The Musical Pencil

One of the milestones on my path to electrical engineering was sometime around 6th grade, when my parents bought me one of those 150-in-1 electronics kits, the one with all the wires and spring terminals, and point to point directions for wiring things up.  I didn’t understand a lot of what the articles said, but I had a lot of fun wiring up sound generators, radios, and light bulbs.  A standard circuit in one of these kits is an oscillator whose frequency is controlled by a variable resistor, usually a CdS cell, so you can play “music” by waving your hand in front of the sensor.

How could you dress that simple circuit up to make it engaging for adults as well as kids?  What if the CdS cell were replaced by a variable length trail of graphite?  What if the person using it was a part of the circuit?  Enter Drawdio, the musical pencil.  With Drawdio you have a small PCB that attaches to the side of a pencil, a strip of copper foil that wraps around the pencil, and a tack that is inserted through the eraser to make contact with the graphite core of the pencil.  The PCB has a 551 (low voltage 555) based oscillator, with the frequency controlled by a variable resistance.  This resistance is the path from the foil strip, to your hand, through your body to your other hand (which is touching one end of the line you’re drawing), through the graphite on the paper, through the graphite core of the pencil to the tack and back to the PCB.  Whew!  That’s a torturous path for the electrons, just to get from one side of the battery to the other.  Talk about single minded!

With Drawdio you can draw a line on a piece of paper and play it like a keyboard by touching the pencil tip to various places along the line.  Put ticks on the line to mark where notes are.  Two people can join in with their own Drawdios by holding hands.  You can also apply the concept to other hand held items:  a hairbrush, water running in a sink, a watercolor paintbrush.

Drawdio is open hardware licensed under Creative Commons Attribution Share-Alike license, meaning that in addition to being able to purchase Drawdio as a complete assembly, a kit needing assembly, or a bare PCB, you can also build your own using the provided schematic, modify it in any way you like, and distribute it as you see fit.

Drawdio is rated easy due to the simplicity of the circuit and the fact that it’s available fully assembled.  The kit is all through hole parts so is also easy to assemble if you own a soldering iron (or have a Radio Shack nearby where you can go pick one up).

Happy Gadgeting,
Steve Ravet