I made a very simple CV output to control an analog filter. (old Frostwave MS20 clone).
I used an NPN transistor, and it's working really well!
I suppose it's not going to work for 1/V per octave, but for controlling something like a filter it works like a charm!
In the proces of making the simple circuit I think I fried PA4... PA5. is still working, but PA4 only outputs very small values in the mV range in stead of the full 0 to 3.3V.
I am very much a beginner when it comes to making these kind of circuits. Here is my simple circuit:
a)Analog GPIO PA4 (or 5) is connected to the BASE of the transistor.
b)The collector is connected to the TIP of my jack socket --- a 10k resistor --- the positive 9V lead of the Axo external power
c)The emitter is connected to the RING of the jack socket --- the GND of the GPIO --- the GND of the Axo power.
So it seems to works like that. But if I disconnect the GND of the GPIO it also still works.
So my question: what is the right and safest way to do this? Do I need to connect to the GND on the GPIO pins or not?
I also connected my new CV-out to the Axoloti input jack, since I don't need an input for this project. I soldered it to the pins just behind the jack input (with the markings T and R) on the Axoloti.
Is this safe or is there a possibilty of frying something?
Thanks very much for community insight!!!
If anyone is interested I'll post some pictures and sound.
Having an analog filter on my Axoloti-synth is a lot of fun and sounds GREAT!!! Also frees up some resources too!