2008-2009 Holiday Gift: A Capacitive Theremin

This year I will give build-it-yourself theremin kits (see the description beneath the signup form) to the first 50 people who sign up to receive my occasional email newsletter about software development techniques and strategies. I won't give your contact information to anybody else, and you can cancel any time after the first newsletter. You will probably get your theremin kit in January or February (sorry, it won't be in time for Christmas). Limit one kit per family. Full and correct contact information required. Offer only valid for shipping addresses in the continental united states.

Sorry, all 50 kits have been claimed!

What's A Capacitive Theremin?

The theremin is a musical instrument that is played by moving one's hands near the instrument. The result is a distinctly unusual sound that has found use on stage, film, and television, such as opening theme for the classic TV series The Outer Limits. The theremin was invented in 1919 by Russian physicist Lev Sergeivich Termen (AKA Léon Theremin) during research on proximity sensors. The player's hand acts like a ground-connected plate of a capacitor. An antenna acts like the other plate of capacitor. Traditionally this capacitance is part of (and therefore affects the resonant frequency of) an LC radio frequency oscillator. The oscillator output is heterodyned down to audio frequencies. That way the position of hand with respect to the antenna changes the pitch of the instrument. Theremins usually have a volume control that's also capacitance sensitive: the electronics are different though. For the volume control, the output of a fixed frequency oscillator is filtered by a volume control LC circuit: as your volume-controlling hand detunes the filter, the output amplitude is used to control the volume of the pitch channel. It's a bit easier to build a theremin-like instrument that senses light, rather than capacitance. These are sometimes called "optical theremins". Theremins are difficult to play. Here is a video of a theremin in action:

Our Kit

The theremin kit we are giving away is doubtless not as good as the professional one in the above video (for one thing, ours lacks a second channel to control the volume), but it's based on the same physics, and you play it the same way, without touching it, by moving your hands. It uses an RC oscillator instead of an LC oscillator. The RC oscillator produces a square wave which is fed to a differentiator. The derivative is zero except for positive and negative going spikes at the transitions between levels. These current spikes are rectified and then averaged by an RC network. When your hand comes closer to the antenna, the capacitance increases, increasing the time between spikes, decreasing the average value. This averaged voltage is amplified and shifted. The resulting voltage controls a voltage-controlled oscillator (VCO). The VCO output is a 3V p-p triangle wave. The triangle wave is attenuated by a volume control potentiometer, buffered by a "power" amplifier, and then fed to a speaker.

The prototype shown in the photo below was wired point-to-point, so the construction looks harder than it really is. This kit contains:

  • All the electronic parts
  • Potentiometers and switch
  • Speaker
  • A printed circuit to simplify construction

You will need to provide:

  • Mechanical parts (case, knobs)
  • Electromechanical parts (antenna, binding posts, wire, solder, etc)
  • Tools (soldering iron, pliers, etc)
  • Battery
  • Hot melt glue
  • Elbow grease

You will find suggestions for tools and assembly techniques mixed in with the instructions for the persistence of vision toy we gave out last year.

Status (as of Jan 1, 2009)

  • The printed circuit board has been designed, and sent out for a quote. Here is what it looks like:

  • The parts have all been ordered:
    • C1 47pF
    • C2 .047uF
    • C3 220uF
    • C4 .1uF
    • C5 .0047uF
    • C6 .001uF
    • C7 2.2uF
    • C8 .001uF
    • C9 2.2uF
    • C10 2.2uF
    • C11 .001uF
    • D1 1N4148DO3510
    • IC1 LM3900N
    • IC2 TS555IN
    • IC3 NJM386BD
    • Q1 FJN3302RTA
    • R1 10k
    • R2 1k
    • R3 5k
    • R4 10k
    • R5 1m
    • R6 1.5m
    • R7 2.7m
    • R8 2m 25-turn trimpot
    • R9 680k
    • R10 43k
    • R11 10k
    • R15 1k
    • R16 10
    • R18 100k 25-turn trimpot
    • R20 1m 25-turn trimpot
    • R21 100k linear taper potentiometer
    • R22 10k audio taper potentiometer with SPST switch
    • SPK 32 Ohm Speaker
    • CLP 9v battery clip
    • Sockets for the chips

It looks like the kits will be shipped late February:

  • A few of the parts are on back order
  • When buying custom printed circuits, you have your choice between fast delivery and cheap. I selected the inexpensive option.

Coming Late February

  • Assembly instructions and photos
  • Your kit!