# How can I build this circuit? (digitally)

Discussion in 'Homework Help' started by ladyfluff2, Nov 16, 2015.

1. ### ladyfluff2New Member

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Hi everyone,

I am a total novice and I am attempting to teach myself. I'm required to design a circuit to take a binary input and display a number on a 7 segment display. I can only use AND, OR, NOT, NAND and NOR logic gates.

The numbers need to be displayed sequentially as follows; 1, 5, 6, 6, 9, 5, 5, 7

I need to use 3 sequencers or bit generators in the circuit as input.

I have uploaded my truth table. I really do not know if this is correct. At this point I am totally stuck and don't really know what to do. If the truth table is correct then I've managed to also obtain Boolean equations from it. However I still don't know how to build a circuit from this.

Any help would be hugely appreciated. Thanks for taking the time to read my query.

p.s. I am being directed to firstly construct a separate circuit for each segment from the truth table you have constructed. Then redraw the circuit to combine the separate circuits to form one complete circuit.

p.p.s I am eventually required to use a 3-to-8 decoder to simplify/reduce the number of logic gates

2. ### KeepItSimpleStupidWell-Known MemberMost Helpful Member

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By inspection #1 isn't right. Only 2 segments should be illuminated. You have more.

You're going to have to decide how your allowed to "drive: these segments or is that not part of the problem? i.e. Open collector inverters with resistors can be used.

Everything can be built from NAND or NOR gates. Everything. Tieing the inputs together of a NAND and using it as one input, gets you an inverter.

Something of this size should probably be done using a Karnaugh map although writig the equations for each output should also work.

"Brute force" cheating - use data selectors.

This http://www.ti.com/lit/ds/symlink/sn5447a.pdf datasheet does most of the work for you.

3. ### granddadActive Member

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Fluff2 , check page 5 of the pdf KISS linked to should point you the right way , seven segments come as common cathode , common anode , if not specified then go with the cathode ( positive ) schematic. ie a 1 at the final gate output turns the led segment on.