pencil (HB’s don’t work very well, 2Bs or 4Bs are better)
small amount of kitchen foil
screwdriver for the terminal block
wire cutters/strippers. You can get away with scissors if you’re careful :)
Start off cutting your terminal block to 2 rows of 8 screws. Bend the legs of your transistors like this and screw them into the bottom row of your terminal block. The BC558 goes on the left with the flat side uppermost. The BC548 goes on the right with the rounded side uppermost:
On the top row:
The red wire from the battery holder goes to the left-most screw (number 1)
The black wire from the battery holder goes to the right-most screw (number 6)
10k resistor between the 3rd and 4th screws
the piezo sounder also goes between 3 and 4
2n2 capacitor between 2 and 4
2n2 capacitor between 3 and 5 It’s a bit of a fiddle getting more than one wire in the screw holes, but persevere – it’s doable!
Finally, to prepare the pencil – take two lengths of wire. One length goes from screw 5, the other end you need to wrap around a drawing pin and push into the pencil at the top. The other length you need to strip a longer length of insulation off (about 5 cm), wrap around the pencil where you hold it, and then cover it with aluminium foil to make a good contact for your hand. The other end of this piece of wire goes to screw 2.
Right, it’s done – put the batteries in, hold them pencil and touch the lead to your other hand. You should be rewarded with a squeaky noise :) Draw a blob on the paper, stick your finger on it and then draw lines radiating out from it. Get someone else to hold hands and then they can do the drawing. The possibilities are endless – have fun!
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)
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: