Lab 3
Building a Flip-Flop and a
7-Segment Display Decoder
Goal
In this lab you will use NAND gates to implement two flip-flop
circuits and then a 7 segment decoder for two binary digits. By
pressing the four input switches in different combinations you will
store 0 or 1s in the flip-flops and then the output wil be decoded
so the 7-segment display will display a 0, 1 2, or a 3.
Flip-Flops
A Flip-Flop is a basic unit of memory that is used to store one bit.
You can implement a flip-flop using NOR gates or using NAND gates.
In this lab you will use NAND gates. The following is the circuit
you will implement. Notice that in this circuit the voltages
produced by the buttons is counterintuitive. It is 1 (+5) when a
button is not pressed and 0 when the button is pressed.
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:
You will need first to implement the
The Initial Protoboard
Remove the LEDs, resistors and buttons from the previous lab and now
put 4 buttons and resistors as indicated in the pictures. The
buttons will be labeled from left to right S1, R1, S0, R0 where S1,
R1 wll be the inputs for the flip-flop 1 an S0, R0 will be the
inputs for the flip-flop 0.
Implement the flip-flop 1 as indicated in the picture. Notice the
pin assignments of the NAND gates.
Notice that pressing the first button (S1) the LED turns ON and
pressing the second button (R1) the LED will turn OFF.
You will implement another flip-flop 0 using the remaining NAND
gates in the first chip. Implement the flip-flop in the space left
above the push buttons and the first chip.
Connecting the display.
Put the seven-segment display in the right of the breadboard as
indicated in the picture.
You will need to remove some tubing from one of the cables to
isolate the terminals of the resistors. Otherwise the terminals of
the resistors will touch each other causing a short circuit. To
remove the tubing cut about 5 inches of wire and cut a little of the
plastic in one of the sides. Then while holding the wire with the
wire cutter, pull the plastic with the other hand as indicated in
the picture.Cut the tubing in multiple pieces large enough to cover
the terminals of the resistor while leaving out abut 1/4 of an inch.
Connect a resistor to each of the segments of the display. Here is
the pin assignment.You will connect the resistors in such a way that
column 48 in the breadboard will have segment A, column 49
will have segment B, 50 will have segment C and so on until the
decimal point is in column 55. See the pictures.
Notice that to turn on segment for example segment B, you need to
put a resistor from terminal 16 in the display to ground and then a
wire from terminal 1 (digit 1) to +5. You can move the blue wire in
the picture from column to column to test the different segments as
you add the resistors. Also, you can move the yellow wire at the
bottom to select the different digits.
Implementing the 7-Segment-Display Decoder
You will implement a circuit that display the digits 0, 1, 2, and 3
in the display when the switches are pressed. When the flip flop
F1=x=0 and F0=y=0 the display will show the digit 0. When the flip
flop F1=x=0 and F0=y=1 the display will show the digit 1. When the
flip flop F1=x=1 and F0=y=0 the display will show the digit 2. And
when the flip flop F1=x=1 and F0=y=1 the display will show the digit
3. To show the digits, 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
F1(x)
|
F0(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
|
|
For Next Lab
For next lab you will have to bring:
- Your protoboard implementing the flip-flop 0 and the 7-segment
decoder. Make sure that setting and resetting the values in the
flip-flops will show the corresponding digits in the
seven-segment diaplay.
- Also bring a printout of Table 1 and Table 2 filled in
with all the values and expressions.