1 decoder but connected to 2 seven-segment

Discussion in 'The Projects Forum' started by mahmoudathab, Jun 24, 2012.

  1. mahmoudathab

    Thread Starter New Member

    May 20, 2011
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    hi can u please help me iam trying to connect 1 BCD decoder to 2 seven-segemnts. by enabling one 7 segemnt and disabling the other (but the output on the disabled signal stays displayed with no change)
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    We need a schematic to better understand your dilemna.

    hgmjr
     
  3. mahmoudathab

    Thread Starter New Member

    May 20, 2011
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  4. hgmjr

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    Jan 28, 2005
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    Without a part number for the 7-segment displays it cannot be determined whether they are common-anode or common-cathode.

    What is their part number?



    hgmjr
     
  5. mahmoudathab

    Thread Starter New Member

    May 20, 2011
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    i don't have the part number. but i think it is a common anode one. will it make any difference??
     
  6. hgmjr

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    Jan 28, 2005
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    The hookup and drive for a common-anode is distinctly different from a common-cathode.

    hgmjr
     
  7. mahmoudathab

    Thread Starter New Member

    May 20, 2011
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    will this schematic work if i used it in a common-anode
    how the number will be saved in the 7seg-display if the common-anode pin was not connected to vcc.
    i found this schematic and i don't konw how it's will work and if it will work or not
    i want to make a two multiplexed 7-seg display units like the one in the FEPST3 KIT
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    What you want to do is called multiplexing. It is basically a waste for two digits, the number of chips you will use to multiplex is likely to exceed the number of decoders you are saving. These kind of chips are cheap.

    Where it gets interesting is if you are into manufacturing. There a minor difference in price can make a huge difference in costs.

    Generally multiplexing is done with 6, 8, or more digit displays.

    So why are you trying to save on the cost of a 75¢ chip such as a CD4511?

    Is this a school assignment?
     
  9. mahmoudathab

    Thread Starter New Member

    May 20, 2011
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    i want to reduce the number of the decoders that i will use so i want to connect 1 decoder and choose between 2 7-segment display
     
  10. mahmoudathab

    Thread Starter New Member

    May 20, 2011
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  11. hgmjr

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    Jan 28, 2005
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    So, am I to understand that you are using this board? If so, then I would recommend you post your source code so that we can see how you are setting up you ATMEGA32.

    hgmjr
     
  12. takao21203

    Distinguished Member

    Apr 28, 2012
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    Most 3 and 4 digit LED displays on the market are multiplexed.
    There are few 5 or 6 digit displays if any.

    Nowadays often microcontrollers are used for LED display.
    The price difference/size for controller with I/O for one digit or two are minor, however, it's interesting to use multiplex to limit stock keeping to smaller controllers, and to have more I/O available.

    Using discrete decoders has become a bit out of fashion, especially for HEX display.

    It's great that Microchip opened all documentation to the general public, and flash writers are available for low prices. Unless these Mitsubishi/NEC etc. ROM mask based controllers (datasheets hard to find, incomplete, software virtually unavailable, e-waste since they are ROM mask based).

    I don't think they ever intended to put these controllers on the open market. And you see the result, their market share is only little these days.

    16f54/16f57 controllers are really super simple to use, and so cheap, you can buy 100pcs and use as many as you like, to replace all these discrete logic based circuits.

    Somehow microcontrollers still have this myth that developement kit costs $1000, and they need large complicated PCBs, as well hard to buy them from distributors.

    And Arduino yes, also have one here for reference...
     
  13. takao21203

    Distinguished Member

    Apr 28, 2012
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    If you want to build a circuit like in the document for this microcontroller board (see above in the thread), better use 74hc164 or 74hc595, because these don't need resistors at all.

    If you want to use common anode, you need PNP transistor from VCC, and this needs negative signal on the base.

    Transistors in general are not good for this, especially for 3 volts, you get both brightness loss + brightness variation.

    Better use digital MOSFET, for N-channel (common cathode) = FDV301 or TSM2303, or 2n7002. These don't need gate resistor as well.
     
  14. WBahn

    Moderator

    Mar 31, 2012
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    Normally the illusion of what you are talking about is created by switching back and forth between two displays quickly enough so that even though one of the displays is dark half the time, the human eye percieves that it is continually illuminated via a mechanism known as POV (persisentence of vision).

    You achieve this king of multiplexing by having your decoder drive all of the pins for both displays but you alternately switch power from one display to the other via the common anode/cathode pin.

    If you really need/want the display that is not being used to continue to display its value, then you have to accomplish several things:

    1) You have to disconnect the drive signals from the decoder to the segment pins since the decoder will be driving the wrong signals (i.e., the signals intended for the other display).

    2) You need to store the values that were being driven, so you need some kind of memory element for each segment.

    3) You need to continue sourcing/sinking the necessary current from each of the segment pins.

    One way, conceivable, to accomplish this is with tristate buffers and capacitors. But the current requirements for the LEDs are going to limit you pretty severely on how long you can hold the display.

    The more practical way is to simply use an 8-bit latch for each display. You write the signals from the decoder into the latch for that display and then the latch will hold the information and drive the display until it is updated with new data.
     
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