Back to the Drawing Board

Ever try signing your name with a mouse or a trackball? Not so easy. You could buy a graphics tablet with a pen. [Rahul Ramakrishnan] has a different approach. He took two 10-turn pots, and attached some strings and a washer. A pencil goes through the washer, and a BeagleBone Black reads the pots to determine what it is drawing on the paper. A couple of retractable badge lanyards keep tension on the string.

This ingenious design would be easy enough to replicate with any microcontroller that can read the two pots. The only awkward part is the need to press a button down when you want the device to treat the pencil as down (see the video below). It would probably be easy to rig up some switch on the pencil to make operation a little smoother.

On the computer side, [Rahul] includes HTML code that uses the Processing.js and BoneScript libraries to capture the input drawing. Real tablets, of course, don’t work this way, but it is a very simple way to obtain X and Y coordinates of the pencil. We’ve seen BeagleBones at the heart of logic analyzers and even in a redesign of the Super NES, and the board is known for having a lot of horsepower while still having capable real-world I/O.

26 thoughts on “Back to the Drawing Board

  1. I always thought that this – with retractable dog lines – would make a nice low cost positioning system in larger spaces. It could also use the angle of the line instead of it’s distance for calculating the position. And of course, 3D is also easily possible with a third line.

    1. Ugh. You don’t go out and buy one to do this, it’s about using what you have lying about.

      Why don’t you go and implement it the way you think it should be done and come back then?

  2. You could also make a small device that plugs into the USB, and has your signature stored inside. Press the button, and it will generate the correct mouse movements to insert it in a document.

  3. In eighties(maybe late seventies also) there were several comercial “graphics tablets” working on a similar principle.
    “Touch tablets”(like Koala Pad or original atari CX77) were pricey and/or tiny(koala), so “electro-mechanical” ones were interesting ersatz.

    1. Not quite… The X and Y position of a strings requires some math to plot properly…
      For example, you move the pen from X0Y0.5 to X1Y0.5, you will find that the reported value for Y will actually vary even if you move perfectly straight.
      You need the length of the strings, and the position of their origin. You are dealing with ‘circles’ and ‘radii’ .. not simple Cartesian movements.

  4. Just write something on one of those boogie boards and snap a picture with your smartphone. Someone could create an app that would use a system of little QR code targets on four corners of the boogie board that would correct any angular distortion. Boogie boards are white on black but that’s trivial to fix in software once you have the image captured.

  5. They used a “full size” version of something like this to measure our kitchen for having counter tops made. The main difference was that it 3D polar coordinates. A “pole” in the middle of the room had a wires with a marker on the end. The technician slowly moved the marker around the contours of the old counters. The pole measured the angles parallel and perpendicular to the floor. His laptop made a 3D image in real time. Very much the same principle as this clever hack.

  6. This is interesting, I had no idea this was a useful feature for many people.

    Because it seems like the user would be in front of a laptop, and most laptops have some sort of webcam mounted in a way that offers an unobstructed view of the area in front of the user, I would approach this problem a little differently. I would try to figure out a way to make opencv track a stylus-like object, and capture the drawings that way. Just a personal thing, i prefer electro to mechanical :)

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