# Simple 3 inputs AND Circuit

Discussion in 'Homework Help' started by MSIE, Aug 18, 2014.

1. ### MSIE Thread Starter New Member

Aug 14, 2014
2
0

I assigned the following pin number to all inputs:

Input A - Pin1
Input B - Pin2
Input C - Unsure which pin to use
Vcc - 14
Gnd - 7

Since IC 7408 only accepts two inputs each time. Where do i connect the third input (C)?

Picture Source: From Doctronics

2. ### atferrari AAC Fanatic!

Jan 6, 2004
2,612
738
Why don't you start by looking for what you actually need ?

Much easier.

BTW, are you actually going to use a TTL part? They require too much power. Rather dated. Look for LS or CMOS.

3. ### MSIE Thread Starter New Member

Aug 14, 2014
2
0
The picture embedded in my first thread. I'm not looking for performance parts. Just want to learn something out of a basic circuit.

3 inputs connected in 2 inputs AND gate to generate an output. I'm more concerned on where to connect the third input on the IC itself because i'm a little confused right now

4. ### atferrari AAC Fanatic!

Jan 6, 2004
2,612
738
OK to the performance.

Since this is homework, I am not sure if you MUST consider that part.

The reasonable thing is to use an AND gate with three, four or eight inputs. (Unused ones should go to a logic "high" or "1").

To show you that more do exist.

5. ### atferrari AAC Fanatic!

Jan 6, 2004
2,612
738
Maybe you could use both inputs from gate A, plus one input from gate B (and the unused one, to "1").

Outputs of gates A & B as inputs of gate C. Resulting output is in the output of gate C.

Are you suposed to test this in a breadboard?

6. ### ericgibbs Senior Member

Jan 29, 2010
2,425
370
hi msie,
Consider what the Output state of the 7408 2 inp AND will be IF both inputs are high, logic '1'.
Then think how you would connect a 2nd 7408 to the 1st 7408 in order to get a 3 input AND logic function.

E

7. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,297
482
7408 does not have 3 input AND gates.

Like others said, you have several choices, you simply need to choose one of them:
* use different ic, that has 3 input AND gate
* use multiple 2 input AND gates to produce one 3 input AND gate
Pick one.

8. ### tshuck Well-Known Member

Oct 18, 2012
3,531
675
Looking at this from a Boolean perspective, remember the Boolean associative and commutative properties?

The associative property, essentially, says (AB)C = A(BC), where the commutative property says AB = BA.

So, if you are to implement ABC, you can implement it as A(BC), (AB)C, B(AC), or any combination thereof.

From another perspective, let Y=AB, or the result of one, two-variable AND operation. Then ABC = YC, another operation.

Once you understand the implications of what this means, knowing which pins to use will become apparent.

9. ### atferrari AAC Fanatic!

Jan 6, 2004
2,612
738
My bad. I should have left for him to do part of the effort to solve the problem.

Sorry for actually not helping him.

10. ### djsfantasi AAC Fanatic!

Apr 11, 2010
2,727
793
It's easy to get carried away. I almost posted the full solution schematic.

11. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,297
482
Almost only counts in horse shoes and hand grenades.
☺7