EELE 262 – Logic Circuits Lab
Lab #2 – Introduction to 74LS Logic Gates
Objective
The objective of this lab is to become familiar with the basic gates within the 74LS logic family and understand the
difference between a logic level and a logic state.
Outcomes
After completing this lab you should be able to:
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Read and understand pin out and power supply information from a 74LS datasheet.
Breadboard a basic gate from the 74LS logic family and measure its output voltage for all input codes.
Describe the differences between a logic state and a logic level.
Determine the switching threshold of a basic gate by sweeping the input voltage and observing the output.
Drive the input of a basic gate with the arbitrary waveform generator.
Pre-Lab
A) Prepare your lab notebook for today’s lab (i.e., start a Lab 2 section, tape in this handout, and update your
table of contents).
B) Create a section in your lab notebook called Datasheets. This section will start at the back of your
notebook and will be built toward the front. Download the relevant pages of the datasheets for the 74LS08
(2-input AND Gate), 74LS32 (2-input OR Gate) and 74LS04 (inverter). The relevant pages are those with
pertinent information about our particular part. For example, if you look at the datasheet for the 74LS32 2Input OR gate, you’ll notice that pages 1 and 2 have important information; however, pages 3 and 5 give
electrical information for the SN5432/SN7432 and SN54S32/SN74S32 versions of the part that we are not
using. Since we have the SN74LS32 part, we only need the electrical information on page 4. Also, pages
6-22 contain package drawings and information about the company that we don’t need.
C) Place the three parts on your breadboard and wire up the VCC and GROUND pins. You should wire your
breadboard so that all of the vertical power/ground rails are connected.
Lab Work & Demonstration
1) Logic State vs. Logic Level - You are going to drive in all possible input codes for each gate and measure
the input and output voltages with a digital multi-meter (DMM). You will provide VCC (+5v) and GROUND
(0v) to your breadboard using the power supplies in the lab. You will provide a Logic-0 by wiring the input
of a gate to the GROUND rail on your breadboard. You will provide a Logic-1 by wiring the input of a gate
to the VCC rail on your breadboard. Create the following table in your lab notebook. You can either draw
your own table or printout another copy of the this handout and cut/tape the table in your notebook. Fill in
the table with your measurement results.
TA will check off
EELE 262 – Logic Circuits Lab
Lab #2 – Introduction to 74LS Logic Gates
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2) Determining the Switching Threshold – Now drive the input of the 74LS04 inverter with a DC power supply.
The power supplies in the lab have two outputs so one output can provide power to your breadboard and
the other can be used to drive the input of the inverter. Observe the output of the inverter with the
oscilloscope using a probe. Sweep the input voltage from 0v to 5v and record the input voltage that causes
the inverter to change from a 1 to a 0. Next, sweep the input voltage from 5v to 0v and record the input
voltage that causes the inverter to change from a 0 to a 1. Record the values in your lab notebook and
show the lab instructor the inverter switching.
TA will check off
3) Driving a 74LS Gate with the AWG – Now connect the AWG to the input of the inverter and drive in a
square wave that goes from 0v to 5v at a frequency of 1kHz. Observe the input and output waveforms
using the oscilloscope. Draw the waveforms that you see on the oscilloscope screen in your lab notebook.
TA will check off
Lab Grading
Pre-Lab
___________ / 10
Lab Demo (step 1)
Lab Demo (step 2)
Lab Demo (step 3)
___________ / 30
___________ / 30
___________ / 30
Total
___________ / 100
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