# 4008B to two seven segment displays

#### electronicscameron

Joined Oct 19, 2011
14
Hello, I am attempting to make a 4-bit adder circuit which adds two numbers, up to 15, together and then displays them in decimal on two SSDs.

The obvious problem is that a single SSD can only display logic from 0-9 however
(2^4)*2 is greater than 9.

I really need a clever person to help me design a circuit which allows numbers >9 to be displayed over two SSDs where the left most column represents the tens and the right most column to represent the ones.

For example, I want the right most SSD to represent all logic from 0-9 and when values are greater than 9, I want the left most SSD to display 1 and the right most to display 0 forming a '10' until it carries on to the theoretical limit of 4-bit counting.

Many regards, Cameron

#### electronicscameron

Joined Oct 19, 2011
14
I'll either use latching switches or a DIL switch.

I don't think I have conveyed what I am planning very well

I am basically attempting to make a 4-bit/8-bit calculator which can (at this moment in time) only add though I would love to let it be able to multiply, divide, add and subtract though that would be ridiculously complex to design and manafacture.

I have access to genie Micro controllers and various other ICs such as any chip in the 4000 series and any chip in the 74 series. I also have access to 555 chips

#### crutschow

Joined Mar 14, 2008
34,855
The easiest way to do binary to BCD conversions is with a microprocessor. There are algorithms for this on the web, such as Georacer posted.

#### JDT

Joined Feb 12, 2009
657
So, what you want is a 4-bit binary to 2-digit BCD converter in hardware. A few years ago (quite a few actually) I would have suggested using a look-up table in a UV Rom. But I don't think you can get them any more!

You could probably do this with a comparator (to tell when the binary input is greater than 9) and (probably) a couple of adder/subtractors.

With all these things I suggest you start by drawing up a truth table and seeing where this leads you.

Of course you can always Google - I expect someones done it already!

#### elec_mech

Joined Nov 12, 2008
1,500
Per your last post (http://forum.allaboutcircuits.com/showthread.php?t=60890), you can add or subtract using CD40110s to enter and change the input values which is done by counting up and down and skips the adder IC altogether. I provided a link in your last post to another OP who is currently doing this.

If this won't work for your application or you want to do more, then you'll want to use a microcontroller or create a relatively large logic circuit to convert binary to BCD.

#### elec_mech

Joined Nov 12, 2008
1,500

#### JDT

Joined Feb 12, 2009
657
OK. Here is an answer. Actually easier than I thought. Told you a truth table is the answer.

For numbers 0-9 the input is fed to the units unchanged by adding zero. Tens display is "0".
For numbers over 9, subtract 10d (by adding the 2s compliment) and display as units. Tens display is "1"

And hardly any Googling!
Had to get the old 4000 series data book out though.

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#### djsfantasi

Joined Apr 11, 2010
9,188
This is a great circuit! Thanks JDT

#### electronicscameron

Joined Oct 19, 2011
14
Hey guys! Thanks SO much for helping me out (Special thanks to JDT)

I have the circuit partially working in simulation

Is there any way to get the circuit to 2^5 by using eight inputs?

Also, I seem to have come across something strange in Circuit wizard V2 or I am doing something incorrect; when 2^3+2^0 are added together, a 1 in the ten's column lights up and the units column turns off. Obviously it's supposed to display a 9 in the units column.

Again, massive thanks to everyone who gave their time to this. I really to appreciate it

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#### electronicscameron

Joined Oct 19, 2011
14
Bump up my post

#### electronicscameron

Joined Oct 19, 2011
14
Is it possible to do this or am I being stupid?

#### electronicscameron

Joined Oct 19, 2011
14
Hi everyone, the attached image is my current design.
What I am trying to make is a binary to decimal converter for quickly changing high values of binary into a number on some seven segment displays

I have put together this circuit which can add to 15 however ideally I would like it to be able to convert binary up yo 8 bits long as you can easily do binary to decimal conversions from small numbers on your hands

Is it possible by cascading circuits together to make this?
many regards, Cameron

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#### Georacer

Joined Nov 25, 2009
5,182
Check the first image in page 5 here: http://www.ece.ucsb.edu/courses/ECE002/2B_W11Shynk/Lab7.pdf

What I would do if I were you would be this: Use a microcontroller to do the necessary conversions from binary to BCD and output a 4-bit BCD result each time. Using the decoding technique presented in the link I posted, you can power a very large number of 7seg displays.
Each moment you output only one 4-bit BCD result and direct it at only one active 7seg display. By scanning all the displays very fast, you get a seamless result.

#### electronicscameron

Joined Oct 19, 2011
14
Thanks very much, I'll give this a go

#### electronicscameron

Joined Oct 19, 2011
14
Seems like circuit wizard doesn't have the 4553 chip

#### Georacer

Joined Nov 25, 2009
5,182
4553 wouldn't be suitable for an 8-bit number anyway. I posted the link mainly to show you how an external decoding circuit can drive multiple 7segs one at a time, using transistors at their ground pins.

#### electronicscameron

Joined Oct 19, 2011
14