EE 101
EE Dept., New Mexico Tech
Multisim: The 555 Timer
Prelab Exercises
1. This lab is all about a device known as a 555 timer. Answer the following questions:
(a) In your words, what is a 555 timer? Hint: Ask the Internet
(b) The model we will be using in this lab is a LM555CN made by Fairchild Semiconductor, how
much does one cost when ordered from Digikey (ignore shipping costs)?
(c) Using the datasheet available on Digikey:
i. What is the supply voltage range for the LM555CN?
ii. What are the pin outputs on the LM555CN? Draw the IC (box with 8 pins like that shown
under Physical Dimensions, not the Block Diagram) and label both the pin number and name
for each pin. Be sure to mark Pin #1.
For those wanting to see how a 555 timer works, here is a great animation of the 555 timer operation.
[Complements of Rensselaer Polytechnic Institute www.rpi.edu]
2. Using the general layout in Figure 1, design a circuit which will produce a square wave output signal
that has a frequency range from 1kHz to 10kHz using the following design parameters:
Figure 1: 555 timer circuit
Design Parameters
• C1 is a .1µF capacitor required for operation.
• R2 is a 1kΩ potentiometer (”pot” for short), a variable resistor with a value adjustible from 0Ω
to 1000Ω.
• Using the following equation from the datasheet, determine the values for R1 and C2
f=
1.44
Hz
(R1 + 2R2 )C2
(1)
Hint: There is a connection between the value of the potentiometer and the output frequency. Find
that connection and use it to create 2 equations with 2 unknowns.
3. Once you have determined the values of R1 and C2 , graph frequency output verses R2 value in increments of 100Ω in order to see the relationship. Make note of the frequency when R2 is equal to 0Ω,
300Ω, 600Ω and 1000Ω.
4. Sketch a complete schematic of your circuit, with all circuit elements labeled (including pin numbers
on the IC).
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201509
EE 101
EE Dept., New Mexico Tech
Lab Exercises
Note: Maybe you are wondering what should be in your lab book for this lab? The answer is simple and
always the same. Your lab book needs to be a complete, legible record of what you did in the lab. In this
lab that means it should include a complete and repeatable procedure, a drawing of the circuit created, each
transient analysis screen, each o-scope screen, any math you used along the way, and any notes about things
you learn along the way.
Build the circuit you designed
Using the Multisim schematic capture program, enter the circuit that you designed in the pre-lab. Below
are some items to help you define certain elements of your circuit.
• The 555 timer chip is in the Mixed group, TIMER family. Choose LM555CN.
• The 1kΩ potentiometer is in the Basic group, POTENTIOMETER family.
Note: The wiper, the little arrow, on the pot needs to look exactly like the one in the schematic in
Figure 1. Be sure to only flip it horizontally. Do not rotate it.
Simulating your design
The goals:
• Simulate your design at 0 Ω, 300 Ω, 600 Ω and 1k Ω using both the transient analysis feature and the
occiloscope (o-scope for short).
Note: When you set the slider to 0% or 0 Ω, the 555 timer’s DIS and TRI pins are shorted together,
causing the output to stop oscillating. To get around this, adjust the potentiometer value to be close to
0%. Be sure to compute the theoretical output frequency for whatever R2 value you choose when you
do your percent difference.
• Capture two signals, the 555 output and VC2 .
• Adjust the simulation output to suit our needs.
• Sketch each simulated waveform accurately and in detail in your lab book.
• Measure the period of the waveform, calculate the frequency and compare that to the theoretical
frequency in your prelab.
Hint: Compare means doing a percent difference.
Hint: Not sure what a period is or how to calculate the frequency from it? Google ”period of waveform”.
Transient Analysis
• Since the transient analysis only measures net voltages, you must create a net (node voltage) to
measure. Connect a 1k Ω resistor from the 555 timer output pin to ground.
Note: For better accuracy, check the ”Maximum time step (TMAX)” box and ”1e-005” will appear in
the box next to it. This manually sets the time step for sampling. The smaller the step, the higher
number of samples taken and the better the accuracy.
• Adjust the TSTART and TSTOP times so that 5-10 cycles of the waveforms are captured. Be sure
not to include the first cycles, as the output can take a few cycles to stabilize. Next drag a selection
box around 1-2 cycles. The goal is to adjust the plot to appear similar to the one in Figure 2. Once
you are happy with the plot on the screen, sketch it accurately and in detail.
• Use the cursors to measure the period.
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201509
EE 101
EE Dept., New Mexico Tech
Figure 2: Adjusted Transient Analysis
Tektronix o-scope
• Find the Tektronix o-scope icon located on the right side of the screen and click it. Place it on your
design. Connect input ”1” to your 555 output and input ”2” to correct net to measure the voltage
across C2 .
• Start the simulation by clicking ”Run” (the little green play button).
Note: You can not edit the circuit while the simulation is running but some components have adjustments that can be modified in realtime.
• To use the o-scope, double click on it and turn it on.
• Using the instructions below, adjust the o-scope to that you have 1-2 waveforms on the screen and
measure the frequency.
• Sketch the o-scope screen as described below.
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EE 101
EE Dept., New Mexico Tech
Notes on using the Tektronix o-scope
• By default only channel 1 is enabled. Press the blue CH2 button to enable it.
• Use the CH1 and CH2 VOLTS/DIV knobs to adjust the height of the waveforms. Be sure to watch
the yellow ”CH1” and blue ”CH2” values on the screen and to make them the same so things are
proportional.
• While adjusting the VOLTS/DIV, you can also use the POSITION knob to move each waveform up
and down on the screen. The little color coded arrows on the left side of the screen represent 0v, or
the x-axis, for each waveform. Both arrows should be adjusted to share the same location.
• Use the SEC/DIV knob to adjust the width of the waveform until you see one to two periods on the
screen. The associated POSITION knob allows you to move the waveform left or right on the screen.
• The waveforms are being continually updated and this can cause them to move and/or flicker on the
screen. Pressing the ”SINGLE SEQ” button will capture one complete waveform.
• The end goal is to display one to two periods of each waveform on the screen, that are as tall as possible
while still being completely visible (similar to the example in Figure 3).
• Once the waveform is properly adjusted on the screen, there are many methods of measuring the
amplitude and the period of it. For this lab use the cursors feature. Press the CURSOR button and
select the Type and Source to measure. Next use the CURSOR 1 and CURSOR 2 knobs to adjust
each cursor and read the values displayed on the screen.
• When sketching the screen of an o-scope in your lab book, be sure to do so on a grid of 8 divisions high
by 10 divisions wide. The divisions on the screen have meaning and define the waveforms displayed.
Each vertical division is equal to the voltage noted for each channel and each horizontal division is
equal to the time ”M”. The example square wave below is 4.5 divisions of 2V high, representing 9V,
and has a period of just under 7 divisions of 200µs, representing roughly 7*200µs or 1400µs. Note the
the values for CH1, CH2 and M to give the divisions meaning.
Figure 3: Tektronix o-scope in Multisim
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