7 Segment Display Decoding

Discussion in 'Homework Help' started by CMonster, May 5, 2009.

  1. CMonster

    Thread Starter New Member

    May 5, 2009
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    Hello all, and thanks in advance for the help. I'm trying to hook up two double 7 segment displays as part of a larger project. Unfortunately after hooking up the first one and trying to test it, nothing happened.

    I'm using an HP QDSP-K347 common anode double display (datasheet: http://www.smcelectronics.com/DOWNLOADS/DP04.PDF)

    As well as two 7446 decoders (http://pdf1.alldatasheet.com/datasheet-pdf/view/27428/TI/SN7446AN.html)

    I'm pretty confident that my decoder output is going to the correct pins on the display. I've got both common anode pins to 9v as well as 330 ohm resistors before each segment. My 7446 is connected to 9v and ground on the correct pins. I've also got 3, 4, and 5 high, which the datasheet leads me to believe should put 2 zeroes on the display.

    That's the part I'm not sure about. Also not sure if I'm putting the correct amount of current through each segment since I could only find one rather lacking datasheet for the display.

    Thanks for reading, I appreciate any help in advance!
    -CMonster
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    Did you count the pins of the display correctly ?

    [​IMG]

    It says TOP VIEW !
    Perhaps try to look at the display with the diode test of a multimeter.

    Greetings,
    Bertus
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Are you sure you have a TTL logic IC running at 9 volts? The smoke comes out at about 6 volts. No TTL is rated to run above 5 volts.

    Also, with 9 volts on the displays, the resistors need to be larger than 330 ohms (those are good for 5 volts). Something more like 560 0r 620 ohms would be correct.

    A BCD to 7 segment decoder should light up the segments a, b, c, d, e , and f (for a 0) with an input of 0000 (BCD 0).
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    As beenthere says the 7446 must be powered with 5 Volts.
    The display may be connected to 9 Volts as the 7446 has an open collector output capable to drive 30 Volts.

    [​IMG]


    Greetings,
    Bertus
     
  5. CMonster

    Thread Starter New Member

    May 5, 2009
    4
    0
    Wow, thanks for the quick response! I'll go ahead and give that a shot. I've got extra 7446s, but do you think I could've burnt the ones I tested out?
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    Did you notice them getting hot?
    I have had the mistake before with a whole board with 30 TTL chips connected to the wrong side of the powerstabelizer.
    (It was an test generator for video).
    The board got 12 Volts on all chips and the chips where getting very hot.
    When I noticed that I cut the power wire and connected it to the correct pins of the stabelizer.
    I was lucky that all chips worked afther the incedent.

    Greetings,
    Bertus
     
  7. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,170
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    LED current limiting resistor @ 5V, IC & Disp. @ 20mA, 2V drop,= 5V-.3-2=2.7V / .o2= 135Ω, or 150Ω, 1/4 W. Did you try grounding pin 3, lamp test; should give "8".[ with new IC @ 5V ].
     
  8. CMonster

    Thread Starter New Member

    May 5, 2009
    4
    0
    Thank you everyone. Bernard, I see you mentioned 150ohm resistors. Can you explain a bit more how you got there? So 330 won't work? Uh-oh...
     
  9. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,170
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    Display rated @ 20 mA & 2 V drop, presume you are using 5 V supply; output of 7446 "sinking" 20 mA, will be about +.3V. V drop of resistor will be about 5V [supply] - .3V = 4.7V, 4.7V -2V[display drop] = 2.7V. choosing an operating current of 20 mA, R=V/I, or R= 2.7V/.02. R= 135Ω.Closest " standard" value higher than 135 is 150Ω, giving display 18 mA. 330Ω will work, giving about 8 mA.
     
  10. CMonster

    Thread Starter New Member

    May 5, 2009
    4
    0
    Thanks, that makes a lot more sense. Unfortunately things still don't seem to work. I'm pulling 12mA total (the supply tells me), but nothing seems to come up on the displays. Not sure what to do. I'll replace what I can and keep my fingers crossed. Thanks for all the help, if anyone has any final suggestions, they would be appreciated.

    Thanks again,
    CMonster
     
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