Lecture Circuits 3
Analyzing a Circuit

Look at the following circuit.  What does it do?
 

 Let's build a logic table and see!  We want to see what the output is for all combinations of input values:
a      b output
0        0 ??
0        1 ??
1        0 ??
1        1 ??

First, let's see what we get if both a=0 and b=0, by tracing what happens on the circuit diagram:

So we have, so far:
a      b output
0        0 0
0        1 ??
1        0 ??
1        1 ??

By repeating the above for (1)  a=0 and b=1 (2) a=1 and b=0  (3) a=1 and b=1,  our completed table becomes:
 


a      b output
0        0
0        1
1        0
1        1

Does this logic table look like that of a gate we already know?  Click here for the exciting answer!


 
 
 
 
 
 
 
 
 


It's the logic table for the OR gate!  This proves that (theoretically) we really don't need OR;  we can build it out of AND and NOT!
 
 

Anything any computer can do
anywhere,
anytime,
can be done with
0's, 1's, 
AND gates and NOT gates
Remarkable!