Circuits for teaching physics
- 1 Rationale
- 2 Learning physics with circuits
- 3 Sensors
- 4 Outputs
- 5 Example circuits
- 6 Micro-controllers
- 7 Data-handling
- 8 Supporting apparatus
- 9 Sample lessons
- 10 Teachers using circuits
- 11 More resources
- 12 References
I've taught my students to build electronic circuits, and, separately, I've taught my students to devise their own experimental apparatus. While these two movements in my teaching have been two of the most successful, I've yet to successfully combine them, yet to ask them to build their own circuits as lab apparatus for studying something else. It's so promising that I've decided to ask others to join me in the trial.
Having our physics students build their own apparatus might have multiple benefits:
- Save schools that would otherwise buy commercial apparatus a large fraction of the cost.
- Allow schools that would otherwise miss out because they couldn't afford the commercial apparatus.
- Allow more students of more schools to borrow lab equipment for homework, or even have more students own their lab equipment outright.
- Pedagogical benefits
- We hope to increase student ownership of the process of learning in the laboratory. Students may be more interested in the measurement of a physical variable when they have created the apparatus.
- Allow students to learn a skill to be used in other environments, including students who would not elect to learn circuits or programming, but would elect physics.
- Attract students interested in circuits and programming to physics.
- Learning about identifying resistors and what makes appropriate substitutions helps students understand why orders of magnitude are an important way of making comparisons. After they've started to learn the resistor color code, I ask my students which color-band is the most important, i.e. getting which one wrong would most likely affect the operation of the circuit.
- Specifying or substituting components, for example choosing among diodes or transistors, should help a student learn about the physical characteristics of those components.
- Students should be less iffy about the nature of electrical connections than students who study physics without building circuits.
Learning physics with circuits
What will students learn about physics with circuits they build?
Traditionally, physics students learn about circuits as a topic of physics after learning about electricity and magnetism. Instead, here I am suggesting we teach students about circuits as tools for learning in a wider range of fields of physics, from mechanics to astronomy, with rich opportunities to learn the traditional things about circuits scattered along the way.
So, for starters, students should be expected to learn about the traditional topics in circuits, such as resistance, capacitance, induction, EMF, parallel and series combinations, amplification, and various more advanced topics.
But using circuits they can investigate waves, interference, optics, kinematics, dynamics, electric and magnetic fields, energy transfers, thermodynamics, the spectrum, and more.
In February, 2014, I took a poll of teachers in the Syracuse area, asking which circuits, from a list I thought might be easy yet useful, they'd most like to make in a workshop to take to their schools. The following table of results shows how important photo-gates are to physics teachers. One could argue that volt-meters are so popular that they're universally already-owned. Or, one could argue that they are not popular because they are used for studying electricity, not as canonical a topic as the kinematics studied with the photo-gates.
Some circuits use only discrete components, many use integrated circuits like the 555 timer or the 741 op amp, and some use micro-controllers to gather data or run a complex algorithm or interface.
The micro-controller requires special consideration, because each unit, be it a chip or a board, can cost anywhere between $1 and $40. With the right programming-circuit and software, a $2 chip can be programmed as easily as a $10 chip, and may be versatile enough to do what some are doing with their $30 boards.
There is a multitude of sensors that can be connected to processors or directly to outputs. Sensors can be used to study the physical phenomena sensed, but they also can be used to teach the different ways of processing their signals.
- Humidity sensors
- Humidity sensors may output relative or absolute humidity values, and are often packaged with a temperature sensor so we can convert to the other value through software.
- Light-dependent resistors
- LDRs are usually cadmium sulfide, which changes resistance according to how much light is falling on the cell. Cadmium is a toxic metal, so we usually use non-toxic alternatives like photo-transistors or photo-diodes or solar cells, especially for children's use. They react to changes in light more slowly than diodes and transistors, but they are easy to model as predictable, variable resistors.
- Potentiometers and rheostats
- Variable resistors are usually a knob or a slider that change resistance from zero to some given value. Some, marked α (alpha) or A, are logarithmically tapered so that they can control audio or light over a wide, exponential range of values. Linear variable resistors are marked β (beta) or B.
- Temperature-dependent resistors may have a negative or positive temperature coefficient, according to whether their resistance goes down or up when their temperature goes up. See wikipedia for more information, including a discussion of how they can increase temperature because the circuit is adding energy via the current used to sense its resistance. http://en.wikipedia.org/wiki/Thermistor
- Thermopiles are semiconductor devices that change resistance or voltage in a circuit depending on their temperature, and they are thermally shielded so that they rise and fall in temperature with remote objects they face through a window in their otherwise heavy insulation. The window is usually made of a material that doesn't allow visible light, so that it is tuned to sensing temperatures corresponding to a peak wavelength in far infrared. These are the sensors used for non-contact thermometers, like those medical personnel use to take a person's temperature via ear or mouth.
- Passive infrared motion detectors
- PIR devices often output a voltage between their supply and 1.5V when not detecting changing infrared light, then short their output to ground when they do detect changing infrared light. When using such PIR detectors with a micro-controller or similar logic ICs, pull the output up with a resistor, which may be an internal feature in micro-controllers like the Arduino, so that the output is clearly high or low.
- Switches can be purchased or hand-made from all sorts of materials, like chewing-gum wrappers. A multitude of types of switches can be salvaged from old electronic equipment, like remote controls, tape players, and phones. They connect one or more conduits to one or more other conduits, and may make the connection when pushed, or disconnect when pushed, or toggle between connected and disconnected when pushed and released. They may spring back to a default position or they may stay in any state.
- Tilt switches
- These switches consist of a metal ball or mercury that makes contact only when it is tilted a certain amount. This is how mercury thermostats work.
- Ultrasonic distance sensors
- These sensors send a train of ultrasonic pings when their trigger pin is taken high or low, then set an echo pin high or low for the duration of the flight of the pings to a reflecting object and back.
- Vibration switches
- These switches consist of a stiff wire surrounded by a metal coil that only contacts the wire when shaken.
There are ways to connect sensors from commercial systems, the most well-known companies being Vernier and PASCO, to circuits—Some sensors output analog values on one wire when powered on two others, while some speak a digital code that may be open to the public. For example, Vernier publishes a guide to connecting their sensors to the Arduino.  PASCO explains how the wires on various sensors function.
See #Display for a discussion of displaying characters.
- Circuits can vibrate a speaker to make sound, and the pitch, timbre, and/or volume can indicate criteria.
- Piezo buzzers
- Piezo buzzers buzz a certain tone with an applied voltage, but can be made to make more interesting sounds.
- Piezo elements
- Piezo elements click when the voltage applied goes over some threshold, and the clicking can be very fast, so one may make square waves of many frequencies with them.
All the following transducers may be arranged in character or graphical displays, including light bars, bar graphs, multi-dimensional arrays, and two-dimensional screens.
- Light emitting diodes
- LEDs usually convert a specific voltage to a narrow band of light with a frequency correlated to that voltage. i.e., blue LEDs operate at a higher voltage than red LEDs. LEDs can turn on and off extremely quickly, so dimming is usually handled by running them with a square wave of varying duty cycle. When they're not busy emitting, LEDs can also be used as photodiodes to detect light levels.
- Incandescent lamps
- Incandescent lamps can be had over a wide range of voltages, and their brightness and color depend on the voltage applied.
- Plasma lamps
- Plasma lamps, for instance neon bulbs, require high voltages to light, but can be visible with very little current.
- Liquid crystal display
- LCDs typically use minuscule amounts of power compared to other ways of displaying characters or graphics. At around $10 per display, they are an inexpensive and efficient way to display a short line or two of text. While many displays are proprietary, there are also many with common protocols that allow easy control.
Relays are switches controlled by current, for instance allowing a 5-volt DC system to control a 110-volt AC system without any electrical connection between the two systems.
- Magnetic relay
- Magnetic relays are the switches you may hear in the car when you turn on or off a device, for example when a low-current line to a switch on the dashboard switches a high-current line through the headlights.
- Solid state relay
- SSRs rely on semiconductor properties to allow a small current to light an LED which controls a photo-transistor or light-activated silicon-controlled rectifier (LASCR), thus electrically isolating the two systems through light. They can be ten times the cost of magnetic relays, but can last much longer and act much more quickly.
A photo-gate timer uses light emitters and receivers to detect the passage of an object. The simplest timer would be a single light receiver, such as a photodiode or light-dependent resistor, that darkens when the leading edge of an object passes over it and lightens when that object's trailing edge passes over it. A circuit could be made to count beats of a clock between the two events. Then, for example, the known length of the object could be divided by the time between the two events to determine the average velocity of the object through the gate.
A more commonly used system in the teaching laboratory is a set of two gates, where the source is not the ambient light but a light-emitting diode (LED) paired with a photodiode designed to be sensitive to the color of the LED, that color usually being infrared. Each gate is triggered when the leading edge of the object first blocks the light. By only paying attention to the leading edge, the timing is independent of the geometry of the object and its shadow. By using infrared instead of visible light, spurious signals from visible light and shadow in the lab are removed, and the receiver does not have to be calibrated for laboratories of varying brightness. The disadvantage of infrared is it is harder to tell whether the emitter is operating—Keep an old cell-phone camera handy; because they lack an infrared filter, they can be used to check IR LEDs.
It is possible to time events without micro-controllers or computers—Shading the first detector could lower voltage enough to trip a monostable 555 to reset a bank of counter ICs. The least-significant counter IC could be fed clock signals from an astable 555. The last detector could likewise disable counting.
But, the complexity of watching multiple gates, ignoring spurious signals, counting time, and reporting that time to a display or computer, makes a micro-controller the best tool for the job.
For an example, see photo-gate timer.
See bicycle speedometer.
Most speedometers are really rotational frequency-meters, with readings calculated according to the circumference of the spinning object. See frequency meter.
Exactly the same circuit as wheel speedometer, or may use a lookup table instead of a calculation if it does not have a linear response to wind speed.
Read values from a Hall-effect sensor or an integrated-chip, multi-axis magnetometer. See magnetometer for more discussion.
The circuit design depends on whether measuring DC or AC current, and whether in contact or not with the measured circuit.
Measure the voltage across a thermocouple, the resistance across a thermistor, or use an integrated-chip that responds with data to serial commands.
Measure the resistance of a light-dependent resistor (LDR), the voltage through a photodiode (like a solar cell), the current through a reverse-biased photodiode, or the current through a photodiode.
For each of two sensors behind two crossed polarizers, measure the resistance of a light-dependent resistor (LDR), the voltage through a photodiode (like a solar cell), the current through a reverse-biased photodiode, or the current through a photodiode.
Have the micro-controller write data from whatever input to a memory-chip, or internal memory if sufficient, for retrieval later.
Measure the voltage between a pin and ground, using a divider when appropriate.
There are thousands of integrated circuits that serve specific purposes, such as counting voltage drops on one pin and providing the count as binary on a set of other pins. Micro-controllers allow us to program a chip to serve a custom purpose, for comparison to the previous example, say counting voltage drops on one pin, comparing the period between the last two drops to the average of the last ten periods, to determine if something is slowing down or speeding up, then lighting a green or red LED accordingly. Micro-controllers allow us to customize them through programming, and most can be re-programmed many times, making them a versatile investment.
To learn your commands, they micro-controller must be told them, and there are very many ways this can happen, depending on the model of micro-controller. Here are some methods I've used, with costs for comparison:
- Use PICAXE computer software (free) to write the commands in BASIC, compile it, and send it via serial cable ($10-20) to a PICAXE chip ($3-$10) in a programming circuit ($5), then put the chip in the working circuit.
- Use linux program MCU8051 (free) to write and compile a program in 8051 assembly language, and send it via serial cable ($10-$20) to a parallel-eeprom programming board ($100) to an Atmel 89C2051 chip ($1).
- Use Arduino software (free) to write and compile a program in C and send it via serial or USB to an Arduino board ($10-$30) or an AVR micro-controller ($3-$5), on a breadboard ($5) with supporting components ($1), that has been loaded with the Arduino bootloader.
Many of these scenarios require only very minimal computers by today's standards. In fact, it's best to have an older computer (late 1990s, early 200s) that has built-in serial ports and can easily be outfitted with Linux.
Learning to program micro-controllers is not completely unlike learning to program computers, and should be an enriching experience for students.
There are many ways to read measurements.
Some applications require data to be stored and retrieved later, such as in long-term anemometry or thermodynamic studies.
- A single count, such as in a timing application using pulses from a 555 IC, can be stored in a digital counter IC.
- Micro-controllers can store data in flash memory to be retrieved via a display or serial communications.
- Micro-controllers can write to disk drives, flash memory, or removable flash memory.
- A circuit can communicate with a computer via a serial connection.
Digital circuits can have a numeric display.
- 1-4 digits are best displayed by seven-segment LED displays, which can be directly driven by micro-controllers with at least 8 output pins, or driven by counter or driver ICs.
- More digits and/or text can be displayed by LCD displays ($10-$20).
- Many circuits can communicate with a computer to display and/or store readings, usually through a serial port.
- A voltmeter across two points of a circuit can be used with a lookup table to determine measurements.
Many laboratories will need equipment such as computers for programming micro-controllers or oscilloscopes for visualizing outputs of sensing circuits.
There are many considerations when choosing power supplies
- Batteries are excellent for safety. One thing to watch out for is students putting two 9V batteries together, since their terminals almost beg to be put together. 9V batteries are also dangerous to store, having both terminals so close to each other, leading to inadvertent shorts.
- I use rechargeable batteries, which easily last through a school-day or through a unit, then go into a recharger.
- Some components, like the rugged 555 timer IC, can take a wide range of voltages from supplies. Micro-controllers usually have much stricter operating ranges, and some are incompatible with others, two popular ranges being 4.5V-5.5V, and 3.0V-3.5V.
My students usually use a 2xAA, 3xAA, or 4xAAA battery pack or a 9V battery, all which connect to a breadboard with a standard 9V-style-terminated cable.
My students use and re-use versatile solderless breadboards. It takes a lesson for them to understand how it makes connections in circuits, and I've found students of all ages over 9 are capable, after a few times practice, of understanding the difference between an abstract diagram and the actual placement of components on the breadboard.
Tools for an electronics lab
- Needle-nose pliers.
- Fine wire cutters.
- Chip pullers and inserters.
- Digital multimeters; measuring DCV, ACV, mA, A, Ω, and Hz, preferably.
- Transistor/diode tester (called "Component Tester" at Radio Shack).
Ask each student per pair to make a circuit that produces a tone that they expect to be harmonious with the other student's, then examine the two as X and Y components on an oscilloscope, enjoying the lissajous-figures associated with the harmonic pairs.
Build a spectrometer with a solar cell and a stepper motor, then investigate spectra.
Build a circuit to measure an unknown resistance by comparison with known resistances.
Teachers using circuits
These educators have been kind enough to share their experiences using circuits to teach physics:
- Peter Siegel, California State Polytechnic University, Pomona
- John Liu, St. Cloud State University, Minnesota
- William Baird et al., Armstrong Atlantic State University, Savannah, Georgia
- See their article The Light-Emitting Diode as a Light Detector in The Physics Teacher, v. 49, p 171 (2011). http://scitation.aip.org/content/aapt/journal/tpt/49/3/10.1119/1.3555506
- Glen R., Watsonville, California
- Andy Cave, Polytechnic Institute of New York University
- Jason Harper
- Kyle Forinash and Ray Wisman
- Kyle and Ray have developed many activities that use mobile devices and electric circuits to measure physical phenomena. http://mobilescience.wikispaces.com
EnergyTeachers.org is building a list of where to buy electronics components, tools, and kits: http://energyteachers.org/forum/viewtopic.php?f=26&t=3375
OpenTP is creating lessons for students to study physics in a way to move away from labs merely confirming ancient physics. (in French) https://opentp.fr