Building a 7 segment LED display - Please help

Discussion in 'The Projects Forum' started by Big KJ, May 25, 2008.

  1. Big KJ

    Thread Starter Member

    May 25, 2008
    Sorry to ask, but is there someone kindly enough to help me?

    My goal is to buit a LED display, with 7 segment leds or alphanumeric displays.

    I'm building with a rotary switch with 12 positions a video signal switcher.
    This rotary switch was 4 poles so I do have at least 2 extra poles not used yet.

    The objective is that when rotating the switch across the 12 positions have the 7 segment led (in this case I'll need two, of course) displaying the respective position number, from 1 to 12.

    If the project is easy enough it would be great instead of the 7 segment leds have alphanumeric displays with 4 or 6 letters with the name of the source switched. ;)

    Now the big drawback: I can follow an electronical scheme, draw the board and solder the components, but have no kind of technical knowledge so far in order to draw this kind of circuit. :(

    Is there someone who cam give me an help?
    Many thanks in advance.
    Cheers from Portugal
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    Try using a pair of these:

    Your switch positions "10, 11," and "12" will need to send a high to pin 11 of your MSB chip. I would use one of those left over poles to do this. I would run a resistor to ground from each of the inputs, and route +V through the switch to the appropriate input.

    Optionally, I might just make up a label with the names of the 12 signals, and put a hole by each name to accommodate an LED.
  3. SgtWookie


    Jul 17, 2007
    Well, the CD40147b would get him from 10 lines to BCD, but then they'd also need a cd4511b BCD to 7-Segment Decoder/Driver.

    Then something to drive the strobe/latch, etc... getting a bit complicated for just displaying a number.

    Or maybe a cd4054b or cd4055b or cd4056b, if they're even still available. Haven't seen them for a while.

    Sure would be easier to just use a dozen individual LEDs to light up behind some text printed on transparency film telling you what channel was selected.
    Last edited: May 25, 2008
  4. Big KJ

    Thread Starter Member

    May 25, 2008
    Hi Thingmaker3,

    Many, many thanks for your prompt reply.

    Yes having labels with the name and a led bellow would work too :)
    Is unbelieving simple and believe me, I feel kinda stupid :mad: not to remembered of this, many thanks for this great tip!

    Now, if I do still want to buid the two 7-segment led, and I do not really want to abuse on you or anyone, would it be possible to draw me the circuit scheme so I can succed on this?

    For what I read on the TI data sheet and for your great advise I would need to use 2 of these encoders, but also 2 decoders and two 7 segments leds.
    Sorry but have no idea of the decoders needed.

    For what I understand from computers when you refer MSB means the Most Significant Byte, rigth?

    I do understand that positions 1 to 9 on the rotary switch will drive display numbers from 1 to 9 and position 10 number 0 ond the LSB.

    Positions 10 to 12 will give the number 1 on the MSB while the others will return 0 on the MSB. If this is correct on positions 10, 11 and 12 on the rotary switch these will drive the pin 11 of the 2nd encoder, while all the other wil be connectd to pin 15, right?

    But how do I get the numbers 1 and 2 on the LSB in order to achieve the numbers 11 and twelve? Is having these routed again to pins 11 and 12 of the first encoder?

    What should the not used pole on the rotary switch drive to the encoder? VCC (maybe 5V) or ground?

    Again if possible to point me to a full scheme or who knows, having the time to draw it for me would be a must.
    Sorry to ask for that and believe me, I do not want to abuse.

    Again, many thanks, I really got impressed for your quick reply and help.
    Cheers from Portugal
  5. Big KJ

    Thread Starter Member

    May 25, 2008
    Yes SgtWookie, you are probably right as Thingmaker3 is: Yours solution is incredible simplier.
    The only drawback is the space letf on the enclosure to do it.

    Anyhow with all my lack of circuit designing if I can not get a full circuit scheme my only way to go is with Leds and labels.
    If someone can point me out to one or have the will and the time to do it I would apprecciate very much.

    Again thanks for the fast reply. What a great helping place!!!
    Cheers from Portugal
  6. Wendy


    Mar 24, 2008
    Ever hear of a diode matrix?
  7. SgtWookie


    Jul 17, 2007
    See the attached.

    I didn't have some of the IC's previously discussed in my library, and really didn't feel like making them from scratch - so I substituted.

    As you can see when you look at the schematic, getting a couple of 7-segment displays lit up isn't precisely trivial when your input device is a 12-position rotary switch.

    Power and ground connections to the ICs are not shown; you can get those pin assignments from the datasheet.

    The 220 Ohm resistor values are approximate, as I don't know what 7-segment LEDs you'll wind up with. Note that these displays are common cathode.

    Updated the schematic; suddenly realized that the prior incarnation would've overflowed the BCD to 7-Segment display driver IC.

    In the 1st schematic, the LEDs will count from 0 to 11.
    In the 2nd schematic, the LEDs will count from 1 to 12.
    Last edited: May 26, 2008
  8. SgtWookie


    Jul 17, 2007
    You're right, they could do that too. Lots of soldering practice. :D
    Here's your diode matrix. Everywhere there's a "1", you'll need a forward-biased diode connected to the respective LED display input.
    Code ( (Unknown Language)):
    1.     a
    2.    ___
    3.   |   |
    4. f |   | b
    5.   | g |
    6.    ___
    7.   |   |
    8. e |   | c
    9.   | d |
    10.    ___
    12.   a b c d e f g
    13. 0 1 1 1 1 1 1 -
    14. 1 - 1 1 - - - -
    15. 2 1 1 - 1 1 - 1
    16. 3 1 1 1 1 - - 1
    17. 4 - 1 1 - - 1 1
    18. 5 1 - 1 1 - 1 1
    19. 6 1 - 1 1 1 1 1
    20. 7 1 1 1 - - - -
    21. 8 1 1 1 1 1 1 1
    22. 9 1 1 1 1 - 1 1
  9. Big KJ

    Thread Starter Member

    May 25, 2008
    Hi SgtWookie!

    Well, what can I say?... Many, many thanks for the scheme.

    I'll try now to draw the PCB which, for me, is a great achievment.
    I'll need to have 6 of these circuits on the PCB so all the 6 rotary switches can indicate its position.
    I'll look now for the spec sheets of the ICs in order to know the pin outs.

    Again, many thanks from Portugal!
  10. SgtWookie


    Jul 17, 2007
    This is a fairly complex circuit for a beginner - and you want to build six of them? :confused: Anyway, you'll have to really concentrate on what you're doing. There are a few minor errors which are basically missing power supplies; Vcc, Vdd, and Vee, for the IC's.

    You can download a freeware version of the software I used to create the schematic, and I'll attach the schematic in a format that can be read by it. The freeware version is also capable of doing PCB layout, along with autorouting. Since this particular project is not frequency sensitive, it is a good candidate for autorouting.

    The software is Eagle Layout Editor v4.16. You can download the freeware version from:
    Since you originally specified that you wanted 1-12 displayed, I'll attach that version.

    Note that I have not verified if any of these items (except for the 1N4148) are still available for purchase. They were in the library; I used them. The part numbers on the schematic have an "N" suffix; in this particular editor, "N" is a standard for a DIP (Dual Inline Pins) IC. These types are easiest for a hobbiest to work with.

    There are a couple of added items in the schematic that were not there last night.

    See the attached.
  11. Big KJ

    Thread Starter Member

    May 25, 2008
    Hi SgtWookie!

    Sorry to ask for help again, but I've been looking all over I did not found any of the ICs on the circuit.

    I looked at RS and Maplin and even googled the two references and did not found anything. :(

    The references I looked are 4532N and 4056N. Did not imagine who's manufacturing these or any other equivalent, if there's any. :confused:

    Can you, please, do point me out to find these?

    Again, many thanks
  12. SgtWookie


    Jul 17, 2007
    Look for CD4532B and CD4056B.

    They are CMOS 4000 series ICs.
  13. Big KJ

    Thread Starter Member

    May 25, 2008
    You are the best :), many thanks!

    Well, the CD4532B is currently carried by RS Components, the other one I did not found it yet.
    Tomorrow (now is about 4AM here in Portugal...) I'll went onto my local electronics component shop and try to find it.

    Anyhow what I do need to is to have this circuit for the Six 12 position rotary switch so I know at any time whats on each switch.

    As I told before, they will switch video signals from the 12 sources to the 6 preview monitors.

    Wish I have the money for a matrix swicth :mad:, but I tryed already with the rotary switches and it works quite good.
    The only problem is if more than one switch uses the same source at the same time, but I can live with it.

    The need for the Led displays is only to make sure which source is active on each preview monitor at a particular time.

    I drilled the front panel of a 1 unit rack enclosure to secure the rotary switches and do have just the space to put the 6 sets of two 7 segment Led.

    On this 1 unit rack I'll need to put the PCB for the complete circuit and I hope to have the space for the PSU, otherwise I'll have it on the outside.

    By the way, which voltage is needed for this? I'm asking that as I did not found the specs sheet for the ICs yet.

    I'm currently downloading the CAD Soft Eagle software, hope I understand how to use it. ;)

    Is easy to have the circuit 6 times drawned in auto on a PCB?

    Again, many thanks and sorry for all this trouble.

    Cheers from Portugal
  14. SgtWookie


    Jul 17, 2007
    The datasheets for the IC's you get will tell you what they require.

    But generally, for 4000-series CMOS, it's between 3 and 16 volts.

    What voltage you'll actually wind up using will depend upon what you have available, and what voltage @ current your LED displays will require.

    You may not be able to find LED displays like that, or other components. They were just what was in the library.

    I have no clue as to what is available where you are.
  15. Big KJ

    Thread Starter Member

    May 25, 2008

    I already installed Eagle 5 but it says it can not use the Auto route function in Freeware mode :mad:

    I was trying to put your circuit 6 times on the same PCB with all the components but the rotary switches nor the 7 segment leds, as these are to be on the front panel connected to the PCB using cabling (like flat cable).

    For the PSU, well I guess that the way to go is to use the same voltage needed for the leds. Tomorrow I will check on what's available at my local store.

    Even tough I can draw the PCB I don't have any idea on how to do it six times, unless I use 6 separated PCBs.

    If I can not get the draw for the PCB I guess my way to go is using some kind of Veroboard or try to draw the PCB at hand :( believing I do have the skills for it...

    Any chances your program can output the PCB for me?
    Only one circuit and I'll do 6 PCBs or one PCB with the circuit 6 times on it.
    I don't want to abuse, believe me, just trying my luck ;) ; so far it was great thanks to you!

    Again many, many thanks for your great help and sorry to been taking your time.

    Cheers from Portugal
  16. SgtWookie


    Jul 17, 2007
    It would be premature to do the PCB layout until you have the parts in hand.

    Otherwise, it's a fair bit of work to do it in the beginning, and then a lot of rework to make the parts fit.

    You haven't mentioned what voltage(s) you have available, or if you're going to have to build a power supply to go along with this.

    As things are now, all of the resistors in the schematic are individual parts. This could be simplified quite a bit by using DIP (dual rows of inline pins) or SIP (single inline pins) resistor arrays. You'd have fewer parts to keep track of.

    However, I don't know who your suppliers are over there, or if they have a website with a catalog of what they stock that I could read.
    Last edited: May 27, 2008
  17. Big KJ

    Thread Starter Member

    May 25, 2008
    Hi SgtWookie,

    Many thanks for your help.

    Just arrived from my local electronics store and I'm very happy as they have all the ICs needed in CMOS version like you pointed.

    About the 7 segment Led they have quite a bunch either commom anode or common cathode.

    Yes they have resistor arrays but if not the needed value, what I use to do when not founding some parts I buy from

    About the voltage, I do have a 4A power supply already going from 3V to 18V which I can use, otherwise I can always make one.
    Local voltage is 220V/50Hz is that matters.

    As I told before neither the rotary switches, nor the 7 segment led will be on the PCB as I will put these on the front panel.

    To connect these to the PCB I'll use standard flat cable with, if possible, some kind of plug like the IDC connectors on the hard drives or just have these soldered to regular pins on the PCB.

    I hope that this answear to your post, if not, please do let me know.

    Again, many thanks
    Cheers from Portugal
  18. Big KJ

    Thread Starter Member

    May 25, 2008

    Looking at local electronic stores I got the components (if something is needed to be changed they allow me to do so):

    IC1: 4532N (CMOS)

    IC2: 4056N (CMOS)

    R1 to R9: 10k resistors in SIL Arrays of 9 resistors each or regular 10k resistors

    R10 to R19: 220r I found DIL arrays but only with 150r or 510r (if these values can't be used it got to be regular 220r resistors)

    R20 to R24: 1k in DIL arrays or regular 1k resistors

    D1 to D11: 1N4148 ok

    DIS1 & 3(?): I'll buy from the smallest ones they have and if possible in blocks of two 7 segments on the same case.

    On the PCB I can use standard IDC 26 Male plugs (26 pins) so I can use standard 26 wires flat cable soldered directly to rotary switches pins and to the 7 segment led, and on the other end a female IDC 26 female to connect these to the PCB.

    The 6 sets of 7 segments leds and the 6 rotary switches will be on the front panel, not on the PCB.

    It would be great if all the 6 circuits (this very same circuit repeated 6 times) could be on the same PCB, otherwise I'll do 6 equal PCBs.

    For power supply, well, I'll use whatever you think is ok.

    I still was not able to use the Eagle to draw the board (not available on the freeware version), but using the ERC function I got the following errors:

    no SUPPLY for implicit POWER Pin IC2P VEE
    no SUPPLY for implicit POWER Pin IC2P VCC
    no SUPPLY for implicit POWER Pin IC1P VSS

    Hope that you can help me with this, and, believe me, I'm, really greatfull for all your kindly help.

    Sorry for the trouble.

    Cheers from Portugal
  19. SgtWookie


    Jul 17, 2007
    Wow, I didn't know they removed the PCB feature from the version 5 demo :eek:

    You might be able to find version 4.16r2 out there somewhere still.

    Anyway, version 3 is attached.
    I found suitable resistors in SIP (10K) and DIP (220 Ohm) form from the UK supplier's catalog. Part numbers for those are in the schematic.

    OK, board graphics are improved considerably.
    12Pos'nSwitchTo2-7-SegmentBRD3.png is the board art; it's reversed as is normal for etching the bottom of the board.

    The 7-segment displays show on the far left, with their 10-pin dual row header. In the middle is the main circuit board. The 12-contact rotary switch is just a model I had in the library; very doubtful it'll work for what you have.

    There's no reason you have to use the DIP non-bussed resistors; they would reduce your parts count though.
    Last edited: May 28, 2008
  20. Big KJ

    Thread Starter Member

    May 25, 2008
    Sgt Wookie

    Wow! Many thanks. I'm really impressed!

    I ordered from RS the resistors, thanks for the part number, should have these in a couple days.

    The PCB looks great, the only problem is that the image is in very low resolution and when I tryed to print it out it cames with very bad results.
    Perhaps it should be in other format than PNG, maybe EPS or TIFF.
    Any chance it could came at real size?
    Otherwise is exactly what I need! :)

    Just two doubts:
    1. Which voltage should I use?
    2. Looking on RS and others I couldn't found any 7 segment leds with a similar pin out
    What I found, which if you think that's possible, would be great is the following parts:

    and the:

    These are dual 7 segment in one case, one in red the other in green.

    This would be great as I do need to have the displays on the front panel one on top of the other with no room in between; please see the attached pict (the picture is almost to real scale).

    This way I will have 3 groups of 2 rotary switches one in red and the other in green each two with diferent colors (Repeated 3 times giving the 6 switches I need).

    Also, the space left in the front panel gives me no room to do it in other disposition.

    If using these led displays wouldn't take much trouble it would be perfect, anyhow I couldn't find the others (HD-E103).

    If not, please do point me for a substitute, as all the others I found come with diferent pinouts.

    Again I don't know how to express my gratitude for all of your great help and asking for sorry for the trouble I'm giving you.
    Without your help, now I know I couldn't have it done - Many thanks

    Cheers from Portugal