university of new mexico department of electrical

University of New Mexico

Department of Electrical & Computer Engineering

ECE206L: 555 Laboratory Procedure

Introduction: This Procedure is to demonstrate the LM555 Timer, integrated circuit (IC) and its variants.

These experiments use the free or extended version of the online program, Multisim Live.

To use the downloaded version of Multisim 14.1 to simulate the testing of the 555 circuits, there

is a different document, ECE306L555LabProcedureDwnLdMultisim, that should be used.

Three circuits will be simulated demonstrating the range of functions and various applications

the 555 Timer IC can accomplish. These circuits will be an astable multivibrator (square wave

oscillator), a monostable multivibrator (one-shot), and a pulse width/position modulator.

Laboratory Goals Familiarize students with the 555 IC and its uses

Design and test a free-running oscillator

Design and test a triggered one-shot circuit

Design and test a pulse width/position modulator

Read, understand and use the information from an IC’s Datasheet

Compose an informative report providing necessary information

Equipment needed Lab notebook, pen

Computer running Microsoft’s Windows 7 or newer

Internet connection/access

Software, National Instruments’ Multisim Live

Pre-Lab LM555 Refer to the 555 Data Sheet, found on Experiments page in the appendix of this procedure.

Design an astable multivibrator (free-running oscillator), referring to Figure 1 below and

the 555 Datasheet Application Information for the Astable Operation on page 7.

Choose the oscillation frequency to be the last 4 digits of your student ID number [if the

first digit is a zero then substitute a 2 for it], and C to be 0.01 μF.

Design the oscillator for a pulse width (PW) of 0.60 (60%) of the period. That is a 0.6

Duty Cycle. [Duty Cycle = Pulse Width/Period] See the datasheet for instructions.

Reference Material LM555 Timer Datasheet from National Semiconductor dated July 2006.

Please view these two videos youtube.com explaining the operation of the 555 time.

https://www.youtube.com/watch?v=i0SNb__dkYI

https://www.youtube.com/watch?v=kRlSFm519Bo

Simulation Fundamentals: Cosimulation in NI Multisim,

https://www.ni.com/en-us/support/documentation/supplemental/07/simulation-fundamentals--

cosimulation-in-ni-multisim.html

https://www.youtube.com/watch?v=i0SNb__dkYI

https://www.youtube.com/watch?v=kRlSFm519Bo

https://www.ni.com/en-us/support/documentation/supplemental/07/simulation-fundamentals--cosimulation-in-ni-multisim.html

https://www.ni.com/en-us/support/documentation/supplemental/07/simulation-fundamentals--cosimulation-in-ni-multisim.html




NI Mulisim for Educations:

https://www.ni.com/pdf/manuals/374484g.pdf

Figure 1: LM555 based Astable Multivibrator

Astable Multivibrator Design 1. Choose the oscillation frequency to be the last 4 digits of your student ID number [if the

first digit is a zero then substitute a 1 for it]

2. Design the oscillator for a pulse width (PW) of 0.60 (60%) of the period. That is a 0.6

Duty Cycle. [Duty Cycle = Pulse Width/Period] See the datasheet for instructions.

3. The frequency, f, is one divided by the period: f = 1/T.

From the data sheet we have

4. T = t1 + t2 where

5. t1 = 0.693*(RA + RB)*C & t1 is the charging time of the capacitor. We also have

6. t2 = 0.693*RB*C and we know

7. C is 0.01uF

8. If t1 is 60% of T then t2 must be 40% of T => t2 = 0.4 * T

9. Solve for RB in the equation t2= 0.4 * T = 0.693 * RB * 0.01E-6

10. Substitute this value for RB in the equation for t1

11. Solve for RA from t1 = 0.6 * T = 0.693 * (RA + RB) * C.

Example of finding RA and RB given C = 0.01 uF for an LM555 based Astable

Multivibrator.

Assume that the frequency was 10kHz (Your Frequency will be different based on ID number)

The Period is 1/freq so here T = 1/10E-3 = 0.1ms and C = 0.01 x 10E-6 F

https://www.ni.com/pdf/manuals/374484g.pdf




The time t1 is the time that the output is high and from the requirement for a 60% duty cycle,

that means

t1 = 0.6 x T = 0.6 x 0.1 ms = 0.06 ms = 60 uS.

If t1 is 60% of T, then t2 must be 40%. Therefore

t2 = 0.4 x T = 0.4 x 0.1ms = 40 uS.

The equation for t2 is t2 = 0.693*RB*C. We know all values except RB, so we solve for RB.

RB = 40uS/(0.693 * 0.01xE-6) = 5.772 kOhms = RB,

with this known we solve for RA in the t1 equation:

t1 = 60uS = 0.693 * (RA + RB)C = 0.693*(RA + 5.772kOhms) * 0.01E-6 F =>

RA = [60uS/(0.693 * 0.01E-6)] – 5.772 kOhms = 8.658 kOhms – 5.772 kOhms = 2.886 kOhms.

With these values for the resistors in the simulation circuit of Figure 1, it produced the following

waveform:

Figure 2: Waveform for example calculations

From Figure 2 Graph A, we can determine the period, T, is equal to 101.62 uS (∆X bottom

right). The frequency is 1/T and 1/101.62uS = 9.84 kHz, which 1.62% off the design

requirement. The duty cycle = pulse width (60.54 from figure 2:B; ∆X) divided by the period =

60.54/101.62us = 0.5958 ~= 0.6, which is again the design requirement. The differences from

the specification are mainly due to the rounding off of the resistor values to whole numbers and

arithmetic round off error in the computer.

Also, note that the measurements are not made at the start of the wave form. The initial pulse

width is wider (94.5uS) than the steady state pulse width (60.54uS) because at the start the

capacitor is at zero volts and in steady state the capacitor voltage doesn’t drop below 1/3 Vcc.

The initial pulse width is wider due to the extra time to bring the capacitor voltage to 1/3 Vcc as

can be seen in figure 3, below.




Figure 3: Initial pulse width is greater as capacitor is initially uncharged.

How to build the Astable Multivibrator Circuit in MultisimLive:

Connect to the internet, Open a browser, Go to URL, https://www.multisim.com, Login, click

“CREATE CIRCUIT” in upper right corner.

Go to the ‘File Navigation Menu’ in upper left corner of window, left click to activate the drop

down menu and select ‘Go to > Public Circuits.’ See figure 4: below.

Figure 4: File Navigation Menu in Multisim Live

On the Public Circuits page click the 555 button shown in Figure 5.

Figure 5: Public Circuits in Multisim

Scroll down about 5 rows and find an 555 clock generator circuit see Figure 6.

https://www.multisim.com/




Figure 6: A Public 555 Circuit

Left mouse click it and the browser window will go to the circuit. Now click the

‘OPEN CIRCUIT’ button. See Figure 7, below.

Figure 7: Open Circuit Button.

This should take you to the simulation window with the circuit and the graph of its waveform

displayed. If the Grapher is not displayed, then click the ‘Split’ button (white button in

Figure 8) on the upper left tool bar to produce the proper display. Unselected it is the color of

the tool bar and when selected the background turns white as in Figure: 8.

Figure 8: Multisim Live Simulation Split Window displaying circuit and graph.




If the 555 circuit already has three resistors, two capacitors, a voltage supply, and, of course, a

555 Timer IC, then arrange them as shown in Figure 10. Otherwise add and delete components

as necessary using the components toolbar. See Figure 9. The LED can either be left in the

circuit or removed. It has no effect on the timing.

Figure 9: Multisim Live, Component toolbar (rotated to save space)

Set the component values as determined by your calculation from ‘Astable Multivibrator Design’

section. This is done by placing the mouse on the value you wish to change and left mouse click.

Note that RA and RB will be the values that you calculated to yield the frequency and duty cycle

that were your design targets based on your school ID.

Figure 10: LM555 based Astable Multivibrator Schematic

Run the simulation by clicking the run arrow in the upper left of the window.

After a few cycles of the output have been graphed ( 400ms or 500 ms) stop the simulation using

the square stop button . The simulation timer is displayed just to the right of the stop button.

See Figure 11.

Figure 11: Simulation Time displayed to the right of the Stop Button

Left mouse click in the graph window and click the ‘Open Configuration Panel’ button, , to

display this configuration panel. The button is in the upper right of the window.

You will need to scroll down on this panel. Midway down this panel you will find the ‘Cursors’

controls, Figure 12.




Figure 12: Graph Cursor controls for measuring waveform characteristics

At the bottom of the panel you will find the ‘Time’ and ‘Voltage’ controls Figure 13.

Figure 13: Graph Time and Voltage controls

Using these controls you can adjust the graph appearance to allow you to use the cursors to

measure the characteristic of the output waveform. See figure 2, A and B.

Calculate the frequency from the period measurement => f = 1/T and the duty cycle from the

period and pulse width => duty cycle = tpw/T. Compare these with your target values. Save your

circuit. Save a screen or snippet of the schematic and graph. Put your measurements in the

properly formatted table comparing the measurement with the expected value (target value) and

error information. All the tables, schematic, and graph will go into your report.

Feel free to experiment with different values for the resistors and capacitors to see what effects

the changes have. One change that may be of interest is placing a resistor in parallel with the

bypass capacitor on pin four. Start with 5 kOhms and make the ‘value-adjust’ visible (Figure 14)

and run the simulator. While it is running you can adjust the values with the value adjuster. See

what affect it has on the output voltage levels and pulse width.

Figure 14: Component Value Adjust Pop-up Menus




Monostable Multivibrator Design

With the astable multivibrator file saved, save the file as a monostable multivibrator file.

Now rearrange the components to form the circuit shown in Figure 15. You will need to add the

switch and LED from the component toolbar. Also add an ampmeter, as well.

Figure 15: 555 based Monostable Multivibrator Circuit

From the datasheet the pulse width of the Monostable Multivibrator output is 1.1*R*C = tpw.

1. Calculate the value of RA that will turn on the LED for a number of seconds equal to 0.2

times the last digit of your ID number. If your last ID number is 0 then continue moving

through your ID number from right to left until you have a non-zero number.

2. Measure the pulse width by:

a. Setting the Trigger (in the Grapher’s Configure Pane to Single, Rising Edge,

Level = 0.1

b. Set RA to the desired value

c. Select and open the switch and leave it selected so that a small bull’s eye appears

just under it. See Figure 16

Figure 16, Switch selected indication.

3. Start the simulation and

4. Close then open the switch as quickly as possible when allowed. This is done by clicking

the bull’s eye once to close and then again to open.

5. Let the simulation run for just a little longer than your expected pulse width in simulation

time, then stop it. While the simulation if running, the simulation timer is just to the right

of the stop button replacing the “Interactive” Menu Item. See Figure 17




Figure 17: Simulation Time displayed to the right of the Stop Button

6. Go to the Graph and left mouse click on the graph, then open the Configuration Pane if it

is not open and

7. Adjust the graph start and stop times so you can accurately measure the pulse width. See

Figure 18: Pulse Width Measurement. Compare the measured Pulse Width with the

expected value you calculated and put this information along with the error information

in a properly formatted table in your report. Capture the image and put it in your report.

8. Calculate the current through Rd, which is the same as the current through the LED.

Assume the LED to have a 0.7-volt drop, and the output of the 555 to be the same as Vcc.

9. Compare the measured current (read from the scale on the right as shown in Figure 18)

with the expected value you calculated and put this information along with the error

information in a properly formatted table in your report.

Example of Design for 1 second Pulse Width

From the datasheet, the pulse width is 1 second = 1.1*R*C and C is 68uF, so

1 = 1.1 * 68E-6 *RA. Therefore, RA = 1/(1.1*68E-6) = 1(74E-6) = 13,369 Ohms.

Figure 18: 555 base Monostable Multivibrator 1 S pulse with output current.




In Figure 18, the start of the pulse was 0ms (our Trigger point) where cursor C1 is positioned.

Cursor, C2, shows the pulse width is 0.99461 sec (Cursor 2 readout at bottom) for an error of

.00539 s and a percentage error of 0.539 %. We can check with the customers and determine if

an error < 0.6% is acceptable.

The figure also shows the capacitor voltage (green) charging to 2/3 of the 5V supply voltage.

The current through the LED is graphed with a dotted, blue line and the scale is on the right side.

The current can be seen to be about 19.4 mA. Watch the value at the ampmeter during the

simulation while the output is high. There will be a Current scale setting at the bottom of the

Configuration Pane to be used to adjust this scale so it will fit in the window.

Now compare the output pulse width of your design with the design target (Expected value) and

determine the difference, the percentage error, and any comment that may be needed. Compare

the LED current with the expected value you calculated, determine the difference, the percentage

error, and any needed comments. Capture an image of the graph. Include all this in the report.

Pulse Width/Position Modulation using an Astable Multivibrator Design

Save the Monostable Multivibrator file, then recall the Astable Multivibrator and save it as a

Pulse Position Modulator file.

Now we will rearrange it a little to allow the voltage on the “Control” input of the 555 timer to

control the width of the pulses. See Figure 19.

Figure 19: 555 based Pulse Position Modulation Circuit

This circuit produces the following output waveform in Figure 20. The Green Waveform is the

modulating sign and the Blue Waveform is the position/width modulated pulses. Using the

cursors measure the time between the start of two different pulses in each of the following time

periods:




0 to 100uS, 100 to 200uS, 200 to 300uS, 300 to 400uS, 400 to 500uS, 500 to 600uS, and

600 to 700uS.

Plot these in a spread sheet program and compare the results to the modulating signal.

Put the labeled plot of the time between the start of the pulses in your report.

Figure 20: Pulse Position Modulated Signal from Circuit in Figure 19

This circuit clearly also modulates the pulse widths as well. Measure the pulse widths (Blue

waveform) of your circuit over one cycle of the modulating sign (Green waveform) and plot

them on a graph (use a spreadsheet program). Compare the plot to the modulation signal.

Include the labeled plots in your report. Capture the image of the waveforms (Snipping Tool or

screen capture) and include it in the report. Feel free to vary the component values and see what

happens.

In the Discussion section of your report, note any variations from the procedure you needed to do

in order to obtain your results. Report difficulties, misunderstandings, or errors you encountered

with the procedure. Highlight any interesting results obtained while varying component values

in the simulated circuits.




This is the last lab in ECE206L, I have enjoyed having everyone in the class and I hope you

enjoyed seeing real, physical circuits working. I regret that circumstances have prevented us

from meeting in person for the last two labs and for the last classes. I, also, regret that you did

not have the opportunity to work with the equipment in the lab. In the future, if you would like

to work on any of the circuits in the lab, let me know and I will gladly assist you. I make myself

available to you for assistance during your educational time here at UNM. If you think my

advice can help with your work, please feel free to contact me. I will be happy to assist you if I

can. Good Luck in all your future endeavors.




Appendix A:

university of new mexico department of electrical

Documents