CSCE 211, Digital Logic Design

Circuit 1

Due: Tuesday, February 21, 2012

 

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:  This circuit project will be due on Tuesday, February 21, in class and will be worth 3 points toward your final grade. 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

 

  1. Draw a diagram of an LED circuit with the DIP switch open.  Explain why the LED is ON.
  2. Draw a diagram of an LED circuit with the DIP switch closed.  Explain why the LED is OFF.
  3. 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

  1. Orient the breadboard “horizontally” with the large holes to the right.
  2. Write your name on the “top” of the breadboard in the black area with the pens being circulated.
  3. 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.
  4. 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.
  5. Make sure the wires are inserted all the way.
  6. 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

  1. 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.
  2. At hole-location 40 insert the longer (+) leg of a red LED into the RED bus.
  3. Insert the shorter leg inserted into location 40 below the bus.
  4. 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.
  5. Connect a wire from the “lower” side of the resistor to any BLUE connection.
  6. Install the two outer batteries into the battery pack.  Do no install the middle battery yet.
  7. Install the middle battery and check that the LED is ON.  If not, check the polarity of the LED.

 

Inverter-LED Test

  1. Pop out the middle battery.
  2. 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.  
  3. 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.
  4. Remove the lower 330-ohm resistor wire, previously installed to BLUE, and reconnect it to pin 2 of the 74LS05.
  5. Pop in the middle battery and check that the LED is ON.
  6. Temporarily connect a wire from pin 1 of the 74LS05 to BLUE.  Confirm that the LED turns OFF.
  7. 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

  1. 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.
  2. 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.
  3. Run a short wire from location 60 to BLUE.
  4. 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.
  5. Use a separate 330-ohm spanning resistor for this LED.
  6. 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.
  7. 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 immediately to the left of D’s LED.
  8. 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.
  9. 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.  (You may read “On” as NO to avoid confusion.)
  10. 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.
  11. 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.
  12. Test each LED circuit as you go along.  “Build a little; test a little.”
  13. When you finish, there will be 4 LEDs, each switched separately for the 4 variables.
  14. 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.