CSCE 211, Digital Logic Design
Circuit 1
Goal: Generate 4 switched logic signals (A, B, C, D) and display them with LEDs.
Each
LED is said to be “buffer-driven” by a 74LS05 open-collector (OC) inverter as
shown here.
Each gate in an OC device needs a resistive path to Vcc, which the LED
and resistor provide. The pin layout for
the 74LS05 is the same as for the 74LS04, which is part of the 74xx00 family.
A
fifth LED circuit (with a “floating” input for now) will be used later for
displaying F(A,B,C,D). In effect, you are creating a circuit that
displays a row of a truth table. Your
circuits will be constructed on this breadboard. The details of constructing the first circuit
will be explained in class and are repeated on the next pages.
Materials needed:
1. AA batteries (3)
2. Breadboard with battery pack
3. Blue 8-position, dual
in-line-pin (DIP) switch
4. Light Emitting Diodes (LEDs)
(5)
5. Resistors 330 ohm, Orange-Orange-Brown
(5)
6. Resistors 1.0 K ohm,
Brown-Black-Red (4)
7. Wire kit
The logic signals and
displays you build for this first project will be used in the next two
experiments. Try to make these circuits neat and compact.
Grading: Your grade for each of the circuit assignments will
be based on (1) demonstrating that your circuits work properly and (2) a short
written report that provides a few details about the circuits. You will demonstrate your circuits and turn
in your report to the TA on your check-off day.
Circuit 1
Report
- Draw a diagram of an LED circuit with the DIP
switch open. Explain why the LED is ON.
- Draw a diagram of an LED circuit with the DIP
switch closed. Explain why the LED is OFF.
- For each of the 4 signal LEDs to the left of the
74LS05, identify the pin number of the inverter output used. (For “function value” LED to the right
of the 7405 chip, we have already selected pin #2 as the output.) The leftmost LED of the 4 will be
associated with the most significant signal value.
Circuit 1 Details
Power Busses
- Orient the breadboard “horizontally”
with the large holes to the right.
- Write your name on the
“top” of the breadboard in the black area with the pens being circulated.
- Use the foam tape
being distributed to mount the battery pack above the holes in the upper corner with the wires down.
Put a “pig-tail” loop into the wires to shorten them.
- On the extreme right
side of the board insert the RED
wire into the RED bus (Vcc); insert the BLUE wire into the BLUE bus (GND). In our
orientation, the power busses run horizontally.
- Make sure the wires
are inserted all the way.
- Put a small piece of
masking tape over the wires to secure them more permanently. Do
not remove these wires once inserted and taped; they are fragile.
LED
Test
- Above and below each
gapped row, there are is a column of 5 electrically-connected
contacts. A wire inserted into of
these holes has the same voltage (relative to GND) as the others.
- At hole-location 40
insert the longer (+) leg of a red LED into the RED bus.
- Insert the shorter leg
inserted into location 40 below
the bus.
- Bridge a 330 ohm
resistor (Orange-Orange-Brown) from the hole just below the short leg of
the LED to the other side of the
gap at location 40.
- Connect a wire from
the “lower” side of the resistor to any BLUE connection.
- Install the two outer
batteries into the battery pack. Do no install the middle battery yet.
- Install the middle
battery and check that the LED is ON. If not, check the polarity of the LED.
Inverter-LED
Test
- Pop out the middle
battery.
- Insert a 74LS05 integrated
circuit (IC), often referred to
as a “chip,” with pin 1 two spaces to the left of
the 330 resistor gap. Make sure the “notch” of this IC, also
called a “chip,” f aces the battery pack; pin 1 is then the pin in the upper
right corner.
- To provide power to
the 74LS05 when batteries are installed, connect a wire from pin 7 to BLUE. Connect another
wire from pin 14 to RED.
- Remove the lower 330-ohm
resistor wire, previously installed to BLUE, and reconnect it to pin 2 of the 74LS05.
- Pop in the middle
battery and check that the LED is ON.
- Temporarily connect a
wire from pin 1 of the 74LS05
to BLUE. Confirm that the LED turns OFF.
- Remove the wire from
pin 1 and let the inverter
input “float-high.” This lets
us use the LED as an ON
indicator for now.
Logic-Signal
Generation with DIP Switch and 1.0K Resistor
- Install the blue DIP
switch across the top gap with switch position 1 at location 60. The
edge of the switch overhangs so place the switch edge at location 61.
- At the top of the
switch “daisy chain” location 60
to 59, 59 to 58, and 58 to 57 with the short “U” shaped wires.
- Run a short wire from
location 60 to BLUE.
- Install another LED five
holes to the left of the 74LS05
from RED to board position 52. As
noted below, this will be the LED
for the least significant variable, D.
- Use a separate 330-ohm
spanning resistor for this LED.
- Connect a wire from an
unused inverter output (for example pin 4) to the lower end of D’s
330 ohm resistor. Test that D is ON; confirm that the LED will
go OFF when the inverter input (pin 3) is grounded.
- Now create a logic
signal by connecting a 1.0 K
(Brown-Black-Red) resistor from the “bottom” of switch 4 to Vcc. Use the Vcc hole between the immediately
to the left of D’s LED.
- Connect a wire from the
lower side of switch 4 to D’s inverter input (pin 3). Test by using a retracted ball-point pen
to move switch lever 4 up and
down.
- The LED will be OFF when the switch is in the up
(“On”) position; the LED will be ON
when the switch is in moved downward.
Read “On” as NO to avoid
confusion.
- Install 3 more LED
circuits to the left of the D
LED. Skip one hole between LEDs D and C. There will be 2
holes between C and B because of the board
layout. Then skip one space between
LEDs B and A.
- Install a separate 330
ohm resistor for each LED and remember to use a separate inverter input
and separate 1.0 K resistor for each logic signal.
- Test each LED circuit
as you go along. “Build a little; test a little.”
- When you finish, there
will be 4 LEDs, each switched separately for the 4 variables.
- Make sure that the switch positions and the LEDs
correspond in going from left to right.
Circuit Hints
·
Design circuits, then build. That is, first create the circuit’s logical diagram
(supplemented by pin numbers); then build.
·
Bends chip pins slightly to fit breadboard by pressing gently pins
against a firm surface.
·
Convention: place all IC notches towards the right-side of the breadboard (towards battery pack).
·
Hot chips spell trouble! Remove the middle battery and trouble shoot.
·
Always use a 330 ohm resistor in series
with each LED. A “leg” or segment in a
7-segment display is also an LED and requires a 330 ohm resistor.
·
Neat wiring is helpful.
·
Do not cross wires over chips
so a chip may be replaced easily if necessary.
·
Watch out for bare wires touching each other.
·
Tweezers or long-nose pliers are helpful in inserting and removing
wires.
·
To extract a chip from the board, use the “finger” cap of a ball-point
pen.
Homework Hints
·
Please make your work neat and legible using the drawing template.
·
Use enough paper to avoid “squashing” your work.
·
Show your work; that is, briefly explain the steps and explain what
your are doing.
·
Box-in the answer.
·
Put your name on every page
·
Use the cover sheet and staple
all pages together.
·
Benefits
*
These steps make your homework easier to grade
*
The homework is good practice for tests
*
Later in your career as an engineer, you will be asked to explain your
calculation and designs.