Hi UlrichH ,
Be aware that not observing the absolute maximum voltage ratings when connecting an external voltage source to Bela inputs may permanently damage your Bela board and/or BeagleBone Black.
you can safely connect Bela's analog outputs to a modular synth already. These outputs have a range of 0V-5V, which should be able to control most modular synths (e.g.: if you have a 1V/oct oscillator, this gives you a 5-octave range).
No external electronic components are needed for this connection, just make sure:
- you connect each analog output to CV input you intend to control, and
- you make exactly ONE connection between the ground of your synth and a ground pin on Bela.
See here http://bela.io/belaDiagram/ where you can find a ground pin on Bela.
When connecting CV or signal outputs from your synth to Bela's analog inputs, you will want to make sure they are in the range 0-5V (though the usable range is only 0-4V for inputs). This can be done, e.g.: with a simple zener limiter
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/limiter.html combined, if needed, with a schottky diode which prevents the analog input from going below -0.3V.
When using digital I/O from Bela, care must be taken that the voltage on them is in the range 0-3.3V.
Again, protection circuits like the ones described above should be ok. Be aware, I would not expect digital outputs to be able to gate/trigger all synths, as often the gate voltage is 5V, while some other times it is -5V. In these cases, you may need external electronic components to shift/scale your levels.
To figure out voltage ranges for your modular:
- check your synth's documentation
- verify with a multimeter
- always use protection circuits if unsure