# 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.

Posted in ARM

## 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. Shannon says:

For the way I write the strings would have to be attached right on the tip of the pencil, I don’t make big enough movements otherwise.

2. Add angle measurement to this idea and you get the GameTrak. Since a 2D angle is measured for both strings (in a very clever way), you get the 3D position of the end of each string. It uses two so that it can also measure the 3-DOF orientation of the waldo attached to the strings.

2. Ed says:

I think a beaglebone is overkill in this design…

1. pelrun says:

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?

3. RÖB says:

Nice touch, using 10 turn pots. Those precision rotary encoders are like \$60.

1. Paul says:

And decent commercial stringpots are \$200-\$300. But, sometimes, they are the right thing to use.

4. Artenz says:

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.

5. 12L14 says:

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. DV82XL says:

I think this design has been around since the Telautographs of the late 1800s

6. As for sensing pencil up/down, I wonder if a metal plate under the page with a conductive stylus could act like cap sense to detect when the pencil is being pressed down on the page?

7. Bob says:

Couldn’t you fill in a box on the paper with the pencil and then use the conductivity of the graphite to detect when it’s touching the paper?

8. Do away with the pots and the micro.
Strip an old ball mouse. Put the opto-encoders on long wires and use that with the retractable lanyards.
Done. No software required.

1. Gryd3 says:

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.

9. dynamodan says:

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.

10. 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.

11. Miroslav says:

Nice idea and implementation.

12. Just put the pencil inside some tube, and connect the tube to pencil with switch. Press tube, switch presses against pencil and goes on. Problem solved.

13. Dan says:

I think I’ll file this one under “The journey is more important than the destination.”

14. proxy says:

inkscape does the signature pretty well with the right settings

15. 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 :)

1. maybe something like how the wii senses the position of the wiimote, with two leds inside the shaft of a transparent stylus?

16. Drone says:

Just sign your name on a piece of paper, take a picture of it, crop, white balance – done. 5 minutes and next to zero hardware cost (everyone already has a camera these days).

17. Ken N says:

Related – Canadian author Margaret Atwood conceived and patented a remote signing system called the LongPen, about 9 years ago. (https://www.youtube.com/watch?v=jYHUiNgDgL0 around the 30 sec mark). In her system, input is a stylus on a tablet, but this pantograph thing is interesting too.

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