I searched the Internet for days looking for the perfect solution. At first I considered making the Prism a MIDI device, controlled by an Arduino (like the Laser Harp
seen in MAKE Magazine). Ultimately, I decided to make it completely standalone, like a normal electric guitar, requiring a minimum of external equipment and compatible with everything.
That led me to research various (all-analog) oscillator possibilities. This time simplicity was key, since it all had to fit inside the body of the guitar. There are a number of single-chip voltage controlled oscillators out there, but the only one that is actually in production
is the XR2206. This neat little device takes a control current or voltage and produces either a sine, triangle or square waveform that is proportional to that control. The control can be as simple as a potentiometer, or something more complex like the infrared range finder I used. During my search I happened upon a most excellent design by Thomas Henry.
It had everything I needed to get started.
The Prism is based on Thomas' design, with a few modifications. I removed some of the control inputs, such as the exponential FM and voltage-controlled Skew. I then added my own custom-designed laser-controlled capacitor bank, a hard-wired LFO generator (based on an XR2206 reference design from the datasheet) and a hard-wired "sync" circuit based on a 555 timer. Oh, and a Sharp Infrared rangefinder to control the pitch.
So how does it work?
Well, I mentioned before that the output frequency is controlled by an input voltage. The three input control voltages, Coarse (the range finder), Fine (a trimpot on the board), and LFO are all mixed together and converted to a current (taking the place of a potentiometer). This current, along with a capacitor from the capacitor bank, determines the frequency produced by the XR2206.
Normally you'd only use a single fixed capacitor, but I wanted each laser "string" to select a different frequency range. The Prism accomplishes this by having each laser trigger a phototransistor, which in turn controls a comparator. If the laser is blocked, the phototransistor turns off and the comparator goes low. This causes a solid state relay to turn on, connecting its corresponding capacitor to the XR2206's capacitor input. When no lasers are blocked, no capacitors are connected and the oscillator produces a frequency above the audible range.
To control the pitch, I used a Sharp infrared rangefinder. You've probably seen this used on autonomous robots, and perhaps some theremin-type instruments. This neat little device measures objects between 10 and 80 cm away, and generates a corresponding analog output between 2.4V and 400mV, respectively. This voltage swing is quadrupled with a simple op-amp on the board.
The desired output waveform is selected by turning a rotary switch, that selects between the sin/tri output and the square output. The frequency and amplitude is the same no matter which waveform is selected.
The skew knob causes the triangle and Sine waves to skew - that is, they get chopped up and lean to one side or the other. For instance, the triangle wave can be made into a ramp for a slightly different sound. The effect is even more pronounced with the sine wave, which goes from a nice clean sound to very harsh and metallic.
The LFO effect can be varied by turning a knob, and turned off by pushing the knob (a very clever design, if I do say so myself!) Its effect can be varied from a slow rise and fall, to a nice vibrato, to a high pitched trill sound.
The Sync only affects the Sine and Tri waves. It is also controlled by a rotary knob, and can be switched off. Each time it transitions it causes the main VCO to reset, chopping up the sound in interesting ways.
There are also a few more on-board trimpots, used for tuning the output waveform. These are only touched once when the Prism is first constructed.
The Prism can be plugged into any regular guitar amp, or it can be modified to control a separate synthesizer setup.
A separate power supply is also needed, that produces +15, -15 and +5V. The lasers are driven by a 3V regulator that "spoofs" the voltage the lasers are expecting.