Teaching Electronics and Laboratory Automation ... - ACS Publications

Article pubs.acs.org/jchemeduc

Teaching Electronics and Laboratory Automation Using Microcontroller Boards Gary A. Mabbott* Department of Chemistry, University of St. Thomas, Saint Paul, Minnesota 55105, United States S Supporting Information *

ABSTRACT: Modern microcontroller boards offer the analytical chemist a powerful and inexpensive means of interfacing computers and laboratory equipment. The availability of a host of educational materials, compatible sensors, and electromechanical devices make learning to implement microcontrollers fun and empowering. This article describes the advantages of using Arduino microcontroller boards for lab automation. It also includes lesson plans and exercises for teaching how to use them in the analytical chemistry curriculum. Two of these exercises include acquiring data from simple spectrometers and the control of a thermal cycler for PCR. The versatility of these devices will make them another important tool in the analytical chemist’s toolkit. KEYWORDS: Upper-Division Undergraduate, Analytical Chemistry, Curriculum, Computer-Based Learning, Laboratory Computing/Interfacing, Laboratory Equipment/Apparatus

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Microcontrollers are also a great vehicle for teaching electronics to chemists. But, why teach electronics in an instrumental analysis course? Some basic knowledge of electronics enhances one’s understanding of chemical instrumentation. All modern instrumental methods use electronic apparatus to control various aspects of the experiment whether it is controlling an oven temperature, scanning a monochromator through a range of wavelengths, driving a pump, or switching an injector valve. Most instruments have detectors that convert a chemical or physical event into an electrical signal. Spectrometers have photomultipliers or CCDs that produce current; ion selective electrodes produce voltages; GCs use flames or mass spectrometers that produce ion currents. Consequently, a basic knowledge of electronics forms a foundation for a greater appreciation of the principles of operation and capabilities of a given instrument. As an introduction to the electronics unit in class, I demonstrated for students a microcontroller performing an automated titration.12 The microcontroller turned a pump on and off to deliver acid from a reservoir standing on the pan of an electronic balance. Then it read the pH and sent the average of 5 readings to the computer. The computer fetched the mass from the balance and plotted the pH versus the mass of titrant delivered while the process continued. The students immediately understood that they were going to learn a useful skill by studying microcontrollers. In a survey after finishing the electronics unit every single person in the class said that they enjoyed the experience and learned a lot. Over half used descriptors such as “cool” and “fun”.

rogrammable integrated circuit chips known as microcontrollers have numerous possibilities for laboratory automation and merit inclusion in the chemistry curriculum. In the past two decades their application to industrial equipment, vehicles, and home appliances has proliferated to such an extent that microcontrollers are now the top-selling electronic chip of any kind.1 Recent availability of inexpensive circuit boards combining microcontrollers with supporting communication devices compatible with programming languages that derive from C has made them attractive for interfacing scientific equipment to computers. The aim of this paper is to describe their value and outline a way of incorporating microcontrollers into the undergraduate analytical chemistry curriculum.

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WHY TEACH MICROCONTROLLERS IN ANALYTICAL CHEMISTRY? Microcontrollers provide very flexible ways of controlling experiments, acquiring data, and interfacing instruments to personal computers. They can automate tasks, such as operating spectrometers,2 pumps, and valves in an automated titration system;3 control reagent dispensing systems4 and chromatographic sample injection;5 and provide temperature control of a thermal cycler and CCD read-out for fluorescence detection.6 They play a central role in miniaturizing instrumentation for field work7−9 and facilitate remote sensing.10,11 They can be used for rapid interfacing of routinely used apparatus, such as pH meters and quickly improvised equipment,12 and serve as the control and data acquisition unit for the construction of low cost instruments.13,14 © XXXX American Chemical Society and Division of Chemical Education, Inc.

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dx.doi.org/10.1021/ed4006216 | J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

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of 0.1% relative standard deviation (RSD) is acceptable can be handled by an Arduino Uno. Lots of applications fall within this realm: electrochemical experiments such as monitoring ion selective electrodes,12 voltammetry,21 amperometry, and coulometry; monitoring signals from chromatographic detectors;5,22 temperature and mobile phase control for chromatographs; capillary electrophoresis;7 microfluidic devices;23,24 simple applications of absorption spectroscopy 2,25 and fluorimetry;5 thermometry; control of mechanical equipment such as solenoids, DC motors and stepper motors, and relays; control of heater blocks and cooling units; data logging for remote sensors;26 and wireless27 and Internet communication are all possibilities. There are several other factors that make teaching microcontrollers practical now. Resources for learning to use these microcontrollers are abundant. In a week one can learn to control simple experiments. Furthermore, the low cost of these devices make it quite economical to provide each student in a class with his/her own device with which to experiment. Many electronic parts distributors offer starter kits with accessories for learning the basic principles. These kits usually include the microcontroller board, a prototyping board for easy plug-in assembly of test circuits, a USB cable, a power supply, wire for setting up circuits, LEDs, and resistors. The bigger kits also provide additional components such as variable resistors, switches, a DC motor, a servomotor, a shift register, a thermistor, and a photoresistor. Kit prices range from $50 to $125. The microcontroller board without accessories sells for about $30, and there is no additional expense for the software. (See, for example, Sparkfun28 and Adafruit.29 Parts were added to kits for this course including photodiodes, optocouplers, and operational amplifiers increasing the cost of the kit by about $16.) This article outlines lessons for teaching advanced undergraduates and graduate students how to interface experiments with microcontroller boards. While there is much to learn about computer interfacing, working with microcontrollers offers a valuable introduction to the field and yields knowledge that is directly applicable to scientific research.

Historically, special tools for interfacing such as National Instruments’ LabVIEW have been popular. LabVIEW consists of both sophisticated software and hardware. Its object-oriented software has made interfacing easier for many researchers over the past 20 years.15−17 The hardware is modular with units for data acquisition, signal conditioning, signal analysis, counting and timing, driving motors, and communicating with other devices or the Internet. LabVIEW is very powerful, but it has a very steep learning curve and is expensive. Consequently, many analytical chemistry instructors have found it difficult to allocate the time and resources necessary in order to include interfacing with LabVIEW as part of an undergraduate instrumental course. Microcontrollers now offer a practical alternative for interfacing for two reasons. First of all, newer devices contain much more memory than they did a decade ago and can accommodate higher level programming languages, making them easier to use. The second reason is that several companies are now making low cost, user-friendly microcontrollers mounted on printed circuit boards.18−20 These boards contain connectors and supporting devices for sending and receiving both analog and digital information to computers and other equipment. Arduino is an Italian company that builds several different versions of microcontroller boards based on Atmel microcontroller chips and supports them with a very extensive Web site aimed at educating potential adopters.20 Their devices are called Arduinos by users. Arduinos use an open source operating system and an object-oriented programming language that is a derivative of a language called Processing (related to Java and C++). Furthermore, these boards are inexpensive. Because Arduinos are so well supported by readily available educational materials, an Arduino board was chosen for the projects described here. The Arduino Uno board used for all examples in this article (Figure 1) has 6 (10-bit) analog-to-digital input lines, 14 digital

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INTRODUCING MICROCONTROLLERS INTO THE INSTRUMENTAL ANALYSIS CURRICULUM How might one fit microcontrollers into the chemistry curriculum? A special topics course on electronics and interfacing for chemists is one option.17 Another possibility that will reach a larger audience is to include an introduction to the topic within an instrumental analysis course. The author has taught a two-semester sequence of analytical chemistry courses at a predominantly undergraduate institution for more than 20 years. The enrollment of the instrumental analysis course usually consists of 15−20 junior and senior chemistry majors. The syllabus has always contained a two-week unit covering analog electronics culminating in students building a simple cyclic voltammetry circuit based on operational amplifiers in lab. Recently the electronics unit was reworked to cover some principles of electronics within the context of learning to use microcontrollers as outlined in Table 1. The lessons and assignments draw on material that has been widely tested by others, such as tutorials and exercises that appear online.20,28,29 Those simple exercises have been extended in this course to demonstrate how the same concepts can be applied to control laboratory equipment. The two-week unit outlined here

Figure 1. Arduino Uno microcontroller board.

input/output lines, and a separate USB (universal serial bus) connector. It hosts a number of different communication modes, such as SPI (serial peripheral interface) and I2C (interintegrated circuits) in addition to USB. (A more recent and more powerful model, called Arduino Due, is also now available.) The speed and precision of the Arduino Uno (a 16 MHz clock and 10 bit analog to digital converter) are modest compared to high-end data acquisition boards, but an Arduino can perform many lab tasks economically when high speed or precision is not needed.20 Applications where data acquisition sampling rates are below 10 kHz and where an analog precision B



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provides practical knowledge and gives students an appreciation of the possibilities of microcontrollers. The amount of information to learn may, at first, seem overwhelming. Where does one start? Fortunately, because microcontroller boards are increasingly popular with educators and hobbyists, there are many excellent resources and a variety of inexpensive hardware that can be adapted for teaching their use. The Arduino Web site includes instructions for downloading the free operating software and installing it on the Arduino; a software reference page with a glossary of programming terms; example circuits with diagrams, instructions, and sample programs (called sketches) for demonstrating each circuit; a software library; an extensive user forum; links to videos demonstrating user-built projects; and other resources. Electronic parts distributors often have their own example projects and video tutorials as well.28,29 A few universities have also posted lab experiments and equipment tutorials on line. The art department at New York University has a particularly good Web site with excellent descriptions and tutorials that are open to the public.30 Table 1 outlines a way to integrate several online exercises with class and lab.20,29 The table is based on a course that meets twice per week for 100 min classes and once per week for a 4 h lab period. Most of the material in this unit is covered in workshop fashion combining lecture, demonstration, and short periods in which students work with their microcontroller and assemble simple circuits. The electronics unit begins on the first class meeting of the semester. Only a brief summary is provided here, since details of daily lessons are described in the Supporting Information. The instructor helps the students identify components in the kit and assists them in downloading the software to their own computers on the first day. Beginning (online) instructions for programming and serial communication and the first exercises in controlling LEDs are assigned for homework. The students take the kits home. In the next class meeting the students learn to control motors and devices that require more power using transistors. A review of Ohm’s law as it applies to a voltage divider (and potentiometers) sets up the next subject: acquiring analog signals. The students practice data acquisition as part of another online lesson at home and calibrate (against an alcohol thermometer) the response of a thermistor to a cup of hot water as it cools. In the subsequent lab period students acquire data from a Spectronic 20 and then their own colorimeter (Figure 2) based on an LED as a light source and a photodiode

Table 1. Curriculum Unit on Electronics and Microcontrollers for Instrumental Analysis Course Activity Class 1

Homework 1

Class 2

Homework 2

Lab 1

Class 3

Class 4

Lab 2 Class 5

Topic Introduction to kit Downloading/installing software Introduction to sketchesmaking an LED blink Using the serial monitor Current; voltage Build circuit 1 Control LED Build circuit 2 Control red, green, blue LED using pulse width modulation Build circuit 3 Control multiple LEDs Digital control Transistors as switches Diodes DC motors Pulse width modulation for speed control Servomotors Power Power transistors Controlling DC apparatus with power transistors Relays Ohm’s law; voltage dividers; potentiometers Build circuit 4 Potentiometer to select a voltage for an analog input Build circuit 5 Using a thermistor as a temperature sensor Acquiring analog data Analog signal from a Spectronic 20 Analog signal from simple photometer based on an LED and a photodiode Converting voltage input into an absorbance value Preparing a calibration curve Determining the concentration of an unknown sample Principles of photodetectors Photomultiplier tubes (PMTs) Photodiodes and phototransistors Charge coupled devices (CCDs) Operational amplifiers Rules for predicting behavior Voltage follower Current follower Simple gain amplifier and adder circuit Noninverting amplifier Integrator Control of a simple thermal cycler for PCR Real vs ideal operational amplifiers; reading specification sheets

Figure 2. (left) Photometric data acquisition via an Arduino. (right) Diagram of colorimeter detector block. C



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Figure 3. A simple thermal cycler for PCR experiments. Diagram of apparatus (left). Thermal cycler with circuit components (right). See Supporting Information for construction details.

Figure 4. Temperature in PCR reaction tube for 6 consecutive thermal cycles.

as a detector.31−34 Later classes cover the principles of common light detectors35 and operational amplifiers. The operation of a simple thermal cycler for PCR is the challenge for the final lab exercise of the unit. The task requires both digital control and data acquisition through the microcontroller. The apparatus shown in Figure 3 is provided for the students by the instructor. Students work in pairs and reserve the equipment for a 2 h block of time at their convenience during the week. The students must bring their Arduino, their calibrated thermistor, and their own computer to complete the system. A partial program is provided to get them started, but they need to modify it in order to cycle through the three temperatures. Each team gets a different assignment for an annealing temperature and must hand in an electronic copy of their successful program with their report. The performance of the thermal cycler (using the sketch provided in the Supporting Information) is shown in Figure 4. The program used here consisted of a 95 °C denaturing step followed by a 62 °C annealing step and a 72 °C extension step.

answer four short questions. The questions required that they be able to apply Ohm’s law, to recognize and describe the function of a diode and a transistor, and to predict the output voltage of an operational amplifier circuit for a specific input. While about half could apply Ohm’s law and recognize a transistor, diodes and operational amplifiers were unfamiliar to virtually everyone. The average percentage of correct answers was 25% at the beginning of the course. The students answered similar questions 3 weeks later (after the end of the electronics unit). Then the average score was 83%. Most of the students had some introduction to electronics in a college physics course before taking the instrumental analysis course. At the start of the semester over half expressed some apprehension toward revisiting the subject. The common thread running through their comments was that it was a difficult or frustrating topic for them. Despite some discomfort, every individual indicated that they thought that knowledge of electronics would be useful to them as a scientist. At the end of the unit student opinions were unanimously positive about the experience. They felt much more confident about electronics. They liked the fact that they could make the microcontrollers do so many different things using their own computers. They especially liked being able to take the kits home and play with them. Even one student, who wrote that he still found it difficult to set up circuits properly, took the initiative to borrow extra thermistors and use his Arduino to

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CONCLUSION In the author’s course students clearly learned some electronics by working with microcontroller boards. At the beginning of the semester students were asked to complete a survey about their experience with and attitude toward studying electronics. They were also asked to look at three circuit diagrams and D



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monitor four temperature zones simultaneously in an apparatus that he was building for his research project. Seeing a student independently apply his knowledge from class to a research project of his own was strong evidence that real learning was going on. The students were better prepared to learn principles of instrumentation as a result of this experience. Following the electronics unit the course covers voltammetry, thermal methods of analysis (DSC and TGA), mass spectrometry, molecular fluorescence, Raman spectroscopy, X-ray fluorescence, and methods for improving signal-to-noise (Fourier transformation and signal averaging). (Chromatography, absorption spectroscopy, and potentiometry are covered in the quantitative analysis course; NMR and IR are covered in the organic spectroscopy course.) Their electronics experience made components in many instruments easier to grasp. There are a lot of examples where their previous lessons applied: circuitry for controlling electrode voltages and measuring current responses in voltammetry; monitoring and controlling temperature in DSC and TGA; methods of ionization, mass filtering, ion trapping, and detection in mass spectrometry; light detection in fluorescence and Raman spectroscopy; X-ray generation, detection, and energy dispersion in X-ray fluorescence spectroscopy. Placing the electronics unit first made later material more familiar and easier to assimilate. The purpose of this paper has been to bring the tremendous utility of modern microcontroller boards to the attention of chemists and outline a practical way for introducing them to analytical chemistry students. In addition to their value for controlling equipment and gathering data, they provide a fun and rewarding vehicle for introducing electronics in the context of analytical chemistry. Modern microcontroller boards provide a welcome addition to the field of laboratory automation and computer interfacing and merit a place in the analytical chemistry curriculum.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed instructor’s notes (1) for class, (2) for the exercise with simple spectrometers, and (3) for the thermal cycler experiment. Student handouts for the exercises with simple spectrometry, with the thermal cycler and with operational amplifiers, as well as two homework assignments. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The author is grateful to the University of St. Thomas for financial support and to Tony Borgerding, Joe Brom, and Kris Wammer for comments on the manuscript.

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REFERENCES

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Teaching Electronics and Laboratory Automation ... - ACS Publications

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