# Controling an output without using extra logic gates ?(basic)

#### SmallRedMachine

Joined Feb 25, 2017
48
I have three inputs A B C and there is one case where A=0 B=0 C=1 gives me output 1 and I want it to be 0, I don't want to use any extra logic gates other than one NAND and one NOT gate, also I don't want the output to be 1 when A=0 B=1 C=0. I have thought about using transistors and comparators but they can't satisfy both these conditions.
I just don't know how to tell the circuit to "Output is 0 when A and B are both zero or when only B is 1" without using logic gates... is there actually any ways to this ?

#### Ramussons

Joined May 3, 2013
1,316
Output is 0 when A and B are both zero or when only B is 1
This simply means that Output = A, there is no logic gate

#### SmallRedMachine

Joined Feb 25, 2017
48
This simply means that Output = A, there is no logic gate
I haven't put the full truth table here because they are working just fine, it's only the two conditions I mention above that is giving me trouble.

#### Ramussons

Joined May 3, 2013
1,316
What have you setup using that NAND and NOT gate?
What is the Truth Table of what you need?

#### philba

Joined Aug 17, 2017
959
You can't expect people to come up with changes to your logic without knowing your full truth table.

Do you know how to use karnaugh maps? Once you understand them it's pretty easy to get to the minimal Boolean equation.

By the way, is this a school assignment?

#### WBahn

Joined Mar 31, 2012
27,500
This simply means that Output = A, there is no logic gate
First off, even if this is correct, it would mean that you just jumped in and did his homework for him. That's NOT allowed.

Second, he hasn't said what the output should be for all of the other combinations. Even if it is reasonable to assume that the output should be 1 for any combinations that don't meet either of these two conditions, that would mean that the output should be 1 for A=0, B=1, C=1 since A and B are NOT both zero and B is NOT the only input that is 1.

#### WBahn

Joined Mar 31, 2012
27,500
I have three inputs A B C and there is one case where A=0 B=0 C=1 gives me output 1 and I want it to be 0, I don't want to use any extra logic gates other than one NAND and one NOT gate, also I don't want the output to be 1 when A=0 B=1 C=0. I have thought about using transistors and comparators but they can't satisfy both these conditions.
I just don't know how to tell the circuit to "Output is 0 when A and B are both zero or when only B is 1" without using logic gates... is there actually any ways to this ?
It sounds like you have a black box circuit that is producing a certain output as a function of three inputs and you want to change the output for one of the input combinations but leave the others unchanged by adding some additional circuitry that has access to all three inputs and the output of the black box. Is that correct?

If so, then it is important to know the entire truth table for the black box to ensure that the output for the other input combinations are not affected.

Is your description of "Output is 0 when A and B are both zero or when only B is 1" a description of the behavior you want, meaning that the output is 1 for all other situations?

Since this is in Homework Help, you need to post YOUR best attempt at solving it.

#### Ramussons

Joined May 3, 2013
1,316
First off, even if this is correct, it would mean that you just jumped in and did his homework for him. That's NOT allowed.

Second, he hasn't said what the output should be for all of the other combinations. Even if it is reasonable to assume that the output should be 1 for any combinations that don't meet either of these two conditions, that would mean that the output should be 1 for A=0, B=1, C=1 since A and B are NOT both zero and B is NOT the only input that is 1.
Hmmmmmmm........ Did'nt think on those lines....

#### WBahn

Joined Mar 31, 2012
27,500
Hmmmmmmm........ Did'nt think on those lines....
It's an easy mistake to make and we've all made it.

#### philba

Joined Aug 17, 2017
959
The homework assignments are pretty easy to spot when you think about it. Somewhat academic terminology with no actual application called out. Also, low post count triggers a ping from my studar. Lots of other clues become pretty obvious when you start looking for them - like posting in the homework forum...

#### WBahn

Joined Mar 31, 2012
27,500
In all fairness, many Homework Help threads start out elsewhere and don't get moved until after someone has posted an answer not realizing it was homework. That's one of the reasons why I put a Mod Note post in every thread that I move (anywhere), so that at least there's some context to fall back on.

Where I have typically gotten tripped up are basic questions that seem like natural ones for someone to ask, such as, "Can I leave unused CMOS inputs and outputs unconnected?" without realizing that these kinds of basic questions are also common homework/quiz questions.

#### philba

Joined Aug 17, 2017
959
The part about posting in the homework forum was a tweak at myself because I realized the post was in the that forum half way through...

#### WBahn

Joined Mar 31, 2012
27,500
The part about posting in the homework forum was a tweak at myself because I realized the post was in the that forum half way through...
I've had the same thing happen. I've had more than once where I posted something and, once posted, saw that it was in Homework Help and had to go right back in and edit it.

#### JoeJester

Joined Apr 26, 2005
4,390
In all fairness, many Homework Help threads start out elsewhere and don't get moved until after someone has posted an answer not realizing it was homework.
I'm sure there are unintentional postings from TS that originated outside the Homework Help area, just as I am sure there are are intentional ones.

When I see one in other area's, I simply report it. If the moderator agrees, they will take appropriate action.

#### WBahn

Joined Mar 31, 2012
27,500
I'm sure there are unintentional postings from TS that originated outside the Homework Help area, just as I am sure there are are intentional ones.

When I see one in other area's, I simply report it. If the moderator agrees, they will take appropriate action.
That's probably the most appropriate response. Asking the TS if it is homework, either before or in conjunction with reporting it, is an option many use, too.