# Using a 3-8 line decoder and OR gates

Discussion in 'Homework Help' started by mcc123pa, Oct 12, 2010.

1. ### mcc123pa Thread Starter Member

Sep 12, 2010
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0
Hi everyone:

I'm stuck on this problem. Here are the directions:

An electronic game uses an array of seven LEDs to display the results of a random roll of a dice. A decoder is to be designed to illuminate the appropriate diodes for the display of each of the six die values. The values are 1 through 6, each on a different die.
a) Use a 3 to 8 line decoder and OR gates to map the 3 bit combinations on inputs X2,X1,andX0 for values 1 through 6 to the outputs a through 6. Input combinations 000 and 111 are don't cares.

I am really stuck at how go on this one. I set up a 3 to 8 line decoder but can't get past that. I have all of the numbers, 1 through 6, converted into xyz form (expressing them in forms of X,X',Y,Y',Z,and Z') but don't know how to put them in as inputs to the decoder or how I am supposed to use OR gates at all.

2. ### Georacer Moderator

Nov 25, 2009
5,142
1,266
I don't see a question here... The decoder will do all the work for you. You give it the 3 bit numbers and it will output 1 HIGH output out of the 8 available. The only "challenge" here would be to rig it so that 000 and 111 couldn't go through, but you just "don't care"

3. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,795
951
Work backwards then. Build up a driver for the LED's. you will need 7 SEVEN of them. BECAUSE???

Build some OR logic to 'turn on' the LED driver(1 of 7) when the appropriate output(s) from your 3-8 chip comes clocking along.

More Hints?

4. ### djsfantasi AAC Fanatic!

Apr 11, 2010
2,812
834
Remember he is simulating the random roll of a die. The following text diagram shows an array of seven LEDs; a digit is an LED, = is a space:

1 = 2
3 4 5
6 = 7

So if the input is "001", LED 4 and no other should illuminate. If the input is "100", LEDs 1, 2, 6, and 7 should illuminate. And thus for the other four possibilities...

If this description is correct, the OR gates would be used to select the required LEDs based on the high output of the 3-8 line decoder.

5. ### Georacer Moderator

Nov 25, 2009
5,142
1,266
If the problem is as djsfantasi describes, then a solution is the following: You need to make a Karnaugh map for each LED. But I still don't see the need to use a decoder, as it can only light one LED at a time, so it's pretty useless. If the OP would be kind enough to describe the problem more thoroughly...

6. ### n1ist Active Member

Mar 8, 2009
171
16
Yes, you can do it without the decoder, but the problem requires one. So the input feeds the decoder, the decoder feeds the or gates, one per pip, to determine for which inputs that pip illuminates.

As for The values are 1 through 6, each on a different die , that sounds like a poorly worded question since the first line limits you to 7 LEDs on one die face.
/mike

7. ### Georacer Moderator

Nov 25, 2009
5,142
1,266
No, actually I just got how the exercise wants the problem solved.

First, you assign one multi-input OR gate for each LED of the 7-segment that will be the display of the die.
Next,you get 1 HIGH output for each number from the decoder. You split that HIGH signal to activate the appropriate segment of the display. For example, the output 1 will drive the two rightmost LEDs.
Each output of the decoder will be responsible for lighting the appropriate LEDs of the 7-seg.

8. ### djsfantasi AAC Fanatic!

Apr 11, 2010
2,812
834
GeoRacer - exactly what I was suggesting...

9. ### Georacer Moderator

Nov 25, 2009
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1,266
From what I understand, you suggest the layout of the LEDs to resemble that of a die. I propose to use a 7-segment, to picture the actual number.

For anyone who wonders what a 7-segment display is: http://en.wikipedia.org/wiki/7_segment