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Ignatius Seminar 8/28/2013

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Introduction. Ignatius Seminar 8/28/2013. What do these slides have in common?. Reference: Logic Gates. TRANSISTOR

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What do these slides

have in common?



logic gates
Logic Gates


To understand logic gates you must understand the transistor used in the construction of gates. Without getting into a lot of physics, know that a transistor is made of three parts.

This is shown in the picture as the two red and one blue section.

Most transistors have three wires.

logic gates1
Logic Gates


One of the most basic parts in a computer circuit is a clock.

Now this clock doesn't really keep time (though it can) but is used to produce very fast, accurate pulses or signals. When I say fast, I mean it. Pulses can be anywhere from 1/100,000 of a second to 1/1,000,000,000 of a second.

As humans we can't even imagine anything going that fast so it will suffice to say that computers are useful only because they can do everything very fast.


As can be seen above, one section of the transistor is connected to the clock. In most computer circuits this will be a 5 volt signal.

In "A" of the above picture the transistor is "on" because there is NO signal to the center section of the transistor. So in other words when the clock sends the transistor a pulse the transistor will also send out a pulse from its output.


Now notice in "B" of the picture the center section of the transistor is now receiving a signal. This in turn turns "off" the transistor. This might seem backwards, but in the world of logic circuits it works out nicely.

It is very important that you understand the concept of a switching transistor to understand logic circuits. Make sure you understand the above before moving on to logic gates.

logic gates2

Logic Gates are the building blocks for all digital circuits. There are several of them, and we are going to take a look at the three most used. If you understand these three you will have no problems understanding the remaining ones as they are just extensions of these three. Logic gates are comprised of the type of transistors described above.

representation interpretation
Representation & Interpretation
  • Representation:
  • 3, 9, 10, -34, 999, -92929 etc. (decimal representation)
  • I, II, III, IV, V, VI, etc….
  • 0, 1, 10, 11, 100, 101, 110, 111, ……
  • Can you think of other representation for numbers?
  • Can you think of anything besides numbers that have more than one representation?

A concept we need to understand is the binary system. I will not teach binary math, but knowing that the binary systems contains two numbers: 1 and 0 will suffice.

Binary numbers are the basis of all functions performed by digital computers.

Remember above when we discussed transistors and how they can be switched "on" or "off"?

Well, in binary if the transistor is "off" then we consider that a "0".

If the transistor is "on" then we consider that a "1".

In fact this is the whole basis to digital. Everything that happens in the digital world can all be summed down to being either 1's or 0's, and nothing in between. On, Off, Yes, No, 1,0, no mediums, maybes, or any other numbers. That's it, that is the magic of digital.

not gate
Not Gate

Output 1

Input 0

Input 1

Output 0

not gate1



and gate

Definition: Takes two binary inputs, produces one output according to the

following table.






or gate

Definition: Takes two binary inputs, produces one output according to the

following table.