Lab 3
Building a 7-Segment Display Decoder
Here is the grading form that will be used to grade your lab.
Goal
In this lab you will use NOT, OR, AND gates to implement a 7 segment
decoder for two binary digits. By pressing two input switches in
different combinations, the 7-segment display will display a 0, 1 2,
or a 3.
A 7-segment Display
A 7-segment display is a display of numeric digits (and limited
alphabetic characters) that is made of 7 LEDS in the following way:
Each segment corresponds to a pin in the base of the display.
IMPORTANT: To power each segment you need to connect a 470ohm
resistor in series with the segment. Otherwise, if you power the
segment with +5V straight from the power supply, you can permanently
damage your display. See the note below.
You can build digits 0 to 9
by
powering the different segents:
Example:
The Initial Protoboard
You will reuse some of the work that you did from lab1.
From lab1 remove the cables that go to the AND and OR gates.
Before:
After
IMPORTANT:
Most LED's use a voltage of 2 to 3V. Before connecting one of
these LEDs to a 5 V source, a resistor has to be added in series to
reduce the voltage and to limit the current that flows through them.
Without this resistor the LED will burn out. The red and green LEDs
given in the kit are the exception since they have already an internal
resistor so they can be powered directly from a 5V source. However, your
7-segment display does not have this internal resistor and it can be
burn if you place by mistake the 5V straight into the LED segments.
To prevent
burning the 7-segment display by accident, disconnect the battery.
Before placing the battery again, add first the resistors that limit
the current to the display's LED segments.
Place the 7-segment display at the end of the protoboard so the last
pin is aligned to the last column in the protoboard. See the
ilustration: Notice that the decimal points are located closer to you.
Notice that pin 1 is in E56 and pin 8 is in E63 in the protoboard.
In the other side, pin 9 is at location G63, and pin 16 is at location
G56. Pins are counted counter-clockwise starting at pin 1 at the
lower-left corner.
The datasheet of the 7-segment display with the location of the pins is
located here.
Here
is also a summary of the pin location and coordinates in the
protoboard:
Segment
|
Coordinates
|
Pin
|
a
|
G58
|
14
|
b
|
G56
|
16
|
c
|
G59
|
13
|
d
|
E58
|
3
|
e
|
E60
|
5
|
f
|
G61
|
11
|
g
|
G57
|
15
|
Common Cathode
|
Coordinates
|
Pin
|
Digit 1
|
E56
|
1
|
Digit 2
|
E57
|
2
|
Digit 3
|
E61
|
6
|
Digit 4
|
E63
|
8
|
Connect first the Common Cathod pins for all the
digits to ground. Make sure that the battery is not connected.
Now connect the resistors from the segments to pins E45 to E52 that
will be used to drive the segments.
Choose the 470ohm resistors that come in a tape like the one in the
image below. The colors for these resistors should be
yellow-violet-brown.
Since the resistors will be very close to each other, you will have to
put plastic isolation around the resistor terminals so they do not
touch each other. Cut a 4 inch cable and then pull the wire out of the
plastic. Use the cutter in the pliers to pull the cable but do not
press too hard so the cable is not broken.
Cut the plastic to the size of a resistor terminal leaving about 1/4 of
an inch of the terminal. Slide the plastic around the resistor terminal
like the resistor in the picture below.
Here is the list of the resistors and the coordinates of where the
resistors have to be connected.
Segment Resistor
|
From
|
To
|
R-a
|
H58
|
E45
|
R-b
|
H56
|
E46
|
R-c
|
H59
|
E47
|
R-d
|
C58
|
E48
|
R-e
|
C60
|
E49
|
R-f
|
H61
|
E50
|
R-g
|
H57
|
E51
|
Here is the resistor R-a connected with the plastic isolation in place.
Here are the resistors for segments a, b, and c connected.
Here are the resistors for all the segments connected.
Connect all the resistors to the corresponding sockets. At ths point
ask your instructor or other experimented student to check your work.
Here we are testing segment a using a wire connected to +5V and
the resistor that connects segment a.
You can test every segment in the same way.
If you have problems turning on one of the segments and all connections
are correct, push the display a little against the protoboard. If you
still have problems, see your instructor. At this point I recommend you
to put a tape above and below the display to prevent accidental
connections that may damage your display.
Implementing the 7-Segment-Display Decoder
We want you to implement a circuit that display the digits 0, 1, 2, and
3 in the display when the switches x and y are pressed. x=0, y=0 should
produce the digit 0. x=0, y=1 should produce the digit 1, x=1, y=0
should produce the digit 2 and x=1, y=1 should produce the digit 3. To
produce them, each segment will have its own circuit that implements
the corresponding boolean expression.
The following table summarizes the truth table for all the segments for
digits 0 to 3. Only the truth table for the segment a and the
segment g are
provided.
You will have to fill in the values for all the other
segments:
Table 1
x
|
y
|
a
|
b
|
c
|
d
|
e
|
f
|
g
|
0
|
0
|
1
|
|
|
|
1
|
|
|
0
|
1
|
0
|
|
|
|
0
|
|
|
1
|
0
|
1
|
|
|
|
1
|
|
|
1
|
1
|
1
|
|
|
|
0
|
|
|
Using the "product
of
sums" technique, we can find out that a can be
implemented by the function a=x+y' .
Using the "sum of
products" we can find out that e=x'y'+xy'=y'(x'+x')=y'.
Reducing
this function we find that e=y'.
Table 2 summarizes the bolean expressons for each segment. Only the
expressions for a and e are given. You have to fill in the table.
Table 2
Segment
|
Boolean Expression
|
a
|
x+y'
|
b
|
|
c
|
|
d
|
|
e
|
y'
|
f
|
|
g
|
|
The following picture shows the implementation of the boolean
expression for segment a.
The following pictures shows the implementation of segments a and e.
FIll in Table 1 and also complete Table 2. Use the simplest boolean
expressions you can find. Then implement them with the chips. Here are
the pin assignment for reference:
For Next Lab
For next lab you will have to bring:
- Your protoboard implementing the 7-segment decoder. Make sure
that 0 is shown in the display when no switch is pressed, 1 is shown
when only y is pressed, 2 is shown when only x is pressed,
and
3 is shown when both switches x and y are pressed.
Your instructor will verify that your implementation works correctly.
- Also bring a printout of Table 1 and Table 2 filled in with
all the values and expressions.