Arduino-based Physics Lab

[Spiff]

I teach high school physics. I'm planning to buy a set of arduinos for various student labs and projects. First up: a centripetal force lab. I've built a prototype and it works great, but now I need to make it cost efficient and easy to replicate. I'll make a youtube video soon so it's clear what I intend. Basically, I have a ruler attached to a stepper motor that I scavenged from an old printer. I need the ruler to spin at speeds between about 40 RPM and 200 RPM. I have several questions on design details.

Power: In my prototype I have the stepper motor powered by a separate breadboard power supply (the Adafruit kit version). I have to do this project on the cheap (it's a public school and I need to build 8 - 10 of these). The stepper motor I have says it's a 12 volt motor. Would I likely be okay running it at 13.8 volts? I happen to have a set of regulated 13.8 volt supplies in my lab that someone bought by accident several years ago. Can I also run the arduinos from these 13.8 Volt supplies, or should I have separate and lower voltage supplies for them?

Thanks!

Sean

[adafruit_support_bill]

Hi Sean,

13.8v should be fine for your motors. There will be about a 1.2 volt drop through the motor controller anyway, and a fraction of a volt over the spec is not going to hurt the motor.

But 13.8v is pushing it a bit for the Arduinos. The spec says 7-12v, but the regulator is working pretty hard at 12v. Ideally, you want something in the 7-9v range. 9v batteries are probably the cheapest solution to start with. If you don't need extended run-times, they may be your best bet overall.

[mr_seeker]

What about putting a resistor in front, to lower the voltage? Then he could use the same power for the motor & arduino.

[adafruit_support_bill]

13.8v might be alright if you don't have much additional circuitry attached. The regulator will tolerate up to 20v, but at that voltage it needs to dissipate 75% of the input power as heat.

If you are talking about just an Arduino and a motor shield and no additional circuitry, leds, displays etc, 13.8 is probably workable.

[Spiff]

Thank you. This is very helpful. To be safe, I'll probably go with batteries, or buy some separate 9 V wall warts for the Arduinos. I expect we will be using the Arduinos for other projects, so it might be good to have separate power supplies for them.

Next design question: Displaying data.
In my prototype I have the arduino connected to my computer, so I just send the rotation speed to the serial monitor where a student could read it. But the computers in my classroom are old. They are 10 year old iMacs running OS 9.2, which is not really supported by the arduino software. Here are the options I'm considering:
1) Finding a way to make these old iMacs work with arduino. I found one link about how to do this online, but it isn't really enough info for me, and I'm not sure it is an easy fix.
little-scale.blogspot.com/2007/12/talking-to-arduino-in-os9.html

2) I have access to a set of old PC laptops, but these are running Windows 2000, which also does not appear to support Arduino without a fair amount of fiddling:
http://arduino.cc/playground/Main/UnoWin2k

3) Buying an LCD display for each setup would probably work, but I'd rather find a cheaper alternative, since I have to build 8 to 10 of these.

4) A four-digit 7 segment display would be sufficient. Reading about these, I think I would also need a shift register chip. (Might I need four of these, one for each digit?) Or I could use a LED driver chip, but these seem to be more expensive. And if I go with LED displays, I think I would need extra transistors to provide the necessary current for all the segments in the four digits. So the cost here starts to add up, since I will be building 10 of these setups.

5) Another very elegant solution, in lieu of connecting directly to computers, would be to connect to students' TI calculators. The calculators have a link port built into them, and I have extra link cables in my lab. I found someone on youtube who is able to do this (http://www.youtube.com/watch?v=cMNOGHKQDQk&NR=1), but I'm going to need to learn some z80 assembly language programming to interface the TI calculator to the arduino. In some ways this would be a great solution, though, because teachers at other schools would be able to duplicate it (since nearly all high school students have TI calculators, but many physics classrooms do not have computers) and students could also use the keypad on the calculator to send data to the arduino.

Any suggestions would be appreciated on how to make alternatives 1, 2, or 5 easier, or on exactly what hardware would be required to implement alternative 4.

Thanks again!

Sean

[BruceF]

13.8v might be alright if you don't have much additional circuitry attached. The regulator will tolerate up to 20v, but at that voltage it needs to dissipate 75% of the input power as heat.

Since the Arduino has a linear regulator, you could probably feed it with a pretty crude 555 buck converter, could you not? The one I linked should drop 14V down to 7V, then the linear regulator would smooth things out for the MCU without quite so much heat dissipation. Just a nerdy option.

The last cheap 9V brick I bought output about 12V. If you're going with a dedicated brick, which may be the smartest choice overall, you should probably get them from a reputable vendor.

As an idea, I like the calculator hookup. No idea how hard it would be, but it sounds pretty useful in or out of the classroom.

[adafruit_support_bill]

In my prototype I have the arduino connected to my computer, so I just send the rotation speed to the serial monitor where a student could read it. But the computers in my classroom are old. They are 10 year old iMacs running OS 9.2, which is not really supported by the arduino software.

You don't need to be running the Arduino IDE to display serial data. Any terminal emulator program would work fine. I am sure you can find one to run on your iMacs (if they don't already have one). The Win-2000 machine should have Hyperterminal on it already.

The calculator option sounds like a fun project, but I don't have any experience with that.

@bruceF - a buck converter could work. And you are right, the cheaper wall-warts are not regulated. Most will be measure quite high under no-load conditions.

[Spiff]

Of course! I hadn't even thought of that. I'll use a terminal emulator on the old macs and that should allow both display onto the screen and data input from the keyboard, when needed.

If I get the time, I'll develop the calculator option further, but this should be enough to get my lab up and running for my circular motion unit in a couple weeks.

The buck converter option also sounds good. Thank you!

Last question (for now, at least) is a small detail:
Choice of H-bridges to drive my stepper motor. My prototype uses the L293D, and it works fine. SparkFun advertises the SN754410 and claims its identical but works with higher current. Any experience with it? I'm guessing either would work fine.

Thank you again for all the help!

Sean

[EasternStarGeek]

If you are concerned with Arduino regulator over-dissipation at 13.8 volts, you can get a 5.1V, 1W zener diode to drop the Arduino Vin down to 8.7v. The regulator will run cooler because some of the power will be shed by the Zener, so naturally it will get a little toasty. If it really cooks, you can solder a small copper "batwing" heatsink onto each lead near the diode body. You can safely run 200mA through a 1W 5.1V diode.

If you need more current, try a 3-watt zener, which will take 600mA.

To include the zener in your setup, insert the Zener between the + terminal of your power supply and the Arduino Vin pin (or center of the power jack, depending on what you have). Be sure that the cathode (band-end) of the zener is pointing towards the power supply + terminal. This may sound counter-intuitive if you only know from regular diodes, but zeners are connected "backwards" so to speak. Trust me, it will do the job.

Some zeners:
1 Watt: http://www.mouser.com/ProductDetail/Fai ... oUkZxvY%3d

3 Watt: http://www.mouser.com/ProductDetail/ON- ... M7eUNuA%3d

Good luck!

[Spiff]

Thanks for all the help and suggestions. I finished building a class set of these rotating platforms for my physics students. Here's a short video to show it working, in case you're curious.

http://www.youtube.com/watch?v=LJYKh9kvdTE

Thanks again for your support!

Sean