Little Tikes "SkidDoodle" Drawing Tablet (1999) - Page 1
Forum section: electronics / general

author: FenderBender
author_rank: Super Contributor
timestamp: October 29, 2012, 05:21:29 pm
content: Was going through my old junk again and I found this old toy drawing tablet from 1999. It hooks up to your computer and you can use it to draw on your computer (like a Wacom tablet) using the bundled piece of software.I don't have the pen to it or the software...so I couldn't test it, so I took it apart.I was actually pleasantly surprised. Most of the kids' toys I've taken apart are really cheap and nasty, one hung low all over the place... because they expect you to use it for a month and then toss it in the trash. So they use crap parts to maximize profit since the thing will never get much use anyway.But this thing, on the other hand, is actually not half bad. It uses a real thick PCB, non of that flimsy crap, and the construction, soldering, part selection, layout, and engineering seems to be top class. Almost as if it was meant to be in a professional tablet.After taking it apart, I checked the back panel of the unit, and to my surprise, made in the USA! Great to see that for a change, especially on electronics, and more interestingly on a kid's toy.Lot's of 74HC logic, a PIC16C, and a bunch of discretes...but nothing too fancy. I like it.I believe the big metal foil square you see there is a shield as there is a little spring wire that is connected to ground. I'm not sure how it works too much but perhaps it uses magnetic or inductive sensing and it needs shielding to prevent something from triggering it incorrectly.How do these work? Anyone know?

author: mamalala
author_rank: Unknown
timestamp: October 30, 2012, 01:52:33 pm
content: There are several methods for tablets. Active vs. passive pen, energy vs. frequency change. A tablet using an active pen is rather simple in it's operations. "Active pen" means the ones where you have the pen connected to the tablet. Most simple method is such a pen that generates a high frequency. This is picked up by the row/column strips on the tablet PCB, and the relative field strength that is measured there gives you the position of the pen.That can also work in reverse, where the row/column traces emit the HF and the pen picks that up. In any case, only one row or column is active at a time. A row activated, a measurement is taken, then goes to the next row, etc. until all rows are done, then the same repeats with the columns, until they are all done, then the whole scanning process starts over.Passive pens involve more circuitry in the tablet itself. Two methods are possible. One is a fixed frequency applied to a row/column, while there is a tank circuit in the pen. That way it draws energy from the HF field, which in turn can be measured by the tablet. So basically a row/column is turned on, emitting HF. An adjacent row/column is then used as pickup to measure the strength of the HF. If the pen is in proximity to that row/column, it will measure lower.The second method with passive pens is frequency detuning. You generate a frequency and split it up. One path is fed directly into a balanced demodulator, the other goes into a more-or-less resonant circuit, part of which are the row/column traces. The pen causes a detuning of that circuitry. The result is fed into the other input of the demodulator, which then gives you a difference signal between these two frequencies, which can be used to determine the position. However, i have seen that method only once, and long ago.In any case, the row/column traces are always switched in sequence, you will likely find a bunch of analog switch chips connected to them.Greetings,ChrisEdit: Forgot to mention: Extra function of the pen, like pressure detection, buttons, etc. are usually a modulated variant/signal of the above which is then detected by the tablet. Cheaper variants use a battery inside the pen to drive the circuitry for that, while better ones draw the energy for that from the HF of the tablet itself.Edit 2: Made a mistake, i think. Replace the frequency shifting stuff above with phase shifting instead. Damn, it was so long ago that i ran across that variant, so my memory is a bit faulty on that.

author: FenderBender
author_rank: Super Contributor
timestamp: November 04, 2012, 02:49:43 am
content: Very interesting!I believe this was an active pen unit. It has a connector the the pen to plug into.I also have Wacom tablet that must use a passive pen because it is not connected/wire to the pad.Great info.