EELE 262 – Logic Circuits Lab
Lab #4 –Combinational Logic (2-input) + LED Driver
Objective
The objective of this lab is to begin implementing combinational logic circuits using discreet parts. In addition, this
lab will introduce an LED driver circuit that will be used to display the input and output values of your circuit.
Outcomes
After completing this lab you should be able to:


Breadboard a 2-input logic circuit in a Sum-of-Products (SOP) form using discrete parts.
Breadboard an LED driver circuit that will be used to display the input codes and output value of your
circuit.
Pre-Lab
For the following 2-input truth table:
AB
0 0
0 1
1 0
1 1
F
1
0
0
1
A) Derive the logic function as a Canonical Sum of Products in your lab notebook showing all work.
Refresher on SOPs: Remember that in an SOP, there are only AND, OR and INV operations. You
probably notice that this truth table is an XNOR gate. However, since we are implementing an SOP, we
are not allowed to use the XOR/XNOR operators.
The term Canonical is used to describe a function that is not minimal (refer to your notes on Minterms).
You may notice that this truth table cannot be minimized further using AND/OR/INV, so in this case the
canonical SOP is indeed the minimal SOP.
B) Derive the Minterm list of this truth table and write in your lab notebook.
Refresher on Minterm Lists: Remember that it is very important to write the input variables as subscripts in
a minterm list to indicate not only the variable names, but the number and order of the inputs in the circuit.
C) Draw the SOP logic diagram in your lab notebook
D) Breadboard this circuit using the discreet parts in your lab kit (AND=74LS08, OR=74LS32, INV=74LS04).
Leave the input signals (A, B) and the output (F) disconnected for the pre-lab. We will connect them to an
LED driver circuit in a later part of this lab.
EELE 262 – Logic Circuits Lab
Lab #4 – Combinational Logic + LED Driver
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E) We are going to create an LED driver circuit that will be used to display the input codes and output values
of our combinational logic circuits that we design. Since we don’t want to excessively load our
combinational circuits, we will use the 74LS244 line driver. This line driver can be thought of as 2, 4-bit
buffers each with an enable line. We will use these line buffers to drive the LED array in order to represent
1’s and 0’s. Due to the pin out of the 74LS244 device, the bread boarding of the LED driver circuit is much
easier if we use 2x 74LS244s to drive the LEDs. We will use the 8-bit DIP switch to drive the input codes
for our combinational logic circuits.
Breadboard the following LED driver test circuit. This circuit will simply light up the LEDs corresponding to
the state of the DIP switch and will verify that our circuit is operational before connecting to our
combinational logic circuit.
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EELE 262 – Logic Circuits Lab
Lab #4 – Combinational Logic + LED Driver
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Consider using an organized approach to wiring such as in the following picture:
SOP circuit for part 1 of the
pre-lab with inputs and
outputs unconnected
An optional +5v regulator
circuit which powers your
breadboard from a 9v battery
(see website for schematic)
LED Array
DIP Switch
Bussed Resistor
Array
74LS244 Line
Drivers
Individual
Resistor Array
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EELE 262 – Logic Circuits Lab
Lab #4 – Combinational Logic + LED Driver
______________________________________________________________________________________________________
Lab Work & Demonstration
1) Demonstrate your LED driver circuit to the lab instructor. Each switch should turn on/off a corresponding
LED.
TA will check off
2) Now you are going to use channels 1 and 2 of the DIP switch as inputs A and B to your combinational
logic circuit. Connect these two inputs to your circuit. Turn off channels 3-7 of your DIP switch so that the
LEDs are OFF. Connect the output of your SOP to channel 8 of your LED driver circuit.
MAKE SURE TO HAVE CHANNEL 8 OF THE DIP SWITCH IN THE OFF POSITION. When you are
using your combinational logic output to drive the LED circuit, you need to have the switch in the off
position so that the output of your SOP is not directly connected to the +5v supply through the DIP switch.
The output of the SOP will have enough drive strength to make able to drive the 470ohm pull-down
resistors.
Show the lab instructor the proper operation of your combinational logic circuit. You will use the DIP
switches to create all possible input codes to your circuit (00, 01, 10, 11). The inputs codes will be
displayed on channels 1 and 2 of the LED array and the corresponding output will be displayed on channel
8.
TA will check off
Lab Grading
Pre-Lab
___________ / 10
Lab Demo (step 1)
Lab Demo (step 2)
___________ / 50
___________ / 40
Total
___________ / 100
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