Monthly Archives: July 2010

Solderless Drawdio

The Drawdio (created by Jay Silver) combines drawing with audio. You can get kits to build one, or you can solder one up on stripboard. But I wanted to make one without faffing with a soldering iron – using screw terminal block.

That counts out the use of integrated circuits like the 555 variants used in the traditional Drawdio…

So I had to design something with discrete transistors. I still wanted to work with a single cell, avoiding those nasty PP3 9V batteries, as well.

My natural first thought was a multivibrator, such as I used to build when I started out with electronics. However, the performance with such wildly differing resistances in the two “sides” of the circuit was poor with only 1.5V to play with. Searching around for inspiration, I found the 4qd-tec electronics club, which had some interesting variations on the multivibrator theme.

The series multvibrator in section 4 worked well when I simulated it in LTSpice.

Circuit diagram

A couple of notes – R3/C3 model the piezo buzzer I’m using. R2 is the resistance of me and the pencil – the end of the pencil lead is the top of R2. Also, I ended up using 1.5nF capacitors, not 22nF like the circuit says.

The current in R2 flows like this:

  • down the pencil
  • down the graphite on the paper
  • through my finger (which is touching the graphite on the paper)
  • through me
  • to my other hand, which is in contact with a piece of bare wire taped to the pencil.

This wire is the bottom end of R2.

So I knocked one up on a breadboard:

Finished circuit on breadboard

Bench development

After playing around a bit, I came to the conclusion that a single cell wasn’t going to cut it (there’s not much headroom at 1.5V when the transistor base-emitter junction drops 0.7V already). The 555 guys have it easy with their IC technology :)

Anyway, with two cells, we got some quite nice noises using a pencil with a simple bit of wire down the side for one contact and a pin pushed in the end for the other contact

The desk was quite a state by the time I’d finished plugging various resistors, measuring voltages!

You can see the waveform at the collector of the transistor – we only get 700mV.


I also grabbed a movie of the changing waveform:

There was a funny pulsing quality to the single cell version, which can be seen clearly on this oscilloscope shot:

Interference from the mains power

The time scale is 20ms per div, which matches a 50Hz pulse quite nicely – mains interference gets everywhere!

From this breadboard I was then able to move the components to the screw terminals:

Drawdio built on screw terminals

Success – a good night’s work! Here’s what it sounds like in use:

Now here… how to make your own!

Bah Ubuntu Lucid upgrade

Just upgraded my desktop from 9.04 to 10.04LTS. I’d skipped 9.10 because of the Intel graphics shenangigans – there’s 133 pages to that thread – gives some idea of the level of aggro that seemed to be involved!

And today’s upgrade died horribly part way through with dependencies it couldn’t sort out…

libc6-686 wouldn’t install, and anything depending on it. And some weird looking perl module. I’ve had to hack away, putting extra options into config files, and I think it’s now going to go through the upgrade process. Eventually.

I think I might head back to Debian – I prefer the rolling “unstable” upgrade where occasionally (very occasionally) something doesn’t work after an update to the Ubuntu “have a whole new set of grief all at once” approach!

Anyway, with judicious application of apt-get upgrade -f and dpkg --configure -a and adding APT::Immediate-Configure 0; to the apt conf file, it seems to be going.

Nonlinearity makes people do strange things

Control engineers know that non-linearity causes problems – the rest of the world could maybe take note…

What is non-linearity?

Non-linearity is where the response of something is out-of-proportion with the change that’s just been inflicted on it. If you tweak something and get a huge change all of a sudden, that’s non-linear. Light switches are non-linear – there’s a small change to the light switch, but the light changes abruptly. A dimmer switch is linear – you turn it a bit, the light changes a bit. Turn it a lot and the light turns a lot.


If you want to use that knob to control something, it’s an awful lot easier if the response is linear. Imagine trying to control the speed of a car if the throttle pedal responded non-linearly. You press the pedal more and more, the engine does nothing, and the suddenly as you cross some invisible point, the engine bursts to life and you scream ahead!

Control engineers (some of whom designed the engine management system in your car) take great pains to keep everything as smooth (ie linear) as possible.

So what about the rest of life?

Now you’re wondering what the “rest of the world” I was talking about has to do with a relatively obscure engineering discipline?

Say you wanted to “control” something like the performance of your sales force. Or some money managers… Maybe you’d say “if you do this well, you get this much bonus”.

That’s a non-linearity right there. Small change at the input (between meeting the target and just missing), potentially huge change to the output (the bonus payment). This will encourage weird behaviours – towards the deadline date, a salesman who’s nearly met his target will cut a different deal to one he would’ve done at the start of hte month. Possibly to the detriment of the long term interests of his company. But to his personal gain short-term. Or a banker might be encouraged to take a riskier bet on some market or other, because she knows it will looks good towards the measures that her bonus depends on. Not necessarily in the long-term interests of the company. Or possibly the country… Taxation can also produce these sorts of behaviours – if there’s step changes (like to a 50% tax rate at a particular income), people will suddenly feel it worthwhile to try and avoid that last bit of tax. Whereas a continual change, or even a pure flat rate, will provide less incentive, as there aren’t opportunities for big “savings” for a relatively small amount of effort.

I’m not convinced by bonus schemes as motivators at the best of times, but a linear scheme, where the return is stricly proportional to the gain you’ve given the company, preferably over the long-term, will (IMHO :) provide much more sensible behaviour.

Maybe those specifying bonus schemes as a method of “control” (although I’m sure they’d it hate to be described as such) could learn a bit from control engineering?