Sunday, 29 July 2012

An IR Synthesizer

I had a dream about theremins the other day (those things that make noises in response to you moving your hands) and pondered how to build one, or something similar, or just something that makes noise. Having pondered how to build one, I thought of some stuff and had absolutely no choice but to try it out :)

My initial plan was to use 1 or more IR range finders to measure the distance of an object from a sensor, then use that distance (or some function of it) to choose a pitch. An Arduino then generates a wave and pumps it out through a speaker, so the very simple circuit would be along the lines of this:

Simple circuit for Arduino to drive a speaker using IR sensor readings

In case you're wondering, that little speaker circuit is genuinely all you need to generate noise from an Arduino - a little 80ohm speaker plus a small resistor and you can send it a wave (potentially using the handy 'tone' command) to generate a noise. However this has some major floors:

  • It's fairly quiet, as you're powering the speaker off the Arduino
  • You can only really generate square waves, as the GPIO pins are either on or off
  • Again, due to the binary nature of the GPIO pins you can't digitally control volume
Anyhoo, it quickly became apparent that this wouldn't do the job. Initially I had a blast at building an R2R ladder digital-to-analog converter (excellent web page on them here), which would allow the Arduino to output a 10 bit binary value, which is then converted to a voltage and pumped into the speaker. Unfortunately once this was working the signal was so weak that it couldn't drive the speaker. Fortunately, I then remembered my Speak Jet chip from this earlier post.

The Speak Jet is a speech synthesis chip, which internally uses a 5 channel synthesizer to produce phoneoms. However you can take direct control over the synth if you prefer, which is exactly what I decided to do. So first step is to expand the circuit to this one:

I haven't shown the whole speak jet setup (which you can peruse at your leisure from earlier posts), but it's basically the speak jet chip plus a *20 gain audio amplifier built with an LM386 op amp. A bit of wiring later and....

That bread board is the speak jet and op amp squeezed into a neat little circuit that I'll probably never dismantle again. Although it looks a little complex, it's mostly just the right organisation of capacitors and resistors to build a simple audio amplifier from an LM386 as specified in their manual. In addition to the circuitry, you can see the oscilliscope attached to the speaker output, with the wave being generated on it.

With a little bit of code to read values from the sensor, convert them into a frequency range and then send over serial to the speak jet (which I might post at some point), I end up with this ear bleeding monstrosity:

If you had headphones on when you played that then I am sorry.

Anyhoo, it makes noise but isn't particularly useful. A real instrument does more than make noise - it is controllable, so the user can choose when it should make noise. I decide the key to this instrument is to add a push button to 'strum' the note. So with a small tweak to the circuitry:

I now have a push switch in there, and update the code so it sends volume commands to the speak jet in addition to pitch commands. When the user is holding down the push switch the note is played at full volume, and after releasing it the note decays for half a second. In addition, the pitch can only change when the button is down, allowing the user to bend the note as they play it. Unfortunately my musical abilities are limited to the listening side of things, so I grab Mark.A from his work and hand him a plank of wood and a push button:

And there you have it! An Infra Red Synthesizer :)


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