Modify a 7-Segment display

Discussion in 'The Projects Forum' started by KevinC, Nov 16, 2006.

  1. KevinC

    Thread Starter New Member

    Nov 16, 2006
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    0
    Hi,
    I am trying to design a 7-segment display (manual binary switch controlled) something similar to the one found on this site, however I need the display to be larger than the average 7-segment display chip. I was wondering if the display could be made of 5-6 LED's per segment to create a single digit. The resulting number of LEDs would be around 39 per digit, thus making a four digit display approximately 3.5" high, and 5.5" wide. I have been out of the electronics field for awhile and need help with chip identification and specs. The current would be significant compared to the conventional 7-segment display chip. I am thinking of powering the circuit with a 6Vdc or 12Vdc supply. I was thinking of the circuit shown on this site, but I don't think there would be enough current to drive the amount of LED's per segment to make them bright enough to see.

    any suggestions are welcome. This circuit is for personal use only.
    Thank you,
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    I've done that. I used strings of 6 green LED's per segment. I placed each string in series and fed them from a 16 volt supply, with 120 ohm current liniting resistors. You only need current in the range of 10 - 20 ma for each LED string. If you use blue LED's, 10 ma will be very bright.

    I made a pc board for the LED's, and drilled holes in a black plastic box so the LED's could poke through. You get good contrast that way, but also tired of drilling holes.
     
  3. KevinC

    Thread Starter New Member

    Nov 16, 2006
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    0
    Hi,

    thank you beenthere, did you use the same chips as mentioned in the "7-segment display" circuit found on this site? can they handle the current needed for all the led(s)? at most the current throught the chip would involve all the leds... of that digit, approx 39 leds? and would this circuit run for a long time with a 6v store bought battery? or could it be done using 12v car/or atv battery I didn't think the chips could handle it though? I also had a very simple design in mind that involved manually switching each segment.. but that would involve alot of switches (28)for a 4 digit display!!!

    Thank you,
    Kevin
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    What I did was use a 7 segment driver to switch 2N7000 hexfets that actually passed the current for the LED's. My big display ran in parallel with a conventional .7" display.

    Green LED's each need about 1.5 volts applied befor they conduct. For 6 in series, that's about 9 volts. For a 12 volt battery, that leaves 3 volts over. To limit current to 10 ma, you need a 300 ohm resistor. An atv battery would run for days. Even with 4 displays, that's 70 ma/digit, or 280 ma total.

    You could use a rotary switch and isolation diodes to send current to the correct segments, so one switch would handle one digit.
     
  5. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
  6. KevinC

    Thread Starter New Member

    Nov 16, 2006
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    0
    thanks mrmeval,

    I think that site is located in the U.K. I would imagine shipping would kill the price... but helpful just the same! thank you both for replying all advice definitely is getting me closer to a finished design.

    Thanks again,

    Kevin
     
  7. KevinC

    Thread Starter New Member

    Nov 16, 2006
    4
    0
    Beenthere,
    I thought of a rotary switch as well, but wouldn't each digit cancel out the other? or am I confusing myself? I thought that if you made a circuit that created the number 8 then ran that to the "8" on the rotary switch would that not make an 8 when a number 1 is required by putting the rotary switch to "1" because all the led segments are essentially connected?

    I hope I'm not confusing the issue too much!

    Thank you,
    Kevin
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    That's what the isolation diodes are for - so you can source the same segment from multiple taps.
     
  9. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
    Not cheap but in the US, probably way too expensive but these are 4" high ones. It has the driver IC, a CPU socket and other interface gear installed. 45$ for the two digits.
    http://www.futurlec.com/Double_7_Segment.shtml

    This one would be cheaper but dumber and is a single digit for 12.90
    http://www.futurlec.com/LED_7_Segment.shtml

    They seem to be a good site and I may get the 8x8 matrix they're offering.
    http://www.futurlec.com/

     
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