7 Segment Display CC vs. CA

Discussion in 'General Electronics Chat' started by dtreschl, Nov 18, 2015.

  1. dtreschl

    Thread Starter New Member

    Nov 17, 2015
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    I asked this question elsewhere: How do I switch a common anode 7 segment display with a common cathode 7 segment display (like the one here:https://www.adafruit.com/products/813) on a circuit I am working on? Is it as simple as switching the power source's positive and negative leads? (I haven't built the circuit yet - I'm trying to make it in a CDS/Circuit Design Simulator first to make sure that it works but I can't find CC vs. CA displays)

    The answer I received to my question of "Is it as simple as switching the power source's positive and negative leads?" was: In some cases that would be all you need to do. You would also need to modify the control of the segments to reflect that.

    What about this though (My response - I need this answered please):
    In that case, the following is the circuit I am trying to base mine off of - any idea if it would still work? I feel as if the IC wouldn't work properly.
    Link to circuit: http://www.ebay.com/itm/New-DIY-AT8...097680?hash=item23547966d0:g:ZYwAAOSwDNdVnoZB
    If the link doesn't work, just try searching AT89C2051 on ebay and make sure to go to the one with six bits.

    Also, Thanks!
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    Switching power supply leads won't work. It really depends on the circuit, but, in general, you need to invert all of the signals.

    I'll look for some examples...
     
  3. WBahn

    Moderator

    Mar 31, 2012
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    Why do you want to switch from a CA to a CC display in the first place?

    How are you driving the display?
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    The choice really depends on what you are using to drive the segments. Generally you use a common anode with a driver that is good at sinking about 10-20 mA. Conversely you use common cathode with a driver that is capable of sourcing 10-20 mA per segment. Changing the design in mid stream requires a redesign from the schematic, to the layout, to the board fabrication.
     
  5. dl324

    Distinguished Member

    Mar 30, 2015
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    This is an example of CD4511 driving a common cathode display, it can source up to 25mA:
    upload_2015-11-18_9-30-50.png
    And driving a common anode display:
    upload_2015-11-18_9-31-22.png

    The kit you referenced uses dual 7 segment displays. The display you referenced is 4 digits, so it's not an easy substitution. Unclear whether either of the displays are multiplexed.
     
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  6. dtreschl

    Thread Starter New Member

    Nov 17, 2015
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    I forgot to mention - The 4-segment display sin't what I was using for it - just a bad example on my part... I'm actually switching the 3 double segment displays with 6 singles, because I want larger digits that are green. I have already bought the digits, but they are CC, while the originals are CA - that is the reason. Here are the ones that I bought 6 of: https://www.upgradeindustries.com/product/64/Green-7-Segment-Display-(Common-Cathode)
     
  7. WBahn

    Moderator

    Mar 31, 2012
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    So why did you buy CC displays instead of CA ones? Seems like you are just making life harder for yourself than it needs to be. If you bought them my mistake, contact the company and see if you can return them and get CA displays.
     
  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    You just need to invert the signal with seven npn transistors (or a single chip with an array of 7 transistors (or 8).

    uLN2803 is one example chip

    WP_20151118_17_27_18_Pro.jpg WP_20151118_17_27_26_Pro.jpg
     
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  9. dtreschl

    Thread Starter New Member

    Nov 17, 2015
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    Very helpful - Thanks! In the cases of Options 1 &2, are the resistors both before and after the transistors the same? And just curious - not exactly sure because I'm relatively new to electronics - Your drawing shows the transistors, but usually transistors are drawn with rings - those are transistors, correct? I just want to make sure that they're not something else.

    Also, I bought CC displays because I mistakenly thought that the ones I had already tested with at home were CC (They were actually CA) (I bought new ones b/c I wanted green - I'm a bit crazy about those things), and the company doesn't sell these as CA's sadly.
     
  10. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    They are transistors - rings are optional but do add clarity. The resistors to the left are between 1k and 10k, not so critical.

    The resistors to the right are calculated based on how much current your units need, the supply voltage for your display and the forward voltage drop of the display. Something in the range of 270 to 680 ohms for a 5V supply. Use the same for all or some segments will be dim/bright than the rest.
     
  11. dl324

    Distinguished Member

    Mar 30, 2015
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    The pinouts for the dual digit displays usually have the pins along the top and bottom edges, while single digits have the pins on the left and right sides. That fact along with the drive changes you need to make means you're probably going to need to make a daughter board with many connections. If you can make your own board, you can simplify the wiring by having the daughter board connect to the holes for the intended displays.

    Did the dual digit displays have independent digits?
     
  12. dtreschl

    Thread Starter New Member

    Nov 17, 2015
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    What do you mean by independent digits? If you mean that each digit has 10 pins, then no - They had 10 pins total.
     
  13. dl324

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    If the digits were independent, they'd have at least 8 pins each; 9 if DP was connected, and 10 if two commons were used.

    With 10 pins, they're multiplexed. That will dictate how you wire the single digits. You should check the schematic to see how the 3 sets of dual digits were wired. They might all be multiplexed to reduce pin count requirement on the driver.
     
  14. dtreschl

    Thread Starter New Member

    Nov 17, 2015
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    That is exactly what happened (Not independent - multiplexed) but I checked it and wired it all out correctly on my prototype. I'm going to try to rewire it and try the transistor thing mentioned above. May take a few weeks (I'm not very smart - I soldered it on a breadboard...) But I'll let you guys know if it works then. Meanwhile, thanks for the great help!
     
  15. dl324

    Distinguished Member

    Mar 30, 2015
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    If you don't have any ULN type drivers, you could use 74AC14; at VCC=5.5V, they'll sink or source up to 75mA. You might only get a third of that maintaining a voltage sufficient for driving LEDs, but it'll be more convenient than wiring a lot of transistors and resistors.

    IIRC, you can't substitute a Schmitt inverter from any other logic family; they won't have the same current capability.
     
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