# 7-segment displays

Discussion in 'General Electronics Chat' started by roflmcwaffle, Mar 28, 2007.

1. ### roflmcwaffle Thread Starter New Member

Mar 28, 2007
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I need to combine 2 7-segment displays to allow me to see 0-15 but i am not sure how to connect them

2. ### Dave Retired Moderator

Nov 17, 2003
6,960
143
Put them side-by-side!

On a more serious note, you need to put this in context. What hardware configuration are you using?

Dave

3. ### beenthere Retired Moderator

Apr 20, 2004
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It sounds more like you need to daisy-chain two BCD counters.

4. ### kender Senior Member

Jan 17, 2007
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What do you want to connect them to?

5. ### electronics wiz Active Member

Mar 15, 2007
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Do you mean that you want the circuit to count in two digits but you don't know how to make them count together?

6. ### CaliusOptimus Well-Known Member

Aug 14, 2005
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i think what he is trying to accomplish is to display a 4bit number on a pair of seven segs. ive tried it a few times myself, with 4, 8 and 16 bit numbers using basic logic chips. no success though, ill be keeping my eye on this thread!

7. ### Dave Retired Moderator

Nov 17, 2003
6,960
143
I agree that this is the intention of the OP, however without some information about the hardware used and configuration we can only give qualitative information.

Dave

8. ### lightingman Senior Member

Apr 19, 2007
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OK..

Let me try and get this rite !!!

I take it that you are wishing to display the decimal numbers 0 to 15 from a 4 bit's of data (BCD).

If this is the case, there are a few of ways of doing it:-

1. Use an EPROM's 8 outputs to drive the displays through a suitable driver (and limiting resistors),
7 to drive the units display and 1 to drive the tens display with the E and F segments in parallel (as only a 1 need be displayed).
Then just program the EPROM to display what you want.

2. Use a CD4511 (CMOS BCD to 7 seg decoder and limiting resistors) to drive the units display and a transistor (with limiting resistors) to drive the tens display with the E and F segments in parallel (as only a 1 need be displayed).
Then work out the logic that you need to drive the 4 bit's of the 4511 and transistor to display the results.

3. There is a device that will do it, but it is an old TTL 74 series device (pre-programmed ROM), and still requires the CD4511.
I can,t remember, but it is a 74184 or 74185 I would not go here, and they are rare!!!

Personaly I would go for the first option, allthough this may sound exessive and a waste of EPROM space, it saves a lot of logic design and chips.
And I have the means of programming the EPROM.

If you need more info or help, I would be willing to work something out for you and program you up an EPROM.

Daniel.

9. ### recca02 Senior Member

Apr 2, 2007
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i hope someone corrects me if i m wrong.
but though i m not good in electronics here is one cheap trick. i dont know if mr lighting man intended to say the same.
units place can be displayed by using bcd to 7 seg decoder which can also be made.
now if i/p are a b c d where a is msb then the input to any two straight led segments for ten's display can be driven by a AND gate having input a.b'.c' that shud do it.

10. ### lightingman Senior Member

Apr 19, 2007
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Yes.. I think I know what you are getting at, But the problem with most 7 seg decoders is that if the input BCD code goes higher than 9, they will produce other display patterns , or just go blank (as with a CD4511).
So allthough the tens may display a 1, the BCD code into the decoder following 9 would be 1010 (or BCD 10), that would not then display a zero in the units display, or the next set of digits up to the 5 to display up to 15....

Hope this helps !!!

Daniel.

11. ### Papabravo Expert

Feb 24, 2006
10,016
1,754
Two seven segment displays will display 00-99 using decimal digits only. One seven segment display will display the values 0-15 decimal as hexidecimal digits using some "styleized" versions of the letters A-F which are used to denote the hexidecimal digits 10-15.

There are also displays which only have a plus/minus and the digit 1 which might be useful in your application. Normally the expense of stocking and holding inventory on two different parts limits the usefulness of this option.

12. ### recca02 Senior Member

Apr 2, 2007
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i find mr P.B's reply as fitting one,
yet since i did post an answer which had practical limitation as stated by mr.
lighting man . i think a ckt which uses dont care condition for 10-15 decimal eq binary codes shud help in overcoming those restrictions.
or
perhaps instead a of using a bcd to seven seg decoder, it wud be better to design a logic ckt for units display.

13. ### lightingman Senior Member

Apr 19, 2007
374
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I have done this many times with 1 to 3 digits. and after condemming the discrete logic aproach because of it's complexity, and as I said before, that is why National semiconductor came up with the 74185 ROM to do it.

Doing 2 digits is easy from 00 to 99 just using a 27C32 or 27C64 (or larger) EPROM and 2 CD4511's. It is just a case of tying the IC'c together.
I can understand that most people would not have access to a programmer for the EPROM. but if enyone is desperate and needs construct a 2 digit BCD to DEC converter, I would quite happily supply the HEX code or pogram an EPROM.

I will try to post all the details, schematics and code within the next few days.

This may help some of you.

P.S. with an EPROM, CD4511 a few gates and a counter, I have full multiplexed display of 10 digits running on my lighting desk.
I know that EPROMS are a bit out of date now (I do most of it with PIC's now),
But they come into thier own for code conversion !!!!!

Hope I can help by posting this info...Good luck to all.

Daniel.

14. ### lightingman Senior Member

Apr 19, 2007
374
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Well there I was opperating lighting for a show this evening, so I thought that during the time between cues, I would knock up a PCB...
along with the schematic..

The trouble is I don't seem to be able to attach the PDF's

I will write the EPROM HEX file tomorrow evening during the show, and try again to put it all here.

Daniel.

15. ### lightingman Senior Member

Apr 19, 2007
374
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Hi lightingman here..

I have been delayed slightly on this one, although I have done all the design work, I wanted to build one and make sure it was working.
Should be done by Thursday, then I will post all the info here !!!.

Daniel.

16. ### cumesoftware Senior Member

Apr 27, 2007
1,330
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I would use two 47LS74 (or 4774) BCD to 7-segment decoders to drive the displays (one per display) and a 74185. I don't recommend the use of CD4511 (or 74HC4511) decoder chips, because they are CMOS. If you use those CMOS chips you will need pull-up resistors to drive them from the 74185, which is a TTL chip. Here is a simplified diagram:

74185 > 74LS74 > First seven segment display
> 74LS74 > Second seven segment display

This is the cheapest way to do it, but you may nof find a 74185 to drive the 47LS74 BCD to 7-segment decoders. You can also use a PROM (or EPROM) (512x8 will do perfectly). Then (after some programming and writing) you can use the 8 data outputs. I really would search for a 74185 chip. They are considered obsolete (don't know why) but they are really helpful. I personally had been in this situation. Worked perfectly.

P.S.: You should prefer TTL to CMOS. They are far more resistant.

17. ### lightingman Senior Member

Apr 19, 2007
374
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Hi and thanks..

Yes your rite, as I said before. The 74185 will do it but no! you can't get them and they pull about 50mA per device (TTL OLD HAT !!!).

The job is done, and it works fine. it is a PCB about 50 x 55mm square and another about 50 x 25mm for the displays, it has on it 1 x 27C512 EPROM and 2 x 4511's, 14 resistors and a 100K x 8 in line resistor pack.

It will handle a 6 bit BCD code and display it from 0 to 99 with leading zero blanking.

I know that the 27C512 is a bit over the top, but it's just that I hapen to have hundreds of them.

I have also designed a 14 bit fully multiplexed BCD to & SEG display driver that uses 1 x 27C040, 1 x 4017, 1 x 4047, 1 x 4511,4 transistors and a few resistors. This will all be here soon....

The only problem that I have now is that no matter how hard I try, I can not seem to be able to attach the schematics, PCB layout and HEX dump onto this site.
After doing all the work, I wish to share it with others!!!

Am I missing something??????

Daniel.

18. ### cumesoftware Senior Member

Apr 27, 2007
1,330
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One more suggestion. I think you should use 74LS47 chips instead of CD4511 chips. Unlike the CD4511 chips, the 74LS47 has the additional logic to blank the display that it is driving if it detects a 0 (at your option). If you use a CD4511, you have to make the necessary logic to blank the leading zeros. Also, a CMOS device like an EPROM will drive them perfectly.

Zip the files in a *.zip file and upload. That should do.

19. ### lightingman Senior Member

Apr 19, 2007
374
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Ah, but not when you use the EPROM. Because if you send HEX "FF" to the 2 4511's it will blank the display (see truth table for the 4511).
So example HEX "87" will produce the display "87" as the 2 halves of the byte are used 1 for each 4511.

HEX "F1" will display " blank 1", HEX "11" will display "11" and so on...

Daniel.

PS. yes I am sensitive, but TTL uses more power, and I am trying to save the world !!!

20. ### cumesoftware Senior Member

Apr 27, 2007
1,330
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Well, that is a better alternative. I forgot that the CD4511 would blank the output if given different codes than the ones corresponding to 0-9 (that doesn't happen with the 74LS47, giving odd symbols instead). So you don't actually need to use it's blanking pin.

Well, my signature is a joke. Never mind, since it is not directed to you. I just don't like very much CMOS tecnology. It does not go with lightning (at the tip of my fingers). Also, my house needs heating, and I don't want to buy heaters or air conditioners. So I'm using TTL chips so my house gets warmer.