# SKE Computer Science Course 2013

Electronics - Devices

Nov 2, 2013 (4 years and 8 months ago)

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SKE Computer Science

Course

2013

Worksheet 7
.

Building Logic gates using Transistors

[CBPrice
.

23
-
06
-
13
]

Color Coding

for this Worksheet

Information

Instructed Learning

Challenge, from me to you

Homework

Purpose

(a) To
learn
how to

create logic gates using transistors

(b) To
understand the limitations of current transistor technology

Resources

Mini
-
lecture
discussing transistors, their limitations and the need for multi
-
core CPUs

Transistors, resistors, LEDs, circuit board, ju
mper wires, power supply.

Activities

Info

Transistors are like Switches.

Remember as discussed in class, transistors are like switches and are controlled by the input value.
When the input is Low (0 Volts, Logical 0) then the switch is open
and when the input is High (5
Volts, Logical 1) then the switch is closed like this

Now let’s see how a transistor is used in a simple circuit like this one
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1

Building the Probe circuit

(a)
We need a probe circuit which we can use to test the logic level (voltage) at any point in the
gates we are building. Here’s the circuit diagram and a screenshot of the circuit on the breadboard.
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=
Note1 :
The flat side of the transistor must be on your left

Note2 : The longer leg of the LED must be on the right

(b) Check your circuit works. When the input is connected to 5Volts (High, logic “1”) then the
LED should light up. When it is connected to 0Vol
ts (Low, logic “0”) then the LED extinghish.

2

The Invertor (NOT
-
gate)

(a) Build the following circuit. All resistors are 10 kOhm

(b) Test that this is a NOT gate

(c) Write down in simple English how it works

using the idea that transistors are switches
. (Refer
to the info section up top).

3

The OR gate

(a)
We need two inputs to this gate. Each must have its own input resistor

Here’s the circuit where
all resistors are 10 kOhm.

(b) Build and test the

circuit and write down its truth table.

(c)
Now explain how the circuit works using the idea that transistors are switches.

4

The NOR gate

(a) It’s dead easy to do this. We only need to swap the position of the transistor and the output
resistor,
like this

(b) Again, build, test and write down the truth table.

(c) Can you offer an explanation how the circuit works in simple English?

5

The AND gate

This is a little more problematical to understand. Let’s
have a look at this circuit

(a) Look at the left “totem pole” of two transistors. If both the transistors are off (A=0, B=0) then is
point “X” connected to 5 Volts or to 0 Volts?

(b) Therefore what is the output (o/p) of the second stage (the invertor)?

(c) What combination of A and
B will make the point “X” to be zero volts?

(d)
What is the output of the second stage now?

(e) You should now be able to explain how the AND gate works.

6

The
N
AND gate

Here’s a challenge (quite an easy one).

(a) Work out how to construct an AND
gate, drawing your circuit on paper.

(b)
After it has been checked

build your circuit and test it.

HW

Portfolio time!