# Logic gates and truth table

Discussion in 'General Electronics Chat' started by cuircitboy, Oct 20, 2011.

1. ### cuircitboy Thread Starter New Member

Oct 20, 2011
3
0
struggling with thse I have to make a few of them

The question is draw logic circuit and truth table A or (B and C)

I have a few so can someone explain the answer to this so i can hopefully do the rest.

2. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
Can you make an initial effort to tackle your problem and post it? It will be much easier for us if we see were you are lacking.

3. ### cuircitboy Thread Starter New Member

Oct 20, 2011
3
0
<snip>

I'm not studying to become an engineer, I am studying computering at Uni, I just cant get my head round this stuff

Last edited by a moderator: Oct 20, 2011
4. ### cuircitboy Thread Starter New Member

Oct 20, 2011
3
0
I don't actually know where to begin

5. ### colinb Active Member

Jun 15, 2011
351
35
OK, first read about what gates are (AND, OR, NOT, then NAND and NOR). Try to understand how truth tables show the result of these gates (which perform logic functions like AND and OR, etc.).

You can think of logic functions as a kind of one-bit addition and multiplication. The OR operation is like addition (and is often written as “+”). The AND operation is like multiplication (often written “⋅”).

Truth tables are a way of writing down EVERY possible input to a logic function, and showing what the output would be.

Draw the truth table for AND and OR and you will see how AND is like multiplication (0⋅0=0; 1⋅0=0; 1⋅1=1; ...). If you don't know what a truth table looks like, look in your textbook. It must be described there. READ YOUR BOOK!

6. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
Check this wiki article in the section Symbols: http://en.wikipedia.org/wiki/Logic_gates
and also here: http://www.allaboutcircuits.com/vol_4/chpt_3/index.html

Then try to convert your literal expression into a symbolic one, using the "distinctive symbols".

A truth table is a table that has columns for each input of the Boolean expression and a column in the end for the result.

Each row has in its input section a binary number, starting from 0 and counting up. The corresponding output section is filled with the result of the expression.

An example of a truth table can be found here: http://www.allaboutcircuits.com/vol_4/chpt_7/9.html

Come back with your questions.

7. ### djsfantasi AAC Fanatic!

Apr 11, 2010
2,900
876
For an example of a logic gate, consider the following example. You have two food items, represented by item A and item B. You want to know if both A AND B are fruits. There are several general cases that can be considered. Let the items be an apple, pear, carrot or potato. There are several possibilities, of which I will show four.
1. A is a carrot, B is a potato.
2. A is a carrot, B is a pear
3. A is an apple, B is a potato
4. A is an apple, B is a pear
Basically, the phrases can be simplified to one simple question, Is the item a fruit? This defines the verbal statement into a logic statement. In Boolean logic, True is represented by a 1; false is represented by a 0.
But I am getting ahead of myself. Lets consider the logical function AND. This function returns a true value if both inputs are true. Verbally, let me repeat the above table, adding in the AND function:
1. A is a carrot, B is a potato -> A AND B are not both fruits
2. A is a carrot, B is a pear -> A AND B are not both fruits
3. A is an apple, B is a potato -> A AND B are not both fruits
4. A is an apple, B is a pear -> A AND B are both fruits!
This table is similar to a truth table, where all possible inputs are enumerated and the desired result of the function/logic is shown in the last column(s). Here attached is a formal example of the truth table for an AND gate, including its electronic symbol.
This is an example of the information that you will find in the resources that Georacer and colinb have supplied. Use their wisdom to attack your problem.

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