MCS-170 Project 7: Logic Circuits - Part I
Due: In Class
In class, we have begun a discussion of how basic logic gates can be
used to create complex logical and arithmetic circuits.
In this lab, you will use a logic circuit simulator to investigate
the design and analysis of logic circuits. During the lab you
will be asked a series of questions.
Record your answers and turn them in to the instructor at the end of
the lab period.
The three basic logic gates are the NOT gate, the AND gate, and the OR
gate. These are symbolized as follows:
We will use these basic gates as building blocks for more complex
circuits. We will build our circuits using an on-line logic circuit
Open a new browser window and go to the page
to run the simulator.
To use the simulator one can click and drag logic gates and other
elements onto the page and connect them to form
Notice that the cicuit loaded initially involves a single AND gate.
Click the upper toggle switch. The top-most LED (the A LED)turns red to
that current is flowing through the switch (which corresponds to a
value of 1). No current means a value of 0 is fed to that input of
the gate. Notice that when both switches are depressed, the rightmost
LED turns red. This corresponds to two 1s being input to the AND
gate, which produces a 1 (as it should). (Note: LED stands for
Light Emitting Diode; you can think of it as a little red light that
glows when current is fed to it.)
Notes on the use of the simulator:
- To create a new circuit element, simply left click on the
element in the window on the left-hand side of the interface (note: you
can use the scroll bar to scroll in this window). Hold down the mouse
button and drag the element into the white drawing area.
- To delete an element, left click it. While it is surrounded by a
black box, click on the scissors icon in the top menu bar.
- You can connect the output of one element to the input of
another by simply left-clicking on the output box, holding down the
mouse button while dragging, and then letting go when you reach the
input box of the next element.
- To delete a connection, left click once on the input box
involved in the connection.
- Every circuit will need a 5V power source (the pink box). The
power source should be connected to each input switch (normally toggle
- You should have one toggle switch and LED for each input to your
circuit to clearly indicate whether that input is a 0 or 1.
- You should also have an LED to indicate the final output of the
- To change the label on a LED, double-click near the end of the
label of the LED, using the left mouse button. A dialog box will popup
for you to type in a new name.
Starting with the initial configuration, replace the AND gate
with an XOR gate (labeled EOR in the simulator). Construct a truth
table based on setting the two toggle buttons of the simulator to 0 or
1. Do the results match
the truth table we had in class for the XOR gate?
Use the Circuit Simulator to build the
circuit indicated below
and then complete the truth table given below the circuit diagram.
B C | Z
0 0 0 |
0 0 1 |
0 1 0 |
0 1 1 |
1 0 0 |
1 0 1 |
1 1 0 |
1 1 1 |
EXERCISE 3: Use the
simulator to construct the half-adder on page 294 of the text.
Create two output LED's for the circuit and label one of them "sum" and
one "carry" so that they represent the correct carry and sum outputs.
Print out the circuit to hand in for this exercise.
EXERCISE 4: Use the
simulator to construct the circuit for the logical statement
(A and B) or ( not B or A). Print
out the circuit to hand in for this exercise.
EXERCISE 5: What did
you find most confusing in
working with the Logic Circuit Simulator?
Hand in your answers to the exercises above.