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.
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:
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:
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:
Success – a good night’s work! Here’s what it sounds like in use:
Now here… how to make your own!