The PASCO Fan Accessory: Hardware Hacking for Physics Education – Part 1

$60??!?!?! and it isn’t programmable?!?
Time to fix that.

A little under a month ago, my physics teacher ran into a few limitations of the astronomically priced PASCO Classic Dynamics System & Fan accessory (above)  — the fan accessory is not programmable, neither is it intuitively adjustable in any way!

(Mar 9th: Updated!)

Certainly, when demonstrating Newton’s laws, it’s helpful to have an adjustable source of force! As a student, I can say that watching (or toying with) tangible and adjustable systems helps tremendously in developing an intuitive sense of the physics involved. PASCO does in fact sell a cart with some programmability, but it’s two-hundred-freakin’-bucks!!  Not cool.

The Easy/Quick & Dirty Way

Back to a month ago. My physics teacher, (of electromagnetic fame) knowing how much I enjoy  a challenge, asked me about the feasibility of controlling the speed of the fan. My gut reaction said: Sure! Variable resistor!

• Which is actually terrible, in retrospect – it’s a mistake that everybody makes at some point. A variable resistor capable of dissipating enough heat would be prohibitively expensive, not to mention bulky.

At this point, I remembered investigating the PASCO ultrasonic sensor at the beginning of the year, in an effort to achieve greater efficiency in our class measurements. I was looking for some really low-level documentation – how it generated a signal, how it filtered noise from measurements, how it inferred distance from the return time of a ping. My research turned up this fantastic document (mirror)- which revealed the major source of error was the speed of sound, as it is dependent on humidity (primarily), temperature, and pressure (altitude seems constant between classes). PASCO has some very neat documents – full of schematics and experiments – hidden in their ftp server. I hoped that this would be the case for the fan accessory – but all they had was this (mirror) manual. Not much to work with. It did explain that the two giant metal slugs attached to the fan are intended as a crude method of  slowing the fan down – of the four AA batteries, either one or two could be replaced by the metal slugs, creating 3 different voltages, and thus three different levels of force! (what did we think the slugs were for? Extra mass?!?) I was worried however, that there was neither flexibility nor any kind of dynamic control. There has to be something better. Aha! Something caught my eye:

(the small circle above “PASCO” and below the 0/1 switch)

Looks like a jack for something! Some quick searching turned up the Time Pulse Accessory – a device that is designed  “to apply a repeatable impulse for use in constant acceleration and constant velocity experiments” Exactly what I needed to make this work! It does the job…..except it’s extremely limited. I could have just stopped there – but I was still intrigued by that jack on the back of the fan – what exactly was this connector intended for? A pulse-width-modulated power source for the motor? A simple logic signal? Whatever it was, I knew that if I understand it,  I can leverage it. Let’s see what treasures the manual has to offer! First things first, we do in fact have an audio connector! A 3.5mm TRS jack – named TRS for its 3 conductors, the Tip, the Ring, and the Sleeve.

Ah! that looks like a standard audio connector!

The only other thing of interest in this manual is this page – I won’t steal the Aha! moment from you for figuring out why ;)

Need a hint? Mouse this over. We can make several logical deductions from this page. Put your Sherlock Holmes hat on First: If the device is powered by a coin-cell, and has a battery life of a few years, it must draw a truly minuscule current, at a minimal voltage.  Perhaps it’s sending a logic signal? Next, there is a warning about electrostatic discharge,  a telltale characteristic of low voltage integrated circuits, especially the modern CMOS logic integrated circuits. These devices have some incredibly thin insulation between the gate and the substrate/well. Don’t worry about it if you aren’t familiar with digital logic circuits, although I highly recommend looking into  it, it’s unimportant at the moment. What is important is the fact that static electricity, at thousands of volts, destroys that thin layer of insulation, and renders any such device immediately inoperable. More evidence for a logic signal. This confirms what we already believed, that the audio cable can’t actually be carrying current. This however, I really don’t have any idea what is going on here. Seems like a problem with the manual?

Too much ambiguity! Which device is sourcing SEVEN amps?

At this point I’m actually very certain that it is just some kind of logic signal. Time to implement.

Implementation of a novel, and superior solution

Although my mind was already exploding with ideas, I always thoroughly search the internet for anyone who did anything remotely similar to the project in question, so that I have a starting point, and some inspiration. Time to break out the google-fu! If you know where to/how to look, you can find some pretty incredible, well documented,  and all-around awesome hacks on the internet.  In my experience, some of the very best are done by undergrad students & professors, as senior projects (the former) or for their classes (the latter) — and are almost universally hard to find. They are usually PDFs, and linked to by a  class page, that page  perhaps the only link thereto (hence not making the first page of Google search. a few great examples:

•  This resistor sorter (mirror), built by Swarthmore Engineering students David Gordon Gentry and Charles Timblin Sussman  as a senior project. – I sent this one into hackaday, they loved it!
• This computer (mirror) built entirely from discrete surface mount components by Dieter Mueller – nearly 3000 transistors!!
• This device (mirror)to teach binary-decimal conversion via gumball reinforcement, built by: Dennis Schmitt, Curtis Stovall, & Seth Bell for a “Mechatronics” class at Colorado State University – Very well documented!
• This DIY Pick-and-Place machine, Built by gravelrashdotcm (yeah, me neither)
• Thousands upon thousands of Columbia University EE/CS/OtherEngineering papers
• Oh, and Dr. Eric Ayars

The Programmable Fan Cart

(The original PDF)

Dr.Ayars, a Professor at CSU Chico, has been a longtime proponent & facilitator of physics education and any advances therein – came across the same limitations a few years back :

A couple weeks ago I was at a physics teachers’ conference where Mark Masters and Tim Grove, from Indiana Purdue Fort Wayne, suggested modifying a fan cart so that it turned on or off at certain positions on its track. This would allow some interesting lab exercises in which students could look at the change in kinetic energy of the fan cart over a distance interval, relate it to the force applied by the fan, and pretend to learn even more about force and energy. [Mirror]

(Sidenote: Dr.Ayars was a Pasco employee from 1992 to 1994; he worked in the tech support department, and has his name on a great many Pasco documents)

Dr.Ayars quickly hacked together an Arduino/L293D based prototype, with PWM control and Reed Switch triggering. But he wasn’t satisfied. Just a few months later he documented an improved version, the PCB resized to fit in the holder for the Time Pulse Accessory, the breadth of options breadthened, and the code refactored. This iteration won him the AAPT apparatus competition’s First Prize!

Impressed by his work, I forwarded my thoughts on the Time Pulse accessory. Dr.Ayars was ecstatic. Time for phase 2.

changelog:

• Mar 9th: added CU EECS search