The challange is to make something that are light enough to keep the balloon size reasonable but still loud enough to be heard.
We decided to use the Arduino synth, that I and Mads Høbye developed some time ago, as a basis but replacing the Arduino board with the much smaller ATTINY85 microcontroller.
Allso we developed code that allowed the ATTINY85 to receive 1200 BAUD serial data provided by an infrared receiver.
We use piezo transducers to make the sound. These are not very loud on their own so we mount these in a logarithmic horn to amplify the sound. The horns are made from corrugated cardboard to save weight.
The synth is powered by two AAA batteries. These may not be the most weight sensitive option but they are easy to obtain and replace and we will be using a lot of them since there will be around 20 balloons.
An infrared spotlight is converted into the 'infrared remote from hell' capable of lighting up a whole room with modulated infrared light.
The spot is driven from an arduino board via. a MOSFET driver. The MOSFET gate is connected to a constant 36KHz output from the Arduino. A NPN transistor is used to short out this signal while the TX line is high. When TX goes low the transister opens and the 36KHz signal drives the MOSFET.
A computer running MAX/MSP provides the data to be send to the synths. Each synth has a note_id and can thus be individually controlled.
The synth firmware is still under development but i expect to publish it when it is ready.