Circuits for teaching physics

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:

1. Affordability
   1. Save schools that would otherwise buy commercial apparatus a large fraction of the cost.
   2. Allow schools that would otherwise miss out because they couldn't afford the commercial apparatus.
   3. Allow more students of more schools to borrow lab equipment for homework, or even have more students own their lab equipment outright.
2. Pedagogical
   1. Increase student ownership of the process of learning in the laboratory.
   2. 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.
   3. Attract students interested in circuits and programming to physics.

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 canonical like kinematics, studied with the photo-gates.

circuit	votes
photogates-timer	12
anemometer	8
wheel-speedometer	7
magnetometer	7
ammeter	4
digital-thermometer	4
light-meter	3
data-logger	3
volt-meter	3

For details on the wheel-speedometer, see bicycle speedometer.

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.

Example circuits

Photo-gate timer

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.

Wheel-speedometer

See bicycle speedometer.

Anemometer

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.

Magnetometer

Read values from a Hall-effect sensor or an integrated-chip, multi-axis magnetometer.

Ammeter

The circuit design depends on whether measuring DC or AC current, and whether in contact or not with the measured circuit.

Thermometer

Measure the voltage across a thermocouple, the resistance across a thermistor, or use an integrated-chip that responds with data to serial commands.

Light-meter

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.

Data-logging

Have the micro-controller write data from whatever input to a memory-chip, or internal memory if sufficient, for retrieval later.

Voltmeter

Measure the voltage between a pin and ground, using a divider when appropriate.

Micro-controllers

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.

Programming micro-controllers

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:

1. 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.
2. 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).
3. 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.