The writers of dictionaries tell us that a module is an exchangable complex part of a machine which forms a rather closed functional unit. We call the solar buzzers modules because they can in fact be thought of as a voice in an ensemble of buzzers, where each module has distinct colour and lives for its own. The interconnecting machine in a performance is usually a mixing console or more generally the acoustic space in which the colours overlap.
Trevor Wishart's insight that composing computer music is much more difficult than writing for a traditional western instrument since the latter is already well defined and variations between one violin and another are comparibly small - whereas each electronic sound in a piece can be unique -, this aspect is somewhat applicable to the solar modules. When we choose which modules to use in a performance, the choice will never be the same and there's no module that sounds exactly like another. However, one must say that there's an inherent frame to the sound repertoire: Since the ICs are logic circuits, they basically understand two states of voltage called "high" and "low". In an ideal world we would therefore get all sorts of square and pulsewave sounds. Indeed there's a common timbre to many modules and it's impossible that a module could sound like a flute for example. The timbre variation is caused by filtering the sound and by the modulation of the microrhythmic patterns which when looking at a spectral view of recording reveals some similarities to frequency modulation. When huge capacitors are involved, pulse trains can be slowed down so a module would produce a click train instead of a tonal colour.
The soundfile in the left column gives you some idea of the colours that modules produce which are created from the basic building plan. Below are some snapshots of solar modules. It's obvious that one of their aesthetic dimensions is the mere visual appearance. There is a great joy about finding strange looking electronic parts in old television and obscure thrown away devices. (There's even a greater joy seeing Martin slaughtering broken television monitors in the streets of Berlin). The rightmost picture shows a bunch of modules which are connected to each other forming a more complex "network". The second part of this article describes the steps necessary to build solar modules.
The core element of the circuit is a logic CMOS chip. We'll here present the circuit used by Ralf Schreiber which is very suitable for workshops because it is extremely simple. The CMOS family contains all sorts of logic elements, like AND, OR, NOR gates etc. They all can be used for building buzzers but it requires more tuning and adaption of the wiring to different pin connections. The 74HC14 used in this diagram is actually an ensemble of six identical parts which are called Schmitt-Triggers. A trigger compares an input voltage to a control voltage which is called the threshold; if the input voltage is greater than the threshold, current will flow from input to output, if the input voltage is too low, current will be blocked.
The Schmitt-Trigger is an inverting trigger. That means if the input voltage falls below the threshold, the output will have high voltage and vice versa. This allows the creation of feedback loops. Imagine the trigger switches the output voltage to low and the output is connected to another input. So the successive comparator stage recognizes input voltage has dropped to low and will set its output voltage to high. If the circuit is connected in a loop, the switching of the voltages will go on forever. Naturally, triggers don't switch infinitely fast but require some time. If the frequency of the feedback loops matches the range of audible frequencies, roughly 16 Hertz (cycles per second) to 18.000 Hertz, the circuit becomes a sound generator.
Capactors and resistors are socalled passive elements. As opposed to the IC chip they don't require powering to work. If you use a metaphor and think of electric current as water current, a capacitor is a sort of cup that can save a certain amount of charge. When the voltage changes, the saved charge can flow back into the circuit.
Likewise, a resistor is some device you put into the water that obstructs the current flow. Put in a series, resistors can devide the voltage between two points of the circuit. Combining capacitors and resistors in a certain way results in an oscillation that has a resonance frequency. The circuit shown hear creates three oscillators, each using two triggers in a feedback loop.
Because it's rather boring to have a fixed sound with a fixed frequency, we use a variable control (threshold) voltage. Instead of connecting the IC to a fixed voltage source like a battery, we use a solar cell whose voltage is roughly proportional to the amount of light that falls onto the cell. The buzzer needs a few volts so you should choose a cell that can output at least 2V. The last step in the building process is to convert electric oscillations into air oscillations, i.e. audible sound. The cheapest way is to attach a small piezo speaker. Since we effectively built three different circuits, it's possible to attach the speaker to different pins of the chip and choose between different sounds. If there's no sound on some of the connections it is likely that the frequency of the oscillator is outside the audible range or the feedback is unstable.
To simplify the building process in the workshops, platinas can be used which contain already the wiring of step 2 and provide connecting fields for capacitors and resistors so little soldering needs to be made directly on the pins of the chip. Here are a few snapshots of the fabrication of such platinas: