Let's Make Robots!

van Rijn

Writes and draws stuff

I'm afraid I'm STILL on the Write LMR trip. It's not got tired yet.

I've decided to motorise a miniature Etch-A-Sketch.

I decided on steppers. Not servos. Servos hadn't enough rotation. Continuous servos had no positional feedback.

Here's my prototype circuit. Simple ULN2803 driver and a PIC 16F628. There's a max232 on there, too as the RS-232 interface to the PC.

WOW! It draws a circle. The most impressive bit of this is my implementation of Bresenham's line drawing algorithm. The circle is made up of straight lines. Straight lines are more difficult to draw than you might imagine...!

Look out for more swirly patterns coming soon. Maybe even an LMR logo!

What you see in the main photo there is a "Hilbert Curve." It's a space filling curve in that the algorithm causes the "pointer" to pass through all the cells in a given area.


Hey, how weird does the inside of an Etch-A-Sketch look? I HAD to do this. I programmed it to scan back and forth so all the grey dust got scraped off the glass. It took about an hour! Hmmm... Resolution is 750x500 = 375 KiloPixels (there's a new one) x 10ms per step = 3750 seconds = 62.5 minutes.


I presume all those little metal balley things (description specially for Frits) are where the magic grey coating comes from.

Below is my attempt at a Lissajous figure with two phases. I'm happy with that.


I got the machine to draw a picture of my eldest offspring. It's done by making "pixels" which consist of veritcal lines. The lines are closer together (more dense) for increasing levels of darkness. Later I will try to use a Hilbert curve to make up each pixel with an increasing "order" for increasing levels of darkness. The result should be much better, but it will be sloooooow.


This second photo is slightly easier to see the subject because it's so blurry!


For the sake of completeness, here's the front panel of the software I wrote on the PC to control it, including a preview of the image.


I'm off to see if I can get it to draw the contents of a DXF file. How hard could THAT be?

 Can you believe it doesn't EVEN Write LMR yet? How remiss of me...

Update (7-Mar-2009): NOW, it writes LMR. BTW, reading a DXF file is more difficult than I thought.

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fantastik!!!   :-)
Drawing a circle with an X-Y system is a great start :)
But I'm cheating. All the hard sums are done in the PC.

Wow, that's really cool.

have a look at this: http://blog.makezine.com/archive/2008/03/etchasketch_clock.html 

The world's best motto: When all else fails, use a bigger hammer.

Wow! Look at those dinky little steppers compared to my monstrosities!

Ooh, I love that word. Kinda like: Ginormous! Don't feel bad, You have more torque! Though I don't know if that is good for an etch-a-sketch or not. 

The world's best motto: When all else fails, use a bigger hammer.

These are just "big." The torque on them is terrible! I had to use a 4:1 reduction gear just to get the knobs on the etch-a-sketch to turn! Hey - they were cheap!

Ha ha! well, cheap is good. I find several small, high torque steppers from printers myself. There are usually 3-5 steppers in an inkjet and a couple of large brushed motors. I have more than I know what to do with from just printers! Gotta love garage sales. 

The world's best motto: When all else fails, use a bigger hammer.

I like what you've done BOA. I can't see any home position limit switches. I assume you just run the motors in one direction long enough to ensure home position is achieved and let the mechanism work as a stopper.

What resolution are you getting? How many steps from one side to the other? 

In engineering terms, to redatum this vector display we run the stepper backwards to the endstops.

Actually, these things are designed so that kids can wind them in the wrong direction without breaking them, so I'm, happy enough with that. It's basically a programming excercise, anyways.

Here are the sums (Frits, you can skip to the last line):

7.5 deg per step with 4:1 gear = 1.875 deg per step.
1 turn of Etch-A-Sketch knob = 20mm on screen.
With gear => 192 steps per "knob revolution" (192 steps per 20mm).

Screen is 75mm wide, so that's 720 steps across at 20 / 192 => 0.104mm/step.