Technology, Engineering and Math-Science Academy for Advanced High School Students
25.151 Assistive Technology & Electronics
Lab 4: 555 Timers, Data Sheets, and Using Integrated Circuits
Objectives:
1. Experiment with interesting applications of 555 Timer circuits
2. Familiarize yourself with the useful information available in component data sheets, including
power ratings, functionality, and applications
3. Understand the “black box” nature of IC’s—if you can read a data sheet, it opens up a world of
applications to an electronics novice
Introduction:
The “555 Timer” is a well-known chip that has many practical uses for both beginners and experts. For
example, the last circuit in Lab 2 used the 555 timer in “astable mode” to turn an LED on and off in a very
predictable (periodic) pattern. Perhaps a blinking LED is not very useful, but with a bit of thought, you
could think of many ways to use this controllable output for much more useful purposes.
More important is the fact that all of these 555 applications are spelled out in the manufacturer’s data
sheet for the chip (and if you Google “555 timer” you will find hundreds of other resources, circuits, etc.
for the 555 timer). Chips (integrated circuits, of ICs) have been developed to accomplish many useful
tasks, including tasks you may find useful for your ATDF project.
All you need to do to find and use these chips is to:
(a) know how to “Google”
(b) be able to selectively read the data sheets to glean the important information (power
requirements, application instructions, etc.)
(c) be willing to experiment/breadboard
(d) know when to ask for a little help from someone with more experience
Due Date/Scheduled Lab time:
o Hand in on or before Thursday, March 20.
o We scheduled one day in lab for you to experiment.
o Part I is research outside of class (but feel free to ask for clarification in class), and Part II is meant
to be started in the lab, but much the reading, design work, and breadboarding should be done at
home BEFORE you come to the lab.
Key to symbols:
 = a critical concept that you will use over and over again (expect to see on a test!!!)
 = bonus question
 or  = write/draw your answer in the indicated margin.
Part I: Research questions related to data sheets and devices:
A. Use an internet search engine to find a chip that will perform the function indicated. Find at least three of these, and post
the manufacturer and chip number and link to a data sheet on your personal wiki in an easy-to-find location for Lab 4.
Chip function
1
2
3
4
5
Manufacturer
Part number
Unit Price
Amplify an audio signal (perhaps you need to boost the
volume of a device for a hearing-impaired person)
Record and play back a voice message (or other sounds)
Allow you to create an electronic touch sensor
Operate a 7-segment display using a binary number input
(similar to the Cricket display?)
Perform voice recognition
B. Data sheet questions: Manufacturer data sheets are available for not only for integrated circuits [many individual
components integrated into a single chip to accomplish a (potentially very complex) task] as well as for discrete components
such as resistors, capacitors, diodes, transistors, etc. We handed out four data sheets to you a few weeks ago (LM555 timer,
LM741 Op-Amp, a 5 mm LED, and PN2222A NPN Transistor). You should note that these devices could come from many
different manufacturers, each of which publishes a data sheet providing the specifications for their version of the product. We
would hope that for some devices, like the LM555, the manufacturers coordinate and develop a common standard to which
they all build. However, we’ve already found one instance where they do not (remember the transistor that was “backwards”
in Lab 2??). Use those data sheets (links on wiki) to answer the questions below.
1.
5 mm LED data sheet: we picked this data sheet out as a “typical” example of an LED. Note that the manufacturer
provides separate tables for diffused and transparent LEDs (what’s the difference???).
For the questions below, assume that you have a transparent green LED.
a.
What is the part number be for a green transparent LED?_______________
b.
What is the unit price for these (or comparable) LEDs? Look up on web or in catalog in lab. Assume you
purchase 100 at a time. ___________
c.
What is a “cathode”? (define for LED - look up in book or on web)
d.
Name two 2 ways to identify the cathode on an LED.
e.
What is the spacing between the two leads on the LED in inches? ______ in mm?_____
f.
What is the spacing of the holes on your breadboard in inches? ______ in mm? _____
g.
What is the dominant wavelength?________________
Use the results for the next 3 questions as a guideline for any LED you use in a circuit!
h.
 What is the maximum current the LED can handle? _____________Power? ____________
i.
 What is the forward voltage of the LED? ____________
occur? ______________
j.
 Suppose you wanted to drive a one of these LEDs using a 9V battery under the operating conditions from
the last question. What size resistor would you need? Show your calculations & draw a simple circuit
diagram. Use the correct circuit symbol for an LED!
k.
What would happen if you did NOT use a resistor? (go ahead and try this, but wear safety goggles!).
______________________________________________________________
Under what operating conditions does this
2.
PN2222A (and 2N3904) Data Sheet: NPN General Purpose Bipolar Junction Transistor (BJT). When we put
together Lab 2, we (unfortunately) used a data sheet from a manufacturer that reversed the order of the emitter (E),
base (B) and collector (C) pins! What’s the moral of the story?? Note that we posted a link to the 2N3904 data sheet
on the wiki.
a.
What are the two general purposes uses for this type of transistor according to the manufacturer?
b.
These transistors come in a variety of “packages” (why?)…which do you have?_____
PN2222A
c.
d.
e.
2N3904
What is the maximum collector current this device and handle?
What is the unit price? Look up on web or in catalog in lab. Assume you purchase 100 at a time.
What is the DC Current Gain (hfe)? Provide a range of values if necessary.
f.
On your very expensive DMM you will notice that there is a place to plug in BJTs to measure h fe.
 Plug your PN2222A (or 2N3904) in and measure hfe. _______________________
 Now plug the PN2222A “backwards” and measure hfe ______________
 This is a quick way to figure out which leads are the E, B, and C—the correct orientation will yield a much
higher hfe than will the backward configuration.
g.
 Based on your answers to the questions above, when would you decide to use a PN2222A rather than a 2N3904
in a circuit?
h.
3.
What are the circuit symbols for an NPN and a PNP BJT? Label the E, B, and C on each!
LM555/LMC555 Data Sheets. Links to both of these data sheets on the wiki, and we provided the entire LM555 data
sheet for you to use. The 555 is an integrated circuit—look at the schematic diagram on page 1 and you see that the
IC contains many transistors and resistors. However, as integrated circuits go, this is really a very simple IC. A
microprocessor IC on your home computer, for example, contains the equivalent of tens of millions of discrete
components onto a small (perhaps 10 mm x 10 mm) semiconductor substrate.
You’ll notice that the 555 data sheet contains a few pages of electrical specifications similar to the type of data
provided for the two discrete devices you looked at for #1 and 2 (the LED and transistor). However, IC data sheets
typically have many more pages of information relating to the function of each pin, sample circuits, applications, etc.
The 555 data sheet is 12 pages long; the data sheet for a microprocessor may be published in a number of volumes
(books).
We have three types of 555 chips in our inventory (look at your chip to see which you have:__________)
o
LM555 (8 pin DIP) – this is the data sheet we handed out in class.
o
LMC555 (8 pin DIP) – this is a “CMOS” version of the LM555 – a different semiconductor fabrication
process and “architecture” is used. Functionally it is the same chip, but can you identify the differences?
o
LM556 and LMC556 – these are “dual” 555 chips – they contain two copies of the 555 in one 14 pin
dual-in-line package (DIP). Why aren’t 16 pins needed (2 x 8? )
LM555
a.
b.
c.
d.
 What is the maximum current this device can “handle?” (source or sink)
“Source” means the chip (output pin 5) provides current to an external circuit. “Sink”
means an external circuit connected to pin 5 supplies current to the chip. Note that the
direction of the current relates to standard current notation – current defined as positive
charge flow (opposite of what our book uses!).
 What is the maximum source voltage (V S) that you may use?
What is the unit price? Look up on web or in catalog in lab. Assume you purchase 100 at
a time.
There is one other major difference between the two chips—what is it?? If you compare the
descriptions on page 1 for both devices you will find the difference. Then tell us which
parameter captures that key difference and provide the value for both chips to compare.
o Key difference between standard (TTL) & CMOS chips: ______________
o Parameter that capture this difference: __________________
LMC555
Part II: 555 Timer Applications
Monostable operation: single timed pulse
Review before you start: Reading on data sheet page 7 + figures 1, 2, & 3. Very important design details
should be highlighted!!
Design Task:
Using one of your potentiometers and the 555 from Lab 1 & 2, design and breadboard a monostable 555
timer circuit that has an ON time that can be adjusted from approximately 0.5 seconds to 2.5 seconds. Try
running this circuit using two different source voltages : Vs = 6V and 12V. Use a mini tactile pushbutton
switch (SPST NO-MOM) to trigger the device.
Draw your circuit diagram below and show your calculations. Use the “design notes” handout as a guide.
Demonstrate the completed circuit to us! Be sure to use the practice using the oscilloscope to observe the
signals being generated by your circuit.
Loads to try out on your circuit and record your observations:
a. For the “normally off” load, use an LED (and make sure you answer Part I problem 1(k) before you
turn this circuit on! And note that you’re operating at two different voltages. What is happens to the
output voltage (pin 3) before and after you trigger the device?
b. Try shifting your LED + load resistor to the “normally off location” to see what happens.
c. Try running a gearhead motor for the “normally off” load (you can run in parallel to your LED +
resistor load if you like). Measure the current drawn by the motor while it is running. Carefully (and
quickly) touch the 555 chip to see if it is getting warm (or hot) while the motor is running. Run for a
few cycles and continue to check the IC’s temperature.
Questions: (answer on back)
1. According the data sheet, what effect should doubling the source voltage have on the length of the output
pulse?
2. Explain how you trigger (start) an output pulse.
3. Why is it advisable to tie pin 4 “high”?
4. The second paragraph under the “monostable operation” description states that “the voltage across the capacitor
then increases exponentially for a period of t = 1.1RAC, at the end of which time the voltage [on the capacitor]
equals 2/3 Vcc.” Prove that this statement is true based on the RC theory we discussed in class.
5. Explain how the “normally on” load “works.”
6. Can you think of any useful applications for this circuit? Are there any limitations for this circuit? Could you
use this circuit to create a 1 – 60 minute timer? What R & C values would you need (and look at the values we
have in stock).
7. Other observations and conclusions:
Design notes for Lab 4, Part II –
You should be able to answer these questions from your reading assignment you did at home before tackling this
lab.
0. Make sure you draw a neat & properly labeled circuit schematic in your notebook, and include all calculations
on that same page (attach to lab).
1. What does the 555 do when operating in monostable mode?
2. What is max Vs for LM555?
Max Vcc for LMC555? Which chip do you have?
3. How do you “reset” the 555?
So what should you do to prevent false resets?
4. How do you start (“trigger”) a pulse? (what conditions?) Circuit design?
Power
Reset
Trigger
Discharge
VS +
Threshold
or
Q = Output
VCC
Control
Ground Voltage
5. Output:
 Be sure to measure high and low output voltages at the output (for Vs = 6V and 12V), and comment in your
discussion/results.
 What does “active low” and “active high” mean?
 How much current should go through an LED (and what is a diode?)
 What is typical voltage drop on a “lighted” LED?
 Circuit design?
Reset
Trigger
Power
Discharge
VS +
Threshold
or
Q = Output
VCC
Control
Ground Voltage
6. Be careful when using potentiometers! What might happen when you “dial to zero?” When you design your
circuit for Lab 4 part IIA, you should select values such that…
 full counterclockwise = ½ second
 full clockwise = 2 ½ seconds (or vice-versa).
0
0.5
1.0
1.5
2.0
2.5