you've verified that the
    segments actually _are_ LEDs). It's called "multiplexing." The program
    would set up what the segments are supposed to be, then send the
    segment information for one digit, and simultaneously activate that
    digit, for a little while. Then, turn it off, and send the segments
    for the next digit, turn it on, and so on. This happens faster than
    you can see the flicker, so they look steady.

    Hope this helps!
  3. Quack

    Quack Guest

    Not strange at all, but completely normal.

    You have to scan 1 digit at a time, so only 1 is on at any one time,
    but fast enough that your eye wont ever see any digit turn off
    (google:persistance of vision).

    You will need to interface it to some form of micro-controller and
    have a play around, you will find many examples searching google of
    how to use these.

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