Scroll Roller, the continuing story

Progress has been slow but steady on the scroll roller I’m working on with Tims for my final (as opposed to the cube).  Mostly things come in spurts.  But as of this evening I feel that we’re in good striking distance of done.

Over the weekend I learned some important lessons:

1. TIP120s are not very well labeled for BCE.
2. The cheap little potentiometers we got with our kits are… um… not rated for 1A of power.
2a. Trying to do this will break your potentiometer in bad ways.
2b. It turns out that it is possible to cause a potentiometer to flash-combust.  Sadly I have no documentation of this fact and am unwilling, at the moment, to demonstrate it again.
3. 20mA, despite my overly optimistic hopes, is not enough to spin a DC motor of any significant size.
4. A common ground works best if you connect ground to ground rather than 12V to 5V.
5. The junk bins in the shop are filled with some amazingly nice pieces of wood around finals time.
6. There are some engineering solutions which hot glue is not wholly adequate for.  It may end up being an integral part of the solution, but it is not sufficient on its own.

Here’s the basic state of things:

I have a pair of DC motors hooked up to TIP120 transistors which are controlled via Pulse Wave Modulation from an Arduino microcontroller.  Or, more simply, I have two motors which I can spin faster or slower depending on computer commands.  I have a potentiometer, which we’re going to try to build a spring-tensioned lever onto, that is used to control these motors.  When the potentiometer is centered, nothing happens.  As you turn it one way, the motor on that side spins faster and faster.  As you turn it the other way, the other motor spins.  From where I sit the software works great.  The only thing that I think might end up tweaked is that I might use some sort of logarithmic scaling function for speed adjustment instead of a linear one.

I have two paper rolling bars from an inkjet printer which have these extremely convenient gears at one end.  Gears which happen to have teeth which mesh with the gears on my motors.

At this point nothing is really put together, but Tims and I sat down and talked and once again I think he’s provided the solutions to most of our design problems.  The only one left of any significance is specific mounting of the paper rolling bars, and I think we’ve got a general solution there.

On the to-do list:

1. Show current work in class on Wednesday, solicit feedback regarding attachment options, bring up possibility of integrating Processing applet for digital scrolling.
2. Wednesday after class, finish physical mock-up.  Drill mount holes for, and actually mount, the motors.  Decide on working display area size.  Resolve bar mounting issues and figure out how to handle bar-scroll interface.
3. This weekend commence testing.  Problems are sure to arise, but if we’re lucky they’ll be small and manageable.
4. Obtain demo scroll.  I’m okay with demoing the entire thing with toilet paper, but I fear that a lot of the impact will be lost.
5. Clean up wiring.  This is purely optional, but I’d love to see the entire thing on a simple six wire ribbon cable.
6. Start looking ahead for this project.  Ability to identify the part that is being examined?  Memory?  Integrated controls between screen and physical display?

Let’s get cracking.

Thomas

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