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7-segment display 3-to-8 decoder logic circuit (please help, thank you)

ladyfluff2

Nov 23, 2015
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Hello everyone,

I am required to create a logic circuit that uses a 7-segment display with a 3-to-8 decoder to show the number 1 5 6 6 9 5 5 7 sequentially on a single display.

Some notes:
  • You are NOT required to simplify equations - rather, you're required to use a 3-to-8 decoder to simplify/reduce the number of logic gates.
  • You only need to submit the circuit built with the decoder.
Attached is a Word file containing my Truth Tables and derived Boolean Expressions. I'm really hoping somebody might be able to take a quick look at these to make sure I've been doing them correctly. Also included is what I hope/believe to be the start of my circuit containing the 3-to-8 decoder...is it correct?

It is at this point that I am totally stuck. I have ready many tutorials but just can't seem to grasp how this works. I can not derive the circuit from my equations, I simply find it too difficult. Could anyone please show me one or two sections showing how my equations translate into the circuit and then how that circuit fits into the larger circuit as a whole. The decoder has really thrown me off.

I hope that I have provided you with enough material to show you that I've really been trying very hard to understand this task. It has taken a lot of work with my very basic knowledge to even get to this point! For the last two days I've been trying to understand the processes on my own but now I feel I really do need help. If you need any further information then please ask.

Thanks so much for your time and any help you can provide.
 

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Laplace

Apr 4, 2010
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I understood the instructions to say you were not to build the 3-to-8 decoder but rather use the output of the 3-to-8 decoder to drive the logic circuit which you are required to build. So the logic circuit will have 8 inputs and 7 outputs, and only one of the 8 inputs will be active at any one time, since each input represents one of the 8 digits in the counting sequence. I think you need to start with an 8-input to 7-segment truth table.
 

Luke Vassallo

Dec 10, 2014
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The equations you derived, can be used to implement the sequence without using the decoder.

I see two ways how this can be implemented either as Laplace send using the 8 outputs of the decoder, but this will result in more complex equations because you only have 1 variable. Or you could implement those equations into a circuit and use an and gate with the inputs as the 'enable' which will activate the each function.

I'm not sure the latter is fine because basically, it's like adding a decoder without the need for it...

Also your boolean expressions can be further simplified with de Morgans laws
 

Colin Mitchell

Aug 31, 2014
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An absurd request like this simply needs a microcontroller and no messing around.
The secret in life is to do something without having any intelligence and without having to do any thinking.
Look at all the multi-millionaires.
 

Laplace

Apr 4, 2010
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The secret in life is to do something without having any intelligence...
Now I thought that the secret in life was to recognize a source of bad advice and not take any of it.

Do you suppose that the reason for this task is to obtain a unique display driver? Or might the real purpose be to get the student to think about implementing logic functions with decoders? Yes, there will also be a time for the student to think about implementing logic functions with microcontrollers. But until then the problem calls for a solution similar to this:
decoder-display-driver.png
 

Colin Mitchell

Aug 31, 2014
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Firstly, if the answer was simple, someone would have provided the answer by now. It has already been 1,237 minutes since the question was asked.
Secondly you are only providing part of the answer. The simple part. I could have done that in a twinkling.
How are you going to produce the odd sequence?????
That's the real question.
That's why I don't muck around and waste time. I simply produce the program, complete with delays, and get on with the next thing I don't have to do.
 

Colin Mitchell

Aug 31, 2014
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"If the answer was simple, someone would have provided the answer by now."
"I think they have."
I don't see anything close to an answer.
Where is the "random" number generator.
 

Laplace

Apr 4, 2010
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Where is the "random" number generator.
So it is obvious that you don't understand the problem and you would get a failing grade for this homework. There is no "random number generator" called for, but rather for the "sequential display" of a particular string on numbers. So the input to the logic circuit is the sequential count (from 0 to 7) and the display segment driver handles the rest.
 

Colin Mitchell

Aug 31, 2014
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I understand what has to be done.
All you have done is create a table of the requirements.
I call the output a "random"sequence because it does not represent any logical increment or decrement.
No-one has come up with any way of creating this random sequence.
 

Harald Kapp

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I call the output a "random"sequence because it does not represent any logical increment or decrement.
The logic is that this sequence is required by the task description. Who knows what it may be good for - if only for testing the student's skills.
No-one has come up with any way of creating this random sequence.
Of course they have. The circuit simply steps sequentially through a table that just happens to contain the sequence 1 5 6 6 9 5 5 7 to be displayed. The table is here realized as a set of logic functions.
 

GPG

Sep 18, 2015
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for testing the student's skills.
Yep. But there is still the use of gates to achieve the final result. I have scribbled down a solution that takes into account available parts (minimised) and I would like to see the solution
 

Colin Mitchell

Aug 31, 2014
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"Of course they have. The circuit simply steps sequentially through a table "

THE CIRCUIT ?????????

You have created NO solution. You have just given simple theoretical possibilities of how it may be done.
The hard part is actually providing a circuit that ACTUALLY does all the work and produces a RESULT.
All the rest is HOT AIR.
 

GPG

Sep 18, 2015
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"No answers will be given, only guidance"
 

hevans1944

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Jun 21, 2012
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I find it interesting that it's been almost a month since the original post, and the OP has apparently fled this scene without comment, probably to get "help" elsewhere. This thread is yet another example of the GIGO principle: Garbage In, Garbage Out. Too bad the OP didn't more clearly define the "problem" to be solved, but that also seems to be common here. <sigh> Good thing only a circuit diagram was required, so no valuable parts or construction time were wasted... yet.:rolleyes:
 
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