An Introduction to Electronics for Advanced High School Students

Transcription

1 An Introduction to Electronics for Advanced High School Students Shyam Modi, 14 Submitted to the Department of Mechanical and Aerospace Engineering Princeton University in partial fulfillment of the requirements of Undergraduate Independent Work. Final Report May 1, 2014 Michael Littman Daniel Steingart MAE 442D 70 pages File Copy

2 c Copyright by Shyam Modi, All Rights Reserved This thesis represents my own work in accordance with University regulations.

3 Abstract The objective of this thesis was to design and teach a short introductory course on electronics to advanced high school students. While many high school science classes have well-developed hands-on exercises to complement theory from lecture, high school physics classes often lack a significant hands-on component. This thesis was designed to help remedy that problem. To that end, six lectures were developed with accompanying lab exercises. The topics covered in the lectures included diodes, transistors, logic gates, flip flops, the Hall Effect, relays, switches, and the Arduino. Each lesson was structured to have approximately twenty minutes of lecture, followed by twenty-five minutes of lab work. The lectures were designed to align with the topics covered in the AP Physics curriculum. The labs were sequenced with increasing complexity, such that each week s lab built on the lab exercises from the previous weeks while introducing one or two new components. I arranged to teach my course to the two AP Physics classes at Princeton High School. At the time of thesis submission, I have taught five out of the six lessons. I have thoroughly enjoyed the experience, and based on the fact that many students continue to work on their circuits long after class is over, it seems that they are enjoying the work as well. After I teach the last lesson, the students will work in small groups to complete a two week-long design project which will put together the topics they have learned. The design project is derived from the MAE412 final project; students will use the Arduino in conjunction with the electronic components they have learned about to sense, actuate, and sequence a model train set. Through this project, students will learn not just about electronics but also how to write clear project proposals and how to work effectively as a team. iii

4 Acknowledgements I am grateful to Professor Littman for his guidance throughout the year, and for supporting me in pursuing an unusual type of thesis. I am also indebeted to Jon Prevost, who spent many hours helping me in the lab and helped me proofread this report. Additionally, my work would not have been possible without the behind-thescenes work of Jo Ann Love, who made purchasing materials incredibly seamless. Thanks also to Professor Steingart for serving as the second reader for my thesis. I d also like to thank Mr. Mark Higgins, the AP Physics teacher at Princeton High School, for giving me the opportunity to guest teach in his class. Thanks also to Dr. Cherry Sprague, the science supervisor at Princeton Public Schools, for accommodating my thesis into the AP Physics schedule. Additionally, I d like to thank the Department of Mechanical and Aerospace Engineering and the School of Engineering and Applied Sciences for generously funding my thesis. In addition, thank you to Christian Fong, my roommate and friend, for taking time out of his busy schedule to take pictures of me teaching. Finally, I d like to thank my family: my parents, Raksha and Yogesh, and my brother, Vrajesh, for their support and motivation. iv

5 Contents Abstract Acknowledgements List of Figures List of Symbols iii iv vii x Introduction 1 1 Diodes and Transistors Introduction to the course Diodes as one way wires What is physically happening in a diode? The Transistor as a switch and as an amplifier Lab exercise Logic Gates and Flip Flops Motivation Logic gates Flip-flops Making logic gates Lab exercise Magnetic Proximity Sensors Motivation Discussion of E.H. Hall s paper v

6 3.3 The Hall Effect Hall Effect sensors and flip-flops Lab exercise Solenoids and Relays Motivation The solenoid Inside the relay Lab exercise Throwing a Switch Motivation The trickle charge circuit Lab exercise The Arduino Motivation The Arduino Nano Programming the Arduino Example program Other useful functions Lab exercise Final Design Project 48 Conclusions 51 A Trickle Charge Circuit Simulation 58 B Solution Code for Lab 6 60 vi

7 List of Figures 1.1 Simple circuit with resistor; current flow in CCW direction Simple circuit with resistor; current flow in CW direction IV plot of a simple circuit with a voltage source and resistor Simple circuit with resistor and diode; current flow in CCW direction Simple circuit with resistor and diode; no current flow IV plot of a diode PN junction Forward-biased PN junction Reverse-biased PN junction An NPN transistor Detailed diagram showing the inner workings of a bipolar junction transistor Pinout diagrams for 2N3904 and LM7805. The LM7805 takes +9V unregulated as an input and outputs +5V. It is already set up on your breadboard NOT gate and corresponding truth table AND gate and corresponding truth table OR gate and corresponding truth table A NAND gate combines the NOT and AND gates A NOT gate combines the NOT and OR gates A XOR gate: (A OR B) AND (NOT (A AND B)) A XNOR gate combines the NOT and XOR gates vii

8 2.8 Truth Table for S R latch OR and AND gates can be constructed using diodes and resistors Pinout diagram for 74LS The Hall Effect is observed when a magnetic field is applied perpendicular to a conducting plate Pinout diagrams for A1104 and 74LS A picture of a solenoid Ampere s Law can be used to find the magnetic field of a solenoid An current-bearing solenoid and a bar magnet have the same magnetic field The internal structure of a SPDT (single pole double throw) relay Ferromagnetic materials become magnetized in the direction of the externally applied magnetic field Pinout diagrams for 74LS244 and 2N A latching relay can be constructed using two coils and an armature in between Screenshot from DC trickle charge circuit simulation. Adapted from AC trickle charge circuit simulation on MAE412 website Pinout diagrams for 74LS244, TIP122, and 2N A top view of the Arduino Nano Pinout diagrams for 74LS244, 2N3906, and 74LS A diagram of the design project setup. The ends of the loop will be provided. Two teams will share space on each project board viii

9 List of Symbols V Voltage I Current R Resistance I c Collector current I b Emitter current β Current gain E Emitter B Base C Collector A Input 1 to logic gate B Input 2 to logic gate Q Output from logic gate S SET input to S R latch R RESET input to S R latch Q Output of S R latch Q Inverse of output of S R latch Z Output of logic gate V cc Voltage common collector, in practice +5V for this project GND Ground P RE SET pin on 74LS CLR RESET pin on 74LS Q Output pin on 74LS F Lorentz Force ix

10 q Charge of particle E Electric field vector v Velocity of particle B Magnetic field vector I/O Input/Output dl Infinitesimal path length vector µ 0 Permeability of free space I enc Current enclosed by Amperian loop L Length of Amperian loop along axis of solenoid I Current in solenoid n Number of coils per meter N North pole of magnet S South pole of magnet COM Common NC Normally closed NO Normally open x

11 Introduction Between 2010 and 2020, the number of STEM (Science, Technology, Engineering, and Mathematics) jobs is projected to increase at a rapid pace. For instance, while the projected increases of occupations on the whole is 14%, computer systems analyst jobs are projected to increase by 22%, and biomedical engineering jobs are projected to increase by 62% [24]. Simultaneously, interest in STEM careers among high school students is quite low: Only 16 percent of American high school seniors are proficient in mathematics and interested in a STEM career. Even among those who do go on to pursue a college major in the STEM fields, only about half choose to work in a related career [24]. In addition, it is clear that the interests of high school students are highly malleable; of those high school seniors that are interested in STEM, 53.3% became interested after their freshman year. [35] Recognizing the disparity between projected STEM jobs and interest in STEM among American students, educators everywhere are struggling with how to improve STEM literacy and how to encourage more students to pursue college and careers in STEM fields [14]. One approach has been to create specialized STEM schools. These schools typically offer more rigorous curricula, more qualified teachers, more instructional time, and more resources than traditional high schools [14]. The results of these schools, particularly those which are selective, have been very positive: Their graduates pursue STEM fields in college at a rate nearly 50% higher than that of other students [14]. Studies identify challenging curricula, expert instruction, and peer stimulation as key factors which influence whether or not talented high schoolers will study STEM subjects in college [14]. Given the success of special- 1

12 ized STEM schools, it could make sense to replicate aspects of their programs into non-specialized schools. Some recommendations include grounding STEM education in real-life practical problems and student participation in original research projects [14]. This thesis was designed with these recommendations in mind. The objective of this thesis was to expose high school physics students to the fun and interesting applications of the theory they were learning in class. In order to do this, I designed lectures and labs for an short introductory course in electronics. I aimed to integrate topics from the students physics curriculum into the lessons I taught. For instance, when discussing Hall sensors, I discussed the physics behind the Hall Effect, which was a straightforward application of the Lorentz force, which the students had studied in class. Similarly, when discussing the relay, I derived the magnetic field of the coil using Ampere s Law, which was part of the physics curriculum. The purpose of integrating the physics curriculum into my lectures was to demonstrate that the material the students were learning in class had real-life applicability. In addition, to give the students an opportunity to engage in an original project, I decided to have the mini-course culminate in a design project which combines the various electronic components discussed in the lessons. The purpose of this project was to give students a flavor of what engineering projects are all about: proper planning, good design, and teamwork, among other things. It was my hope that my thesis project would give some students the confidence and inspiration to study science or engineering in college. 2

13 Chapter 1 Diodes and Transistors 1.1 Introduction to the course I m Shyam, a senior MAE at Princeton. I took a course last year, MAE412, which I enjoyed. I realized that with some modifications, it was accessible to advanced high school students. I decided to make this my thesis. Over the next two months, we will cover the following topics, while emphasizing both the physics behind the components as well as the applications Diodes Transistors Flip Flops Magnetic Proximity Sensors Relays Switches Arduino After learning about this topics, the course will culminate in a design project after the AP exam. The design project will require you to put all these pieces 3

14 together. (Show video of my MAE412 project: s/k770zets0o0kugt/cimg8276.mov) 1.2 Diodes as one way wires Definition: A semiconductor device with two terminals, typically allowing the flow of current in one direction only [9] Let s compare a diode to a wire. Consider the circuit below: Figure 1.1: Simple circuit with resistor; current flow in CCW direction What is the magnitude and direction of current in this circuit? Recall that by convention, current flows from positive to negative. So the direction of current will be counter-clockwise. To find the magnitude, use Ohm s Law: V = IR (1.2.1) In our case, V = 10V and R = 50Ω, so I = 0.2A. What if we switched the direction of the battery? Figure 1.2: Simple circuit with resistor; current flow in CW direction 4

15 Then I = 0.2A in the clockwise direction. For a simple circuit with only a resistor, the plot of voltage versus current looks like this: [10] Figure 1.3: IV plot of a simple circuit with a voltage source and resistor Now, consider the original circuit with a diode put in: Figure 1.4: Simple circuit with resistor and diode; current flow in CCW direction What is the current? The diode is facing in the direction of current flow, so it behaves (almost) like a wire. The diodes we will be using have a forward voltage drop of about 0.7V [18], so the voltage across the resistor is 9.3V. Since R = 50Ω, I = 9.3V/50Ω = 0.186A, which is close to the current of 0.2A we had before. What if we put the diode in the opposite direction? Figure 1.5: Simple circuit with resistor and diode; no current flow 5

16 Diodes only allow the flow of current in one direction. In this case, the diode is facing opposite to the direction that current would flow, so the diode prevents the flow of current in the circuit. This is an idealization, however. If a diode is reverse-biased with a sufficiently high voltage (called the breakdown voltage), then it will allow current to flow through it. [37] This will usually destroy the diode due to the high amount of heat dissipated. For the diodes we will be using (1N4001), the breakdown voltage is about 50V. [18] So what does the I-V plot look like for a diode, keeping in mind the forward voltage drop and the breakdown voltage? Figure 1.6: IV plot of a diode [4] 1.3 What is physically happening in a diode? Earlier, we defined a diode as A semiconductor device with two terminals. What is a semiconductor? A semiconductor is a material with a varying ability to conduct electrical current. [12] Take a substance like silicon or germanium. These substances are not good conductors in their pure form. However, through a process called doping, the 6

17 balance of charge can be altered. Silicon has four valence electrons, so if we dope with phosphorus or arsenic (which have five valence electrons), then the overall charge is negative, and we call this a N-type impurity. Similarly, if we dope silicon with boron or gallium (which have three valence electrons), we have an absence of electrons and the overall charge is positive. This is called a P-type impurity. [3] Put a P-type material and an N-type material next to each other. happens? What Figure 1.7: PN junction Close to the P-N junction, the electrons from the N-type material fill the holes in the P-type material. This leads to an insulating section in the middle of the diode, called the depletion zone. [3] If we connect a voltage source across the diode, what happens? Figure 1.8: Forward-biased PN junction 7

18 The flow of current coming out of the positive side of the battery pushes the holes in the diode across the P-N junction, and the diode allows the current to flow through it. [3] The effort associated with pushing the holes across the P-N junction is what accounts for the forward-biased voltage drop across a diode. What if we orient the battery in the opposite direction? Figure 1.9: Reverse-biased PN junction In this case, the depletion zone gets even larger, and current does not flow. 1.4 The Transistor as a switch and as an amplifier What happens when you sandwich two diodes together? Figure 1.10: An NPN transistor It might seem that no current could flow through this, since it looks like two diodes pointing in opposite directions. 8

19 However, there is a way to allow current to flow through this structure (called a transistor). If we apply a positive voltage difference across the base (P) and emitter (one of the Ns), some current will flow from the base to the emitter, like in a diode. When holes are introduced to the base, electrons from the emitter are strongly attracted towards it. Some of those electrons will indeed combine with the holes. In practice, the emitter is very heavily doped, and the base layer is very thin. Not all of the electrons which have crossed into the base will combine with holes. The collector is designed with a thin, high resistivity layer close to the base/collector junction (and low resistivity elsewhere), so there is a strong potential gradient which pushes the electrons towards the collector terminal. Therefore, electrons are able to pass from one end of the transistor to the other. [5] Figure 1.11: Detailed diagram showing the inner workings of a bipolar junction transistor [5] 9

20 The reason that a transistor can be thought of as a switch is because we can turn on the current from the collector to the emitter by turning the base current on and off. It is more accurate to think of the transistor as a dimmer rather than a switch, because the greater the base current, I b, the greater the collector current, I c (this is true to an extent; transistors have maximum ratings for I c ; the maximum I c for the 2N3904 is 200mA) [31]. The relationship between I b and I c is given by I c = βi b (1.4.1) where β is the gain of the transistor. 10

21 1.5 Lab exercise In this lab, students will learn about diodes and transistors, by varying the direction that diodes face in the circuit (to see their use as one way wires ) and by varying the base current to the transistor (to see how the transistor can be used an an amplifier). Instructions: 1. Assemble the circuit shown in the diagram (N.b. in the diagram, the terminal of the transistor with an arrow is the emitter) 2. Vary the value of R1 from 250Ω to 4.7kΩ and then to 75kΩ. Notice that the brightness of D2 decreases as you increase the resistance. Notice also that when R1 is increased to 4.7kΩ, the brightness of D1 remains the same, but when R1 is increased to 75kΩ, the brightness of D1 decreases. That is because when R1 is 75kΩ, the transistor is not fully turned on, so the collector current is limited to only about 11mA: I c = βi b (1.5.1) I b = 0.067mA; β is approximately 160 at this current [6]; I c = 11 ma. 3. Make R1 250Ω again. Change the orientation of D2 to the opposite direction. Note that current no longer flows through the diode (demonstrated by the fact that D1 is also off). This shows the one-way wire nature of a diode. [31] [33] Figure 1.12: Pinout diagrams for 2N3904 and LM7805. The LM7805 takes +9V unregulated as an input and outputs +5V. It is already set up on your breadboard. 11

22 Lab 1 - Diodes and Transistors D D +5V R2 250 D2 C R1 C +5V 250, 4.7k, 75k LED Q1 2N3904 D1 LED B B A A Title Lab 1 - Diodes and Transistors Size Document Number Rev A Date: Saturday, March 22, 2014 Sheet 1 o f

23 Chapter 2 Logic Gates and Flip Flops 2.1 Motivation In the model trains projects, we will want to store information. In particular, we will want to know when a train is on a certain portion of the track. To control the trains, we will be using a microcontroller (a small computer) called an Arduino, which has on-board memory. This would be one way to store information. However, we want to understand how memory works. Fundamentally, memory in computers consists of millions of flip-flops, which are capable of having two stable states (0 and 1). Rather than using the on-board Arduino memory, we will use external flip-flops (74LS76) to store train locations. Before discussion what flip-flops are and how they are made, we need to have a solid understanding of logic gates, which are the one of the building blocks of electronics. We will then combine logic gates to create a flip-flop circuit. 13

24 2.2 Logic gates There are three main types of logic gates which we need to know about: NOT, AND, and OR. Each of these performs a different boolean function. Each logic gate has an associated truth table, which tells us how different combinations of inputs correspond to the output. A Q 1 0 [16] 0 1 Figure 2.1 & Table 2.1: NOT gate and corresponding truth table A B Q [16] Figure 2.2 & Table 2.2: AND gate and corresponding truth table A B Q [16] Figure 2.3 & Table 2.3: OR gate and corresponding truth table 14

25 We can combine these three logic gates to create other logic gates: [16] Figure 2.4: A NAND gate combines the NOT and AND gates [16] Figure 2.5: A NOT gate combines the NOT and OR gates [16] Figure 2.6: A XOR gate: (A OR B) AND (NOT (A AND B)) [16] Figure 2.7: A XNOR gate combines the NOT and XOR gates Logic gates have the potential to be very useful in train projects. For example, we may want Train 1 to move only when Train 2 is not on a certain part of track. In this case, we could detect if Train 2 is on a certain part of track, have this signal go through a NOT gate, and have the output control power to Train 1. 15

26 It can get pretty cumbersome to put together large sequences of logic gates. In practice, we will do most logic in the Arduino. 2.3 Flip-flops We can put together two NAND gates to create a memory cell, which we call a flip flop. There are other ways of making memory cells, but this is one of the simplest. It is called an S R latch ( S stands for set; R for reset). The bars above the S and R mean that they are active low. S R Q Q remembers [15] 0 0 undefined Figure 2.8 & Table 2.4: Truth Table for S R latch Let s walk though how this flip-flop would work. We would set it up so that by default, both S and R are HIGH (though a pull-up resistor). Now, take S LOW. What happens? The output of the corresponding NAND gate has to be HIGH. The output of this NAND gate feeds into the other NAND gate. Since R is HIGH, the NAND gate outputs LOW. So Q is LOW. This means Q should be HIGH. Indeed, we can verify this by noting that Q is fed back into the other NAND gate, and since Q is LOW, the output of the NAND gate corresponding to Q is HIGH. Now, bring S back HIGH. What happens? The NAND gate corresponding to Q has inputs of HIGH (from S) and LOW (from Q), so the output of that 16

27 NAND gate is HIGH. Q has kept the same value. Now, to verify that Q has stayed LOW, note the inputs to the other NAND gate: HIGH (from Q) and HIGH (from R), which outputs LOW. Now bring R LOW. The corresponding NAND gate has inputs of LOW (from R) and HIGH (from Q), so it outputs HIGH (i.e. Q is now HIGH). The inputs to the other NAND gate are HIGH (from Q) and HIGH (from S), so the output (i.e. Q) is LOW. By setting R LOW, we have reset Q. We can now bring R back HIGH. The inputs to the corresponding NAND gate are HIGH (from R) and LOW (from Q), so the output (i.e. Q) stays HIGH. That output is fed into the other NAND gate, and along with HIGH from S, the output (i.e. Q) is LOW. So Q and Q have retained their values. This is how a flip-flop functions as a memory cell; it can remember the previous state. Why is it illegal to have both R and S be LOW? Both of the NAND gates would output HIGH. But this is a problem; Q and Q cannot both be HIGH (by definition, Q means NOT(Q)). What would happen if both R and S were brought back HIGH simultaneously? The outcome is indeterminate. Depending on whether R or S becomes HIGH first (even by a fraction of a second), the resulting values of Q and Q will be different. 2.4 Making logic gates What is happening inside a logic gate? The logic gates we use transistors (called TTL, or transistor-transistor logic). These circuits can be quite complicated to analyze. However, it s possible to create simple logic gates using just diodes and resistors. Here are OR and AND gates using just diodes and resistors. For the OR gate, note that if either A or B is HIGH, Z will be HIGH, but if neither of them are HIGH, then Z will be low. For the AND gate, if either A or B are LOW, then 17

28 Z will also be LOW. However, if both A and B are HIGH, then there will be no voltage drop across the resistor and Z will also be HIGH. [26] Figure 2.9: OR and AND gates can be constructed using diodes and resistors 18

29 2.5 Lab exercise In this lab, students will learn how flip-flops work. They will change the inputs to the 74LS76 flip-flop in particular sequences and see how this affects the output. Instructions: 1. Assemble the circuit shown in the diagram. Note that not all pins on the 74LS76 are noted on the circuit diagram. Specifically, the pins for V cc and GND are not labeled. You will need to consult the datasheet for the 74LS76 to find which pins correspond to V cc and GND (and this will generally be true for future components that we use). 2. Connect Pin 2 (P RE) to GND and Pin 3 (CLR) to +5V. Henceforth, we ll refer to these pins as SET and RESET. What is Q? It should be HIGH. 3. Now connect both SET and RESET to +5V. Keep the circuit powered on the whole time as you perform this sequence of inputs; unlike last week s lab, the manipulations you are performing in this lab are not independent. What is Q? It should still be HIGH (the flip-flip remembers ). 4. Now keep SET connected to +5V but connect RESET to GND. What is Q? It should be LOW now. 5. Once again, connect both pins to +5V. What is Q? It should still be LOW. [19] Figure 2.10: Pinout diagram for 74LS76 19

30 D D Lab 2 - Logic Gates and Flip Flops C +5V +5V C RESET SET R5 1k R4 1k U1A J K CLK CLR PRE 74LS76 Q 15 Q 14 D3 LED D4 LED R7 270 B B A A Title Lab 2 - Logic Gates and Flip Flops Size Document Number Rev A Date: Saturday, March 22, 2014 Sheet 1 o f

31 Chapter 3 Magnetic Proximity Sensors 3.1 Motivation In the projects, we will want to know where the train is at different points in time Last time, we talked about flip-flops, which can store a state (e.g. the train is on a certain part of track or it is not) Why is this important? Maybe you only want train 2 to move if train 1 is not nearby (to avoid a crash). There are many different ways to sense that a train has entered a certain part of track. One option is to use optical sensors. Another type of sensor, which we will use, varies its output voltage in response to a magnetic field. It is called a hall sensor. 21

32 3.2 Discussion of E.H. Hall s paper The paper can be found here: html [11]. Here is a summary of the paper: Hall s inquiry started when he noticed an inconsistency between Maxwell and Edlund. Maxwell wrote that The mechanical force which urges a conductor carrying a current across the lines of magnetic force, acts, not on the electric current, but on the conductor which carries it. This didn t make sense to Hall since a wire not bearing a current is in general not affected by a magnet and...a wire bearing a current is affected exactly in proportion to the strength of the current, while size, and in general, the material of the wire are matters of indifference. When Hall read Prof. Edlund s Unipolar Induction paper, he noticed that Edlund assumed that a magnet acts on a current in a fixed conductor just as it acts on upon the conductor itself when free to move. The difference between Maxwell and Edlund motivated Hall to conduct his experiment. In his first experiment, Hall placed a spiral of silver wire between the poles of an electromagnet so that the magnetic field lines were perpendicular to the spiral. Regardless of the whether the electromagnet was on or off, the resistance didn t change (contrary to what Hall had expected). Hall then postulated that there would be a difference in potential across two sides of a conductor when current was passed through it with a perpendicular magnetic field. When he used a metal plate, he didn t detect any current using his galvanometer, because the plate was too thick. His advisor, Prof. Rowland, advised him to try to same experiment with a gold leaf (much thinner). Hall found that the current in his galvanometer varied proportionally with the strength of the magnetic field. More generally, he found that the electric field in a conductor is proportional to the magnetic field strength and the current in the conductor, and inversely proportional to the area. 22

33 3.3 The Hall Effect Figure 3.1: The Hall Effect is observed when a magnetic field is applied perpendicular to a conducting plate [27] Pass a current though a conductor, with a magnetic field applied perpendicular to the plate. How does the trajectory of an electron get affected? The Lorentz force tells us F = q(e + v B). When we first start passing current through the conductor, there is no electric field across the short side of the plate, so that term goes away and the equation simplifies to F = q(v B). [30] If we imagine an electron moving through the plate, in which direction does the Lorentz force act on it? It is moving in the opposite direction of I (recall conventional current points I in the direction of the movement of positive charge). Since B is pointing up, and v is pointed back, by the right hand rule, the force on a positive charge would be to the right. So the force on the electron is to the left. As electrons get deflected to the left, we generate an electric potential across the conductor. This is called the Hall voltage. [30] By measuring this voltage, we can detect the strength of the magnetic field. Why doesn t charge just keep on building up so that the Hall potential becomes infinite? Recall the other term in the Lorentz force, related to the electric field. 23

34 We have a force of F = qe oriented in the opposite direction to the magnetic force. When E = v B, the net force is zero and charge stops accumulating. 3.4 Hall Effect sensors and flip-flops Hall Effect sensors can be analog or digital. In analog sensors, the output voltage varies proportionally to the magnetic field strength, whereas in the digital sensors, the output is binary (5V or GND). We will be using digital sensors. The sensors we are using have three pins. One needs to be connected to an input voltage (in our case, +5V), and a second pin is connected to ground. The third pin is the output. Because it is an open collector output, we need to connect the output to +5V via a resistor. When a magnetic field is applied across the sensor, the output will be pulled down to 0V. [22] If we combine hall-effect sensors with flip-flops, we can store whether or not a train is on a certain part of a track. Connect the output of one hall sensor to the SET pin, and the other hall sensor to the RESET pin. RESET the flip-flop to start. When the train (with a magnet attached to it) passes over the first sensor, it will SET the flip flop (so that Q is HIGH). When the train passes over the second sensor, the flip-flop will be RESET and Q will go back LOW. In a later lab, will we use the Arduino to read in Q, and make sequencing decisions based on its value. 24

35 3.5 Lab exercise In this lab, students will learn how hall effect sensors work and how they can be combined with flip-flops. They will vary the magnetic field in the proximity of the hall sensors and see how it affects the output of the flip-flop. Instructions: 1. Assemble the circuit shown in the diagram. Remember that the V cc and GND are not labeled for the 74LS76. You will need to consult the datasheet for the 74LS76 to find which pins correspond to V cc and GND. 2. Start by resetting the flip-flop. You can do this by holding the magnet next to the hall-effect sensor which is connected to the RESET pin (Pin 8). The LED should be off. 3. Now put the magnet next to the other hall sensor, which is connected to Pin 7. The LED should turn on because the flip-flop has been set. 4. Reset the flip-flop by placing the magnet next to the other hall sensor. [22] [19] Figure 3.2: Pinout diagrams for A1104 and 74LS76 25

36 D D Lab 3 - Magnetic Proximity Sensors A1104 HALL EFFECT SENSOR 5 C C U1B D5 +5V +5V SIGNAL GND +5V 9 Q 11 Q 10 J 12 R8 250 K LED 6 CLK R21 1k CLR PRE LS76 A1104 HALL EFFECT SENSOR B B +5V SIGNAL GND +5V R22 1k A A Title Lab 3 - Magnetic Proximity Sensors Date: Friday, April 04, 2014 Sheet 1 o f Size Document Number Rev A

37 Chapter 4 Solenoids and Relays 4.1 Motivation We will be using a microcontroller called an Arduino to control the power on the train tracks. However, the maximum current that can be sourced from an individual I/O pin is 40 ma. [1] An N Scale train can consume upwards of 250 ma.[23] We could destroy the Arduino if we tried to use it to power the tracks. Instead, we can use the Arduino as a signaling mechanism by using it to turn on a transistor. By powering the tracks through a transistor, we don t have to worry about the current limitations of the Arduino. We want to do more than simply power the track in one direction. Sometimes we want to reverse the direction of the train, or turn off power on the track. To do this we can use a relay, which is the topic of today s lecture. 4.2 The solenoid A solenoid is a coil of wire wrapped around in a cylinder. 27

38 [17] Figure 4.1: A picture of a solenoid What happens when we pass current through the solenoid? From Ampere s Law, we have B dl = µ 0 I. Sufficiently far from the solenoid, B = 0. Because we are taking dot products, and we know that B will be along the axis of the solenoid, only one side of the closed loop contributes to the integral. We get that BL = µ 0 I enc, or B = µ 0 I enc /L = µ 0 In, where n is the number of coils per meter. [36] Figure 4.2: Ampere s Law can be used to find the magnetic field of a solenoid [13] What does the magnetic field look like? All the vertical components cancel each other out, but the horizontal components add. The magnetic field looks the same as that of a bar magnet. 28

39 [29] Figure 4.3: An current-bearing solenoid and a bar magnet have the same magnetic field 4.3 Inside the relay Figure 4.4: The internal structure of a SPDT (single pole double throw) relay [7] In the figure above, connect one of the coil terminals to the signal, and the other to ground. When the signal is LOW, there is no current going through the solenoid. 29

40 When there is no current going through the coil, the spring holds the armature in the normally closed position. This results in the common terminal being connected to the normally closed terminal. When the signal goes HIGH, current goes through the solenoid, resulting in a magnetic field. The field strength is further amplified by the iron core, which has a high magnetic permeability. [29] The contact on the armature is made of a ferromagnetic material like iron. When subjected to a magnetic field, the iron contact becomes magnetized in the direction of the magnetic field. The iron therefore gets attracted to the solenoid, regardless of which way the solenoid is oriented. Assuming this force is strong enough, it overcomes the force from the spring, and the common terminal gets connected to the normally open terminal. Figure 4.5: Ferromagnetic materials become magnetized in the direction of the externally applied magnetic field [28] Note that the armature in the relay stays in the normally open position only as long as current flows through the solenoid. There is another type of relay, called a latching relay, which does not require this constant flow of current to maintain the normally open position. An example of a latching relay is a light switch on the wall. 30

41 4.4 Lab exercise In this lab, students will learn about two new components: the 74LS244 (buffer), and the DPDT (double pole double throw) relay. They will build a circuit which reverses the direction of power on the train tracks, causing the DC trolley to reverse direction when the input to the relay is changed from HIGH to LOW. Instructions: 1. Assemble the circuit shown in the diagram. Remember that power and ground are not labeled on the circuit diagram, so you will need to consult the datasheet. Also note that the input to COM1 is unregulated power, not +5V. 2. Once you have assembled the circuit, verify that it works properly by changing the input from HIGH to LOW. You should hear a clicking sound from the relay (this is the sound of the armature moving from normally closed to normally open). Using the multimeter, verify that the outputs from Pins 11, 9, 6, and 8 are as you expect them to be. 3. When you are ready, come to the test board and connect your circuit to the train tracks. Change the input from HIGH to LOW. The DC trolley should change its direction. Because of the pull-up resistor (R9), the input is HIGH by default. Figure 4.6: Pinout diagrams for 74LS244 and 2N

42 Lab 4 - Solenoids and Relays D D D7 1N V +5V +9V Unregulated R9 1k U3 R10 C U2 C 2 18 Q2 INPUT 4 A1 Y1 16 2N3906 NC A2 Y A3 Y3 12 1k COM A4 Y4 9 NO1 13 A5 Y5 7 NC A6 Y A7 Y7 3 COM2 8 A8 Y8 1 NO2 1 A 19 1OE 2OE 74LS B Relay DPDT B B D6 1N4003 A A Title Lab 4 - Solenoids and Relays Size Document Number Rev A Date: Wednesday, April 16, 2014 Sheet 1 o f

43 Chapter 5 Throwing a Switch 5.1 Motivation Switches are used to divert a train from on train track to another. They allow us to make more complicated railroad designs. In order to throw a switch, that is, to change the orientation of a switch from one direction to the other, we need to provide two pieces of information to the switch: the direction in which the switch needs to be thrown (DIRECTION input), and when to throw the switch (TRIGGER input) Throwing a switch requires a significant amount of current. At the moment when a switch is thrown, the switch momentarily draws several amps of current. Our power supplies are only able to provide 1A of current; therefore we need a different way to provide this current. 5.2 The trickle charge circuit This week s lab is longer than that of previous weeks, so the lecture today will be focused on explaining the different parts of the circuit diagram. 33

44 Let s start with the upper left part of the circuit diagram. We use the unregulated +9V power supply to charge a capacitor. Notice that the current that is drawn from the power supply is limited by the 1k resistor. Also note that the capacitor is polarized. This means that the capacitor must be put in the circuit with the correct orientation, or you will destroy the capacitor. The longer lead on the capacitor is the positive side. Another indicator is a white ribbon on the side of the capacitor, which denotes the negative side (cathode). The capacitor (and the +9V power supply, through the 1k resistor) are connected to one of the common terminals of the relay. In practice, when current flows into this common terminal, it will be coming from the capacitor, not the power supply. This is because the capacitor is not current-limited by a resistor. This is how we avoid drawing too much current from the power supply. The NC and NO terminals corresponding to this common are connected to two of the terminals on the switch. I ve called them NC and NO. However, unlike the relay, where a constant supply of current across the coil is required to maintain the NO position, the switch does not require a constant supply of current in the coil. If the switch is in the NO position, it will stay there until current is passed through the NC coil of the switch. A simple way to think about how to build something like this is that the switch consists of two solenoids with a piece of iron in between. When one solenoid, the NC solenoid, is energized, the iron is pulled in that direction. When the NO solenoid is energized instead, the iron is pulled in the opposite direction. There are also springs which are oriented in manner that keep the iron in place once it has reached either the NC or NO positions. 34

45 [21] Figure 5.1: between A latching relay can be constructed using two coils and an armature in The third terminal of the switch is the common. It is connected to ground through the TIP122, which is a Darlington pair transistor. It consists of two NPN transistors connected to each other, so that the emitter of the first transistor is connected to the base of the second. [34] This results in a greater overall current gain, which is necessary since we want to sink several amps of current. Note that current can only flow through the transistor when it is turned on, that is, when the input to the base of the transistor is HIGH. We call this input the TRIGGER. When the TRIGGER is LOW, no current can flow through the coils of the switch, so the switch cannot be thrown. When we want to throw the switch, we make TRIGGER HIGH. This discharges the capacitor and throws the switch. Then we bring TRIGGER back LOW so that the capacitor can recharge. To control the direction that the switch is thrown, we use the DIRECTION input. Through the 2N3906 transistor, this input controls whether or not the coil in the relay is energized, and therefore whether current will flow into the NC or NO terminal of the switch. 35

46 Finally, a couple small things. The capacitors on the switch help reduce the magnitude of the transients when the switch is thrown. Without these, the transients on the power supply can affect other components (like the state of a flip flop). Also, the two pull-up resistors on the DIRECTION and TRIGGER lines coming out of the buffer are there to eliminate chatter when the circuit is first powered up. (Show Falstad circuit simulation; see Appendix A for instructions) [8] [25] Figure 5.2: Screenshot from DC trickle charge circuit simulation. Adapted from AC trickle charge circuit simulation on MAE412 website 36

47 5.3 Lab exercise In this lab, students will build the DC trickle charge circuit, which is used to throw a railroad switch. They will integrate many of the components they have used in past labs, including transistors, relays, and buffers. In addition, two new components are introduced: the switch and the Darlington pair transistor. Instructions: 1. Assemble the circuit shown in the diagram. Remember that power and ground are not labeled on the circuit diagram. Also note you are asked to use both +9V unregulated power and +5V in different parts of the circuit. 2. This is a complicated circuit which you should build in pieces. Build the DIRECTION part of the circuit first, and make sure that the relay switches from NC to NO when you change the DIRECTION input. Then build the TRIGGER part of the circuit. Before you come to test your circuit, make sure all three switch wires can be easily plugged into your breadboard. 3. To test whether or not your circuit works, start with both DIRECTION and TRIGGER LOW. Then, after charging the capacitor for a couple seconds, make TRIGGER HIGH. Depending on the original orientation of the switch, the switch may change direction, or it may not. If it does not, try it again, but this time set DIRECTION HIGH. Don t forget to make TRIGGER LOW once the switch has been thrown in order to recharge the capacitor. Figure 5.3: Pinout diagrams for 74LS244, TIP122, and 2N3906 [20] [34] [32] 37

48 Lab 5 - Throwing a Switch D D +9V Unregulated RR Switch U6 N.C. (Green) R14 NC1 11 C uF COM1 9 Common (Black) 1k NO1 C3 NC2 6 10uF + C1 4 N.O. (Red) 4700 uf +5V COM2 8 C +5V 1 A NO2 C DIRECTION TRIGGER U7 A1 A2 A3 A4 A5 A6 A7 A8 1OE 2OE Y1 Y2 Y3 Y4 Y5 Y6 Y7 Y V R15 1k 1k R16 2N3906 Q5 16 B Relay DPDT B 74LS244 R17 B 1k R18 4.7k Q4 TIP122 A A Title Lab 5 - Throwing a Switch Size Document Number Rev A Date: Wednesday, April 16, 2014 Sheet 1 o f

49 Chapter 6 The Arduino 6.1 Motivation The goal of the design projects is to build a robust system which senses, actuates, and sequences the model trains. The projects need to work automatically, without any manual inputs. In previous labs, we manually moved around signal wires (to turn on the coil in a relay, or to throw a switch). We are going to make this process automatic by using a microcontroller called an Arduino. Today s lecture will focus on the different pins on the Arduino Nano and how they are used, as well as a primer on the Arduino programming language. 39

50 6.2 The Arduino Nano Figure 6.1: A top view of the Arduino Nano [1] The Arduino has 14 digital I/O pins and 8 analog input pins. The digital pins are numbered 0 through 13, and the analog pins are numbered 14 through 19. These pin numbers will be useful when writing your Arduino code. Note that the pin numbers do not correspond directly to the physical location of those pins on the Arduino. For example, the pin which we would usually call pin 4 is connected to GND, not digital pin 4. [1] In addition to the I/O pins, the Arduino needs to be connected to power and ground. Since the recommended input voltage is 7-12V, we will connect +9V unregulated power to pin 30 (V in ). Note that there are two ground pins, pins 4 and 29; both of these must be connected to ground. [1] For power and ground, we can connect to the Arduino directly. For the I/O pins, however, we will always connect to the Arduino through the 74LS244 buffer. The reason is that each I/O pin can supply only 40 ma of current, and if we accidentally draw more than that (by using the Arduino to turn on a relay without a transistor, for example), we could damage that pin or even the entire Arduino. By using a buffer, we ensure that too much current is not drawn from the Arduino. If we make a mistake, we will break the buffer (cheap) rather than the Arduino (expensive). 40

51 6.3 Programming the Arduino To program the Arduino, we will use the Arduino software, which has already been installed on your computers. The Arduino software uses the Arduino programming language, which is simple to learn and is described here: http: //arduino.cc/en/reference/homepage Once you write the code that the Arduino is supposed to run, you will upload it to the Arduino using a USB cable. To do so, go to Tools Board select Arduino Nano w/ ATMega328. Then press the check mark in the upper left part of the window to verify that your program has been written correctly. Finally, upload it to the Arduino using the right arrow that is next to the check mark. 6.4 Example program One of the simplest programs you can write is to make the Arduino behave like a wire. Such a program would take in an input on one pin and immediately output the same value on another pin. Here is what this program would look like: int in = 5; int out = 6; void setup() { pinmode(in, INPUT); pinmode(out, OUTPUT); } void loop() { int val = digitalread(in); digitalwrite(out, val); } 41

52 Let s analyze this program. There are three main parts. In the first part, we define two integer variables, in and out, to equal 5 and 6, respectively. This means that whenever we refer to in in our program, the Arduino will know to go to digital pin 5, and whenever we refer to out, the Arduino will go to digital pin 6. The second part of the program is the setup() function. In this function, we define the pin modes. The Arduino needs to know which digital pins will be inputs and which will be outputs. To do this, we use the pinmode() function, which takes in two parameters: the number of the pin, and the mode (either INPUT, OUTPUT, or INPUT PULLUP). [2] The last part of the program is the loop() function, which contains the instructions for the actions that you want the Arduino to perform. As the name of the function suggests, the actions in the loop() function repeat indefinitely. In the case of this program, we want the Arduino to continuously read in the input on pin 5 and write it to pin 6. To do this, we use the digitalread() function, which takes one parameter: the pin number to be read in. We store this in a variable called val and then use the digitalwrite() function to output val on out, which we earlier defined as pin 6. [2] 6.5 Other useful functions There are many operators, variables, and functions that are part of the Arduino programming language, but here are a few more key functions that you may find useful in writing your programs: The boolean data type can store two values, true and false. If you are alternating between two things (for instance, a switch being NC and NO), using a boolean variable to store the current state of the switch can be useful. [2] 42