From BEAM Robotics Wiki
This text was taken from Ralf Schreibers Homepage. The solar sound module was created in 1996 by Ralf Schreiber. It’s a simple analogue circuit an attached piezo speaker and a small solarpanel. The module generates tiny and various sound pattern which sometimes reminds you to birds or insect sounds. The kind of sound and the intensity depends on the amount of light that touches the surface. Every module is special and unique and it’s impossible to build exact equal modules. The solar sound module starts in the morning with the first sunbeams and stops in the evening. It will work several decades. The circuit provides the basis for the "living particles" installations and workshops.
The solar sound module circuit is designed with a Hex Inverter IC (Schmitt Trigger Type 74HC14). Every oscillator is assembled with two inverters, one resistor and one capacitor. The finished module has three oscillators, which compete for the unbuffered, lack energy of the solar cells. The variable sound pattern represent the internal astable switching status. The sounds are experimentally adjusted with alligator clips and different resistance and capacitor values. The weak energetic circuit concept permits this experimental work, which is generally forbidden in electronic circuits.
The sound variety of the modules is very wide, they can chirp, whistle, buzz or sweep. For an interesting sounding module you need to experiment for a while. You can use different 74HC14 types and change the values. Furthermore you can fit additional parts like diodes, resistors, potentiometer, piezos, Solar_Cells, photodiodes or capacitors.