Explorations = In Engineering Workshop Series: Electrical Engineering=

Title of Less= on: Count Your Blinkings

Grade level /= course: Middle School (half day) and optional high school (full day)

Objectives / Concepts

Lesson Abst= ract: In this lesson, students will be introduced to the basic concepts of electric circuits and use resistors, capacitors, and integrated circuits to build a working electro= nic model. Students will use a computer to simulate the circuit.

Standards Addressed: NA

Objectives:= Students should learn about basic electricity and resistive and capacitive components and apply knowledge to build a working model. Applications using integrated ci= rcuits will be examined. Computati= onal computing modeling techniques will be utilized to show circuit behavior p= rior to actually building the circuit. Students will explore concepts involving energy, power, resistance, and capacitance. Full day students will further e= xplore concepts in voltage regulation and digital logic circuits.

Prerequisite knowledge: Prior to this lesson, students must be able to:

calculate using = basic mathematical operations
perform basic mo= use manipulations such as point, click and drag
use a browser for manipulating web based computer applications.

Course Outline and Sch= edule:

(Half Day and Full Day= )

Time	Activity<= /o:p>
9:00-9:20	Introduction, Units, Voltage, Current & Power
9:20-9:40	Circuit Elements and Diagrams
9:40-10:15	Prototyping Exercise: Light Emitting Diode (LED)
10:15-10:30	BREAK
10:30-11:00	Capacitance
11:00-11:20	Applet Exercise: 555= Timer
11:20-12:00	Prototyping Exercise: Flashing LED (and Reflections for half day students)
12:00- 1:00	LUNCH (for full day students)
1:00- 1:20	Digital Logic
1:20-1:40	Zener Diodes and Vol= tage Regulators
1:40- 2:00	Applet Exercise: Vol= tage Regulator
2:00-2:15	BREAK
2:15-3:45	Prototyping Exercise= : LED Counter
3:45-4:00	Wrap Up and Reflecti= ons

Note: A basic circuits primer and simulator primer can be found at:

http://www.shodor.o= rg/~rbroadnax/ohms_law.html

Required Equipment and Sup= plies

· Cell Phone

· Household Light-bulb, 60 watt

· 6 hex nuts, any size

· plastic pipe

555 Timer Kit= (half day session)

watt resistor (x3), see parts= list for more details

&nbs= p; solid insulated, 2 inch length

black
red
one other color

Note:

Add 10K ohm resistor, LED, and Red wire for optional d= ual flasher circuit.

555 Timer + C= ounter and voltage regulator Kit (full day session)

Drinking straws (x2, one t= hin, one fat, for illustrating resistance)

see parts list
see parts list

Light Emitting Diode (LED) (x4) Kingbright Sup= er Bright Red Water Clear , see parts list for more details

Texas Instrum= ents (TI) SN74HC393AN or equivalent
black (x8)

2 in. (x6)
3 in. (x2)

red (x2)

2 in. (x2)

orange (x5)

2 in. (x3)
3 in. (x2)

other colors (x5)

3 in. (x5)

= I. = &nb= sp; Introduction

Ask students to introduce themselves. Instructor may introduce himself in a similar manner as Dr.Rhett Davis of NC State University did in this lesson as follows.

· Good morning. I’m Rhett Davis, and you can= call me Dr. Rhett. I’m an Electrical Engineer, and I love my job, because I get to play with cool stu= ff almost every day. I’d l= ike to spend the next 3 hours introducing you to some of this cool stuff and give = you an idea of what it’s like to do what I do. In fact, I’m not only an electrical engineer, but I’m a professor of electrical engineering at North Carolina State University in Raleigh, just about 30 miles south of here. My students are off in = the world working for lots of companies, like the one that designed this cell-p= hone here (how many of you have a cell phone?). Now, that’s a little of an exaggeration, because it actually t= akes thousands of engineers to make this thing,= So, we all have to work together, and the part that I work on in particular is the microchip inside the thing, which is like its brain. I’ve brought an example chip= with me, it’s one of my most prized possessions, one = of the first chips that I ever designed. (passes it around). It has hundreds of thousands of transistors on it that allow it to carry out a function much l= ike what happens inside this cell-phone. I’d like to give you a taste of what it’s like to design this chip, but we’ve only got three hours, and it’s not quite enough time to introduce the transistor, but we can get close by introducing some similar circuits, including such elements as resistors and capacitors.=

· Start of class material: Can anyone tell me = what an engineer does?
Engineers are Problem-Solvers, solve problems de= aling with how we relate to the world. Electrical engineers focus on how we solve problems dealing with electricity and magnetism, for instance, how to build a little box that we = can carry around and use to speak to anyone in the world with a similar box (mo= tion to the cell-phone). At the he= art of all of this is an understanding of how electricity works and how to get it = to do what YOU WANT IT TO DO.

· For instance, let’s create an example = of a problem that we can actually solve in 3 hours. Let’s say that we want to ha= ve a light that can stay lit for 30 seconds without any battery connected to it,= and then turns off.

= II. = Units

· Need to use very large and very small numbers. We use abbreviations= to make our lives easier

<= span style=3D'mso-list:Ignore'>&nbs= p; &= nbsp; &nbs= p; &= nbsp; &nbs= p; i. &nb= sp; 1,000,000,000 Giga (G) (as in gigahertz or gigabytes of m= emory)

<= span style=3D'mso-list:Ignore'>&nbs= p; &= nbsp; &nbs= p; &= nbsp; &nbs= p; ii. &n= bsp; 1,000,000 Mega (M)

<= span style=3D'mso-list:Ignore'>&nbs= p; &= nbsp; &nbs= p; &= nbsp; &nbs= p; iii. &= nbsp; 1,000 Kilo (k)

<= span style=3D'mso-list:Ignore'>&nbs= p; &= nbsp; &nbs= p; &= nbsp; &nbs= p; iv. &n= bsp; 0.001 mili (m) (as in millimeter)

<= span style=3D'mso-list:Ignore'>&nbs= p; &= nbsp; &nbs= p; &= nbsp; &nbs= p; v. &nb= sp; 0.000001 micro (μ)

<= span style=3D'mso-list:Ignore'>&nbs= p; &= nbsp; &nbs= p; &= nbsp; &nbs= p; vi. &n= bsp; 0.000000001 nano (n)

= III. = Energy & Power (Optional)

· ability to do stuff. More energy more stuff= . Examples:

<= span style=3D'mso-list:Ignore'>&nbs= p; &= nbsp; &nbs= p; &= nbsp; &nbs= p; i. &nb= sp; sound

<= span style=3D'mso-list:Ignore'>&nbs= p; &= nbsp; &nbs= p; &= nbsp; &nbs= p; ii. &n= bsp; light

<= span style=3D'mso-list:Ignore'>&nbs= p; &= nbsp; &nbs= p; &= nbsp; &nbs= p; iii. &= nbsp; motion

<= span style=3D'mso-list:Ignore'>&nbs= p; &= nbsp; &nbs= p; &= nbsp; &nbs= p; iv. &n= bsp; heat

· Drop a nut for 1 second. How much energy?
E =3D ½ m v2 =3D &fra= c12; (0.01 kg) (10 m/s)2 =3D 0.5 J (kg m2/s2)

· =

· Power is energy per unit time. P =3D E/T We use the unit of Watts to represent power. You can use a lot of energy over a long time, but it’s not as = big a deal if you drop them all at once. If I drop all 4 of these over 1 second, how much power have I used?<= span style=3D'mso-spacerun:yes'> 2 Watts. If I drop them over 4 seconds, how= much power? 0.5 Watts.

· All of these things, sound, light, motion, h= eat, have a certain amount of power associated with them. Electricity has power associated w= ith it to, and electrical engineering is all about how we convert it into these different forms, such as light when you turn on a light switch, or heat when you turn on your stove, or motion when you turn on an electric motor (like a garage door opener), or sound when you play music on your I-Pod, or listen = to a phone. This is important, bec= ause if you start working with too little power, what will happen? nothing= . What will happen if you work with = too much power? Too much. For instance, the easiest thing to= turn electric power into is heat. = How much heat is enough to burn you? (60 W light bulb). How many of yo= u have ever touched a light bulb when it’s on? It’s HOT. It can burn you. How much power is dissipated in th= is light-bulb? It’s actual= ly written at the top. This one says… Actually, 1 W of = heat is enough to burn you, and 100 W is enough to start a fire. We don’t want to burn down t= his building today. So now, you h= ave to pay attention and understand how much power you’re handling so that y= ou don’t start a fire.

= IV. = Charge & Current (Optional)

· The force of gravity causes masses to move towards each other. In almost= the exact same way, the force of electricity causes charges to move towards each other. The difference is that charge can be negative or positive. Negative charges repel, positive charges attract.

· We use the name “Coulomb” to ref= er to units of charge, and use the symbol Q to represent that charge. Think of this nut as a positive ch= arge of 1 C. Thus, Q =3D 1 C. Now if I have two nuts, Q =3D ? (2= C) 7.5 nu= ts? Q =3D 7.5 C. So, you can think of charge as bei= ng like mass.

· Now, think of the earth as a big negative charge. When I let go, where = is it going to go? Down. We can think of the height from wh= ich we drop the nut (or, more accurately, the force with which we move it) as “Voltage”. ItR= 17;s a measure of how much energy the charge will pick up if we drop it. The actual relation is E =3D QV.

· If I I let a cha= rge of 1 C drop through 1 V, how much energy is expended? E =3D (1 C)(1 V) =3D 1 J.

· If I let a charge of 2 C drop through 1 V, h= ow much energy is expended?
E =3D (2 C)(1 V) =3D 2 J

· If I let a charge of 2 C drop through 3 V, h= ow much energy is expended?
E =3D (2 C)(3 V) =3D 6 J

· If I let a charge of 1 C drop through 5 V in 1/10^th of a second, how much power is expended?
P =3D (1 C)(5 V)/(1/10 s) =3D 50 W

· It’s convenient for us to talk often a= bout how much charge we have per unit of time, and we’ll use the quantity “current” to refer to how much charge we have moving per unit of time. We use the unit “ampere” to refer to the amount of charge that moves through a given spot per second. One am= pere means one coulomb moves in each second.&nb= sp; We can then calculate Power by multiplying the Current and Voltage (P=3DIV).

· If I have 0.5 amperes of current moving thro= ugh 5 V, will I burn myself? (possibly, yes… P =3D (0.5 A)= (5 V) =3D 2.5 W, which could be enough to burn you. Will I start a fire? (Probably not)

= V. = Wires & Resistance

· Ok, one last quantity to define before we can get to the fun stuff, and that is resistance. Before we talk about resistance, let’s talk about wires. Can anyone tell me what a wire is? (something that conducts electricity, a piece of metal)= . Yes, a wire is all of these things= . But primarily, a wire is something= that guides electrons to flow where we want them to. So, you can think of a wire as bei= ng a piece of pipe, like this. We = put electrons in, and the voltage creates the force to carry them through, but = the pipe guides it, so that it doesn’t go straight down. In the same way, if we take a wire= , and we set up a voltage between the two ends, current will flow from one end of= the pipe to another.

· What is a resistor? A resistor is something that resis= ts the flow of electrons. To illustr= ate this, let me pass out these straws. Everyone take one large and one small straw. You can think of a resistor it as = a very thin pipe or straw. The thinn= er the pipe is, the more it resists the flow of electrons. So now, everyone blow as hard as y= ou can into the little straw. Blow h= arder harder!!! So you feel how the= straw is resisting the flow of air through the straw. Now take the large straw and do th= e same thing. Was that easier or har= der to do? Which one provides more resistance? The smaller one.<= /p>

· We use the symbol R to represent the amount = of resistance in a resistor. The= units that we use are called Ohms, and we use the symbol Ω to represent it.<= span style=3D'mso-spacerun:yes'> Now, the very interesting thing ab= out a resistor is that when electrons flow through it, it converts their energy i= nto something else. Sometimes it’s heat, sometimes it’s light, sometimes it’s motion, sometimes it’s sound. You can calculate how much energy,= with a relation that we call Ohm’s Law (named after the German scientist, Simon Ohm, who discovered it, right about 200 years ago).= That relation is V =3D I R. That means if you know the voltage, and you know the resistance, then you can find the current and power. So, for a voltage of 3 V and a resistance of 10 ohms, how much curre= nt flows? I =3D (3 V)/(10 Ω) =3D 0.3 A. How much p= ower is dissipated? P =3D (0.3 A) (3 = V) =3D 0.9 W.

= VI. = Electricity and Water: An Analogy

One way to think about cir= cuits is to relate them to a water wheel used for grinding grain. Start with a list = of all of the things you need to use a water wheel to grind grain.

list of 5 items

▪ Water

▪ Hill

▪ Pipes/channels

▪ Water Wheel

▪ Mill

list end

Waterwheel image

Image from: http://www.muraltown.com= /sculptures4.html

Why do we need these compo= nents?

list of 1 items

▪ We need the water to turn the wheel, but it has to be moving to reach the whee= l. Thus we need a hill to cause the water to flow. We also need to make sure t= hat the water flows into the wheel, so we need channels or pipes to direct it. = We need the actual water wheel for the water to fall onto. Finally, we need a = mill or a pump that can transform the spinning of the wheel into usable power. <= /p>

list end

How does this relate to el= ectrical engineering?

= VII. = Circuits are very similar to waterwheels, but inste= ad of moving water we have moving electric charge. There is a connection betwe= en each component of the water w= heel and circuits:

= VIII. =

list of 4 items

Water =3D Electric charge

Hill =3D Battery (causes the electr= ic charge to move)

▪ Pipes/Channels =3D= Wires (Carries the electric charge where it needs to go)

▪ Water Wheel/Mill = =3D Light bulb or motor (object that needs power to do some work)

list end

= IX. = Circuit Diagrams

= X. = Ok, now lets talk about circuits. A circuit is a coll= ection of electrical components to do something useful. Let’s draw a simple ci= rcuit that uses 4 components. Two o= f them you already know: The wire (represented by a line) and the resistor (represented by this jagged shape). The resistor is like a pipe that has a dent in it that will slow the flow of water. There are two = more components that we need to know about and use.

· One being a battery, which uses these two lines. What is a battery? A battery sets up a voltage betwee= n it’s two ends or ter= minals.

<= span style=3D'mso-list:Ignore'>&nbs= p; &= nbsp; &nbs= p; &= nbsp; &nbs= p; i. &nb= sp; look at battery

<= span style=3D'mso-list:Ignore'>&nbs= p; &= nbsp; &nbs= p; &= nbsp; &nbs= p; ii. &n= bsp; pos & neg ends

<= span style=3D'mso-list:Ignore'>&nbs= p; &= nbsp; &nbs= p; &= nbsp; &nbs= p; iii. &= nbsp; voltage

<= span style=3D'mso-list:Ignore'>&nbs= p; &= nbsp; &nbs= p; &= nbsp; &nbs= p; iv. &n= bsp; note the positive & negative ends (terminals) in diagram

<= span style=3D'mso-list:Ignore'>&nbs= p; &= nbsp; &nbs= p; &= nbsp; &nbs= p; v. &nb= sp; battery like an elevator= that lifts up charge and lets it drop. Corresponds to the height of the water in the river.

· Other component is a switch. A switch is like a valve and has t= wo positions, open & closed. When closed, current flows through it. When opened, no current flows.

· Let us draw a Circuit with a 9 V battery= , a switch (open) and a 1 k Ωhm resistor. How much current flows? (none). C= lose the switch (how much current flows)? 9 mA

= XI. = Applet Exercise

· Go to http://www.fals= tad.com/circuit/ (java)

Or to http://lushprojec= ts.com/circuitjs/circuitjs.html (JavaScript)

An L-R-C circuit may appea= r with battery, switch, resistor, capacitor, and inductor.

Modify the circuit to cont= ain one battery, switch, and one resistor. (Right-click over a circuit component and choose delete, create resistors by right-clicking and choosing “Add Resistor”, then d= rag a resistor. Can also add wires to replace deleted components in the same manner). Edit the value of the resistor to = 1 K Ohms. Edit the battery voltag= e to 9 volts DC. You should have a c= ircuit with a battery, switch, and resistor, all connected by wires.

· Leave switch open. How much current flows? (none)

· Close switch. How much current flows? (9 mA) This is the same as we calculated i= n our previous circuit. What do the moving dots represent? (Ans. Current flow, or flow of electrons)

· How much power is dissipated? Is it enough to burn you? Let’s calculate the power. Power P equals voltage V times cur= rent I.

· (P=3DV*I =3D 9*0.009 =3D 81 mW. Might get warm but probably won= 217;t burn you!)

= XII. = Diodes

· Current flows in only one direction through a diode.

Modify your circuit (from the http://www.falstad.com/circuit/ applet) or (http://lushprojec= ts.com/circuitjs/circuitjs.html)

· by chang= ing the resistor to a diode. (Open the switch; Remove the resistor first and then add the diode). Then close the switch.

· How much current? How much power?= (Should be so much that it’s= off the scale).

<= span style=3D'mso-list:Ignore'>&nbs= p; &= nbsp; &nbs= p; &= nbsp; &nbs= p; i. &nb= sp; enough power to fry the diode!

<= span style=3D'mso-list:Ignore'>&nbs= p; &= nbsp; &nbs= p; &= nbsp; &nbs= p; ii. &n= bsp; if your diode doesn’t work, it may be because it burned out!

How do w= e keep it from burning up? (Put a re= sistor in series, effectively limiting the amount of current that will flow through the diode.

· Add a resistor in series. To do this, set the switch to Off; you will need to remove a portion of wire and add= the resistor using the mouse. Edi= t the value of the resistor to 1 K Ohms. <= /span>Set the switch to On and note how the current and po= wer decrease.

= XIII. = Building circuits-The Physical model

· Resistors and color codes: Brown-Black-Red (1kΩhm), Brown-Black-Orange (10kΩhm)

· Light Emitting Diode (LED): Long lead (+), s= hort lead (-)

· battery: red wire (+), black wire (-)

· Safety… what will happen if you connec= t the two leads of the battery together? R =3D 0.1 Ohm, I =3D 90 A, P =3D 810 W – definitely enough to = burn you! might start a fire!!&= nbsp; Battery may explode! DON’T DO IT! This is called a ‘short circ= uit’.

· Breadboard Basics: If you are in the half day class, = you should be using the smaller breadboard; if this is the full day class, use the = larger board. (If you have one of the transparent breadboards, use it as a “show and tell” demo or passs it around for the students to view the connecti= on points). The breadboard is us= ed to make quick, solderless connections between components. Connections are made by inserting = the wire or component lead into the appropriate location. If you view the board with the cen= ter divider space running horizontally, then 5 holes in each column above the m= id section of the board are connected by the internal wiring of the board. So, if one lead of a resistor is inserted into a hole in a particular column, and a wire is inserted into another hole in the same column, the resistor lead and the wire will be connected. The entire column = of 5 holes is called a node, or connection point. The columns located below the mid section of the board work the same way. Each column of 5 holes is independent of the next column beside it.&= nbsp; The double rows of holes located along the top and bottom edges of t= he board are connected in the same way, but in row format instead of by column= and can be used as voltage connection nodes, sometimes call= ed rails.

· Note: If you hold the breadboard vertically with the letters A-E and F-J across the top, then the rows become columns and the columns become rows.

= XIV. = LED Circuit Prototyping Exercise

· Build the LED circuit and see it working

· Bend both leads of a 10 K ohm Resistor at right angles and plug the resistor into the breadboard. Leads must be= on different nodes

· Bend the long lead (+) lead of the led outwa= rd and then downward to make a small knee in the long lead. This will effectively make both le= ads plug much easier into the breadboard.

· Plug the long lead of the LED to the same no= de as one end of the 10 K Ohm resistor.

· Connect a red wire from one of the outer rai= ls to the other end of the 10 K Ohm resistor.= Let us call this rail the Positive + rail.

· Connect a black wire from a different rail to the free end (-) end of the LED. This rail is the Negative, or Ground rail.

· Connect the red wire from the battery snap to the Positive rail where the red wire is connected.

· Connect the black wire of the battery snap to the Negative rail where the black wire is connected.

· Connect the 9 volt battery to the battery sn= ap and check that the LED illuminates. You are making progress!

· (Optional) How long will this LED remain lit? (Battery Life Chart)
Energizer Alkaline batteries claim a lifetime of 625 m= Ah =3D 2250 C
How long will it take to use up 2250 C of charge?
(9V/1000Ohms) =3D 9 mA, 2250/9 mA =3D 250,000 s =3D 4166 min =3D 69 hours = =3D 2.9 days Disconnect= the battery.

· LED with 10 KOhm resistor,

· 2 LED’s(optional)

· Fan circuit/ Fan and light (optional)

= XV. = Capacitors

· Capacitors store charge, in some sense, like= a battery.

Capacitance, C, Farad F, The letter symbol for a capac= itor is C; the unit of measure is in farads, named after its founder, Michael Faraday. The symbol for farad= is F; it is usually measured in uf, or microfarads, f= or many applications. Charge Q = =3D CV, I =3D CV/T

· Current through a capacitor depends on the r= ate of change of the voltage

<= span style=3D'mso-list:Ignore'>&nbs= p; &= nbsp; &nbs= p; &= nbsp; &nbs= p; i. &nb= sp; voltage changing quickly, high current

<= span style=3D'mso-list:Ignore'>&nbs= p; &= nbsp; &nbs= p; &= nbsp; &nbs= p; ii. &n= bsp; voltage changing slowly, low current

<= span style=3D'mso-list:Ignore'>&nbs= p; &= nbsp; &nbs= p; &= nbsp; &nbs= p; iii. &= nbsp; voltage not changing, no current flowing.

<= span style=3D'mso-list:Ignore'>&nbs= p; &= nbsp; &nbs= p; &= nbsp; &nbs= p; iv. &n= bsp; Applet Exercise:

· Go to http://www.falstad.com/circuit/

or to http://lushprojec= ts.com/circuitjs/circuitjs.html

· -> Circuits -> Basics -> Capacitor<= /p>

· Flip the switch to On. Note that , after the switch is flipped On, it takes a while for the voltage across the capacitor to reach the voltage across the battery. When the switch is initially turne= d On, there’s no charge on the capacitor, so what’s the voltage across the capacitor? (0V) How= much current is flowing? (50 mA) Because current is flowing , the capacitor starts to charge, and the voltage across the capacitor becomes, say, 2 V, w= hat happens? What is the voltage = across the resistor now? (3V) What is the current (30 mA).Since the resistor R (100 = Ohms) and capacitor C are connected in series, the current will be the same through b= oth components. Now, because the = capacitor is charging up, the current is flowing more slowly and the voltage changes = more slowly.

· How long does it take for the capacitor to g= et to half of its final value? (= about 15 ms).

· Time Constant: Notice for this circuit that if you multiply the capacitance times the resistance, y= ou get a number of seconds. In this = case, 100 ohms times 200 micro-farads =3D 20 ms.= We call this number the RC time constant, and it’s roughly equ= al to the amount of time it takes to charge or discharge a capacitor.

· How can we increase the amount of time that = it takes to charge or discharge? (increase the capacitance or resistance). Do that now… mouse over the resistor or capacitor until it turns blue, then right-click and choose edit. = Set the value to something larger and click OK. Then click reset to start the simulation. What happens?

· Pick some values to set the time constant to about 60 ms. What values did = you choose?

· Now, click the switch, note that we discharge the capacitor.

Try to make the time constan= t 20 s. How long does it take befo= re the light is very dim?

· If you’re really adventurous, try to m= ake the time constant 50 s. How l= ong does it take for the light to become very dim?

· Disconnect the battery from your circuit.

= XVI. = The 555 timer

Suppose we wanted to automatically blink our LED so as to make it become a flasher.&nbs= p; Let’s take a look at another circuit component. It is called a 555 timer and is us= ed in many applications in home appliances and entertainment, automobiles, and industry. This 8 pin integrat= ed circuit (IC) has more than 20 transistors inside along with several resistors and capacitors. The 555 timer chip, as it is calle= d, got its name from the unique fact that it has three 5K Ohm resistors in its internal construction. It can= be used as a timer to turn things On or Off for a designated time period, or can be used as a clock for other circuits, or as= a lamp flasher. We will use the= 555 to flash our LED. When we loo= k at the 555 timer module, we note that it is a rectangular package with 8 leads= , or pins, 4 on each side. This arrangement is known as a Dual Inline Package, commonly called a DIP, since= it has two rows of 4 pins each. = The package has a notch or circle to help us locate pin 1. If we hold the module horizontally= with the notch or circle facing to the left and the pins down, pin 1 will be at = the lower left corner. If we hold= the 555 timer chip vertically with the notch or circle at the top, then pin 1 i= s at the top left. The pins are nu= mbered counterclockwise. Each pin ha= s a unique function as we will observe later.&= nbsp; The spacing of the pins exactly match the spacing on our breadboard, making it easy to plug it into the breadboard for connecting resistors, capacitors, wires, and LEDs. = It will be plugged across the board divider, leaving 4 pins on either side of = the divider. This will connect the eight pins to eight separate nodes.

= XVII. Applet Exercise: The Timer Circuit<= /p>

Before we start working with our physical 555 timer ch= ip, let’s go back to the circuit simulator and click the following.Circuits>555 Timer>Square Wave Generator.

If you need help in running the simulator, click one o= f the following links.

For Mac computers:

http://ww= w.shodor.org/~rbroadnax/circuit_applet_Mac.html

For Windows computers:

http://www= .shodor.org/~rbroadnax/circuit_applet_pc.html

When the 555 timer circuit appears, note that the pin numbers along side the IC chip are not listed in sequence. This is done to mak= e the schematic (circuit diagram) much easier to follow. Oftentimes, pin 1. Called GND, or ground, is shown as being connected to a ground symbol, drawn using three progressively shorter horizontal parallel lines with a vertical line attach= ing midway of the longest (top) horizontal line. (the in= structor can illustrate what the symbol looks like.) Several other connections througho= ut the circuit may connect to a GND, or ground, symol. In reality, all of these connectio= ns are connected together at the same node, usually the – (negative) rail or= negative terminal of the battery. This convention helps to maintain clarity in the schematic diagram by reducing t= he number of wires needed to represent the circuit. Note that there is no LED pictured= in the schematic diagram. We will connect our resistor and LED to pin 3 (output) of the 555 timer chip as we build the circuit.

Make the following changes in the component values in = the circuit.

Change battery voltage to 9 volts DC.

Change resistor values to 10KOhms.

Change capacitor value to 10uf.

Select and connect the positive end of an LED to pin 3= of the 555 timer chip.

Connect the negative lead of the same LED to one lead = of a 10KOhm resistor. You may have= to add a wire to make this connection.

Connect the other end of the same 10KOhm resistor to G= ND.

Reset the circuit by clicking Reset.

Run the simulator and check that the circuit operates.= The LED should blink.

Stop the simulator.

Student Exercise: Changing the blink rate of the LED

What effect will increasing the capacitance have on the blinking rate of the LED? All= ow students to answer. (LED will blink slower.)

Ask student s to edit the value of the 10uf capacitor = and increase it to 100uf.

Reset and Start the simulator.

What happened to the blinking rate of the LED? (Answer: slower)

Building the circuit:

Now, let’s have more fun! Locate the parts to be used in the= project.

Let’s start building our circuit by plugging our= IC into the breadboard. First, g= rasp the breadboard and locate the strip that separates the breadboard into two halves. There will be pin hol= es above the divider and pin holes below the dividing strip. The 555 timer IC will straddle the divider. Place the IC so that= four pins will be on either side of the center of the board. If you are using the large board f= or the full day class, place the IC closer to the left side of the board (if holdi= ng the board horizontally) so that there will be room for adding additional components as the class progresses. Orient the 555 timer chip so that pin 1 is closer to the edge of the circuit board. Make sure that= the pins line up with the holes on the board.&= nbsp; You may have to bend the pins slightly to make them line up correctly. Press the IC into = the board, being careful not to bend any pins or to stick pins into your finger= s. A slight side to side rolling moti= on sometimes helps in plugging DIP chips.&nbs= p; We are now ready to start wiring up the circuit. Remember, The<= /span> IC chip should be oriented so that pin 1 is located at the left corner. After the IC has been plugged onto= the breadboard, connect a red wire from the Positive rail along the edge of the breadboard to pin 8 (+VCC) of the chip, and a black wire from the Negative = rail on the edge of the breadboard to pin 1 (GND). Connect the red lead of the batter= y clip to the rail where you connected the red wire coming from pin 8, and the black lead into the rail where you plugged the black wire coming fr= om pin 1. DO NOT CONNECT A BATTE= RY TO THE battery CLIP yet!

connect a 0.01uF capacitor (if you have one) between p= ins 5 and 1.

Connect one of the capacitors between pins 6 and 1, or between pins 2 and 1. Make su= re that the negative lead (short) of the capacitor is connected to pin 1.

Connect a 10kOhm resistor between pin 6 and 7 or betwe= en pins 2 and 7. Resistors are not polarized, so either end can be connected to either pin.

Connect a 10KOhm resistor between pins 7 and 8.

Use jumper wire to connect pins 4 and 8 to each other = (red) and pins 2 and 6 to each other (yellow or other available color).

Attach the positive lead (long) of an LED to pin 3 of = the 555 and connect the negative lead to a 10KOhm resistor.

Connect the other end of the resistor to pin 1 or grou= nd, or to the negative rail.

In some cases, you may need to use jumper wires to ext= end to connection to the appropriate locations.&n= bsp; Remember that pin 1 is connected to the negative rail, or ground, or= 0V, or –V, and are electrically the same point. Pin 8 and the positive rail are al= so electrically the same point, and sometimes referred to as Vcc or +V.

Re-examine the circuit to ensure that all components a= re connected correctly. Failure = to connect the circuit correctly may cause damage to some of the components. Ask the instructor or helper to ch= eck that the circuit is connected correctly before connecting the battery.

Now, connect the 9V battery to the battery clips and c= heck that the LED begins to blink continuously.=

Disconnect the battery from the battery clip.

Now, remove the capacitor from the circuit and replace= it with the other capacitor. Be = sure to check that it is plugged in with the correct polarity.

Re-connect the battery and check that the LED blinks continuously.

Does it blink faster or slower? Why?

Disconnect the battery and remove the capacitor.

Connect the first capacitor back into the circuit.

Re-conne= ct the battery and check that the LED blinks. Disconnect the battery.

Do not remove any other components from the circuit. <= /p>

Without = reading the labels, which is the 100uf capacitor?&= nbsp; Why?

OPTIONAL EXERCISE for Half Day session:

By adding another resistor, LED, and Red wire, you can create a circuit that blinks two LEDs alternately, such as located at a railroad crossing.

Do the following.

Disconnect the battery from the battery snaps.

Connect one end of the 10K ohm resistor to Pin 3 of th= e 555 timer IC.

Connect the other end of the 10K ohm resistor to an un= used location on the breadboard.

Connect the negative end (short lead) of the LED to the resistor.

Connect the positive lead of the LED to 9 volts. You may need to use a Red wire to = make the connection

Reconnect the 9 volt battery. The The two LEDs should blink alternately.

Disconnect the battery.

If this is the half day session, go to the “Conc= lusion” section.

For full day Students:

Now that you have learned about several circuit compon= ents and how they interact with each other in a circuit, we will investigate fur= ther into digital circuitry. We wi= ll use the 555 timer circuit that you have successfully built to provide input to a binary counter.

Digital Logic

Now, let us talk a bit about digital logic and binary counters.

Practically every digital computer, including laptops = and tablets, can be made from basic building blocks called gates, specifically,= AND gates and OR gates. The opera= tion of these gates is usually based on base 2 mathematics. The output of the gate has one of = two states; either it is On, or 1, or Off, or 0. The same is true for its inputs. If a high lev= el,for example, 5 volts is place on the input of the gate, the 5 volt level is usu= ally considered to be high, or On, or True, or 1 state; a low voltage, such as 0 volts, is considered to be Low, or Off, or False, or 0 state. There is no in between states. For example, in your home, when you switch a lamp to On, the light comes On;set the switch to Off, and the lamp goes Off.&= nbsp; Similarly, gates will either be On or Off= , High or Low, True or False, 1 or 0. The internal circuitry of the gate defines whether it acts as an AND gate or an= OR gate. The action of the input= s and output of the gates is shown bya diagram called= a truth table. For a two input = AND gate, the truth table will have three columns ; = one for input x, one for input y, and one for output z (x, y, and z are arbitra= ry labels). There will be four r= ows to represent the four states of x and y. for example,

If x is False and y is False, then z is False

If x is True and y is False= , then z is False

If x is False and y is True= , then z is False

If x is True and y is True,= then z is True

The truth table may look like the following.

X.y=3DZ

F F F=

T F F

F T F

T T T=

Similarly, for the OR gate, if either input ,x or y equals 1 or True, or high state, then the output z will be T= rue. For example,

If x and y are both False, then z is False

If x is True and y is True,= then z is True

The truth table looks like the following.

X+y=3Dz

F F F=

T F T

F T T

T T T=

If we examine the truth tables closely, we notice that= the AND and OR gates are complements of each other.= If we complement the inputs x and = y and out z of the OR gate, it will act like the AND gate. The complemented AND is called a N= ot AND, or NAND gate; the complemented OR gate is called a NOR gate. NAND and NOR gates are widely used= in modern computers. They will h= ave two or more inputs and can be grouped together and configured to create many computer functions such as flip flops, memory, and counters.

Inverter

The inverter is another basic but useful building bloc= k of a modern digital computer. If we invert the output of the NAND gate, we realize that if either input is Low,= the output will be High. If both inputs are High, the Outpu= t will be Low.

An inverter, on the other hand, has one input which is inverted at the output. If the input is Low, the output is High; If it is High,= the output is Low. A NAND gate is equivalent to anINVERTER if we tie the two inpu= ts of the NAND gate together.

Flip flops

Flip flops are used in many ways in computers and comp= uter related applications. They are considered to be storage devices because they remain ‘flipped’ until they are reset, or flopped. One of the simplest types of flip flops is called the Set/Reset, or = SR flip flop. It can be made by fashi= oning two NAND gates as follows.

Connect the output, which we will call P, of NAND gate A to one of the inputs of NAND gate B.

Connect the output of NAND gate B, called Q, to one= of the inputs of NAND gate A. The remaining input of NAND gate A is for signal input X and the remaining input for NAND gate B is for signal input y.

If x is Low, P is High and = the input to NAND gate B is High

If y is Low, Q is High and the input to NAND gate A is= High,Now, if x goes High, then the output P goes Low,= and the input to NAND gate B is LOW, holding output Q High.

If x goes High again, output P will go Low, again hold= ing out Q high. As long as input = y is Low, output Q will remain High. Q has been set by input x.

If input x goes Low again, output q still remai= ns High. Now, if input y goes High, output Q will go Low. It has been reset by input y. This is called a Set Reset or SR f= lip flop.,

The Delay, or D flip flop h= as built in delay of the input signal. It has an input called D, a clock input, an output called Q, and a complemented output called Q not. If the s= ignal on D is high when a transition on the clock input occurs, the state of Q will change or flip. If the signal on the D = input remains high when the next clock transition occurs, the output Q will change states again. This will conti= nue as long as D is high and the clock keeps running. If D goes low, output Q will not c= hange state. If the clock stops run= ning, Q will not change state regardless of the state of input D. The flip flop ‘remembersR= 17; its’ output state.

Binary Counters

Flip flops can be connected together to form what is k= nown as a binary counter. Using additional circuit elements and gates, we can cr= eate a ripple counter = ;that will count in binary (base 2) mode. For our purposes, we will use a 4 bit counter IC (integrated circuit) which already has the circuits set up for counting. We will use a 74HC393 four bit cou= nter, but we will use only three of the four available bits. Each internal flipflop comprises a bit.

Applet Exercise

Now, let’s go to the circuit= s applet

Click on Circuits>Sequential circuits>Counters>4-bit ripple counter

The counter circuit that appears uses a 7493 counter, = which is an older variety of the 74HC393 counter that we will be using. The wiring diagram is different bu= t the idea is the same. We will use the schematic diagram included in our handout for our mode= l.We will be using only three of the four flip flops in our counter. Our 555 timer circuit = will drive the clock input of the 74HC393 counter and serve as the least signifi= cant bit (LSB) of our 4 bits.

Each pulse from the 555 timer will turn on the LED as = we saw earlier. This pulse is connec= ted to the input of the 74HC393 counter. The first stage of the counter will divide by two the number of puls= es from the 555 timer and pass it to it output and = also to the next flip flop. The se= cond flip flop will divide by 2 and pass on its output and to the third flip flop. The third flip flop will repeat this process. LEDs wil= l be connected to the outputs of the 3 flip flops to show when the bits are present. The fourth flip flop= is not used in our circuit.

When everything is connected, the counter will count i= n binary, or base 2.&nb= sp;

One important thing to notice is that our 555 timer IC= can be powered from a DC source of 3 volts to 15 volts, so the 9 volt battery c= an be used with no problem. Howe= ver, our binary counter chip requires 5 volts to run it. This requires us to somehow figure= out a way to provide 5 volts for the counter chip. We can do this by using what is ca= lled a voltage regulator.

Applet Exercise

Voltage Regulator

The voltage regulator allows us to derive 5 volts from= our 9 volt battery to safely power both the 555 timer and the counter chips. Let’s explore how the voltage regulator works.

Go to the circuits applet a= nd click on the following.

Circuit>diode>zener&= gt;voltage reference

Our voltage circuit will be composed of a 5.1 volt Zen= er diode, a 249 ohm resistor, and a 100 uf electro= lytic capacitor. Change the values = in the applet to match yours. The Ze= ner diode is a special kind of diode that can be connected in reverse to act as= a voltage regulator. By design,= when the 5.1 volt Zener diode is connected with its positive lead to GND (-) and= its negative lead to the 249 ohm resistor which is connected to the 9 volt batt= ery positive terminal, the Zener diode will drop 5.1 volts across it. The 249 ohm resistor is used to li= mit the current that will flow through the circuit. The Zener diode will not conduct u= ntil at least 5.1 volts appears across its leads. If the voltage rises above 5.1 vol= ts, the Zener diode will clamp at 5.1 volts regardless of how much voltage is applied. In our circuit, the = Zener diode will clamp at 5.1 volts and the other 3.9 volts from the 9 volt batte= ry will appear across the 249 ohm resistor.&n= bsp; The positive lead of the second 10uf capacitor is connected to the negative end of the Zener diode (where the 249 ohm resistor is connected to= the Zener diode’s negative lead). The negative lead of the capacitor is connected to GND. The capacitor is used to filter out noise caused by the Zener diode.

Since the Zener diode will clamp at 5.1 volts, the 555= timer and the counter chips can be safely connected across the Zener diode and run off of 5.1 volts.

Putting It All Together

Hands On Activity-Voltage Regulator

Now, let us get the circuit components from the kit and begin to build the circuit on our breadboard. You should have a Zener diode, a 2= 49 ohm or 250 ohm resistor, and a 10 uf capacitor.

Be sure that the battery is disconnected from the circ= uit.

Find an unused location on the breadboard and plug in = the Zener diode. Be sure to posit= ion the positive lead near a GND location.&nbs= p; Connect a black wire from GND to the positive lead of the Zener diode (R= emember that the Zener diode is connected in reverse!).

Connect the 249 ohm resistor to the negative lead of t= he Zener diode.

Connect the other lead of the resistor to the 9 volt b= attery or to the 9 Volt Positive rail using a red wire.

Connect a wire of a different color from the junction = (node) of the resistor and Zener diode to an unused rail on the breadboard. This will become our new 5.1 volt = supply location.

Now, remove the red wire connecting 9 volts to Vcc (pin 8) of the 555 timer from the positive 9 bolt= rail and connect the wire to the new 5.1 Volt supply rail. The other end of the red wire shou= ld still be connected to Vcc (pin 8)= of the 555 timer IC.

If there are other red wires connected to the 9 volt r= ail, move them to the new 5.1 volt rail.

Connect the battery and check that the 555 timer circu= it continues to blink the LED as before.

Gently touch the Zener diode to check whether it is ge= tting hot. If it is, quickly remove= the 9 volt battery and for help.

If the Zener stays cool or gets a little warm, disconn= ect the battery. We are ready to proceed to the next section.

Now, let us hook up the counter. Remember that the counter runs on 5 volts. If we connect the coun= ter directly to the 9 volt battery, we will risk the chance of burning it out. Be careful to connect the counter’s pin 14 to the 5 volt output of the voltage regulator. In fact, both the counter and 555 = timer should be connected to 5 volts.

Do the following.

Install the counter onto the breadboard.

Use a black wire and connect pin 7 of the counter chip= to GND (ground, or the negative side of the battery).

Use a black wire and connect pin 2 to GND.

Use a different color wire and connect the output of t= he 555 timer, pin 3, to the 1clkA input, pin 1 of the counter chip.

Connect a 10K Ohm resistor to pin 3 of the counter IC.= Connect the other end of the resis= tor to the positive lead of an LED. = Connect the negative lead of the LED to GND. Use a black wire if available

Connect a 10K ohm resistor to Pin 4. Connect the other end of the resis= tor to the positive lead of another LED. Connect the negative lead of the LED to GND.

Connect a 10K ohm resistor to Pin 5. Connect the other end of the resis= tor to the positive lead of an LED. = Connect the negative lead of the LED to GND.

Connect pin 14 to a 5 volt supply. The 555 timer<= /span>, Zener diode, and positive lead of the capacitor should also be connected to this node.

Check your connections according to the schematic diag= ram included in the student handout. If you are reasonably sure that the circuit is connected correctly, reconnect = the 9 volt battery.

Check that all LEDs start flashing in binary mode. The LED that is connected to the 5= 55 timer should flash faster because it is our least significant bit. It will serve as the 1’s digit. The next LED, which is connected to the counter chip, will flash half as fast, and is the 2’s digit. The third LED is the 4’s digit; the fourth LED is bit 4 and is the most significant bit; i= t is also the 8’s digit. Our circuit counts in base 2, or binary. It will count up to decimal 15, or binary 1111, and start over.

Congratulations! You have accomplished a remarkable feat by building a working model = of the counter circuit. You have completed the basics of the building blocks used in the modern computer. Have fun!

Conclusion

Electrical engineering covers a vast area of electrical technologies used in today’s environment, from bio-medical to space travel to consumer electronics. Engineers will continue to push forward in making new discoveries th= at enhance and influence our lives.

After completing the prototyping project, disconnect t= he 9 volt battery. Do not disassem= ble the circuit.

You may keep the completed model for your enjoyment (p= er the instructor’s OK).

Return any left over parts= , wires, ICs,and tools to the instructor or teacher’s aid.

Clean up any excess paper and dispose of it properly.<= /p>

Go to ww= w.shodor.org/reflections and fill out a Reflections. Under the drop down menu, select ‘Explorations In Engineering’ and an= swer the questions. Please use com= plete sentences. Submit your Reflec= tion.

Acknowledgements:

A major portion of this lesson pla= n and circuit design were done by Dr. W. Rhett Davis, Associate Professor = of Electrical Engineering at North Carolina State University, located in Ralei= gh, NC. In addition to his contri= bution to this lesson plan, Dr. Davis has taught many middle and high school stude= nts the materials contained in this lesson plan through his participation in Shodor’s Explorations In Engineering (EIE) work= shops.

Our appreciation goes out to Dr. Davis for his contrib= utions he has made in the lives of many young people and to our engineering progra= m.

Additional thanks = and acknowledgements are extended to Mr. Paul Falstad= for the use of his JavaScript based Circuit Simulator, located at

www.falsta= d.com/circuit/

and to the folks at Lush Pr= ojects, who have created a java script version of the simulator, located at

http://lushprojects.com/circuitjs/circuitjs.html

which are used in this less= on plan.

Sincerely,

Ron Bro= adnax

Engineering Workshop Coordinator

Shodor