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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Have you thought about some multiturn trimpots?

Well, NOW I have! That's a very real possibility.

Here's a question: my experience of these is that the "mechanical interface" is a slotted screw. I don't see an easy way to attach to it. Any thoughts? Have you seen others with a more useable interface?

Now THAT is the million dollar/pound/yen question! How to mount things to other things that arent meant to be mounted there?


I have 1 here that has a tiny brass cap with a slot in. I don`t know what it takes to solder to brass but maybe you could solder or araldite a pin into the slot, which would give you a basis for fixing a sprocket on.

Brass, you say? I'd use ordinary lead/tin solder! I just need something to which I can solder it. Hmmm.... A brass 3mm bolt...
Isn't a million yen like 10 USD? a million pounds is like 4 million dollars...

But not necessarily do alike. I spotted a similar toy myself this week and pondered about servos as well. Great to see you actually hacking into it, satisfying my curiosity.

As before with writers, the conondrum is about range vs precision. Or size vs resolution. (she said what!??) 

Changing te gear ratio should give you full reach across the tablet. Worry about precision later. It takes only a very few vectors to write "LMR".

Welcome back my dear toy hacker!


Yes. In fact, I've already decided that the gear seen in the photo above is not going to be suitbale. In fact, I think I'll make it a 1:4 instead of a 2:1. Can't immediately see a way of attaching the damn small gears to the servos, though.

Has some various gears in various pitch :


Stepper motors will goive yu the most accuarte positioning. 

Here is a version Etch CNC:  http://axis.unpy.net/etchcnc 

For a small Etch a Sketch, cheap hobby type stppers driven by a microprocessor will work just as well. 


Good point. I wanted to do this on the cheap, with stuff I already have, though.

Wait a minute. I think I HAVE steppers...