I want add some CV Inputs for Axoloti: is it not possible just to take a simple resistor to reduce the CV range to +/-5 Volt (or whatever Axoloti ist able to handle) ? I guess this is a newbie question 
I saw this solution for trigger but don't understand the need for an optocoupler:
Assuming you want to use analog inputs on the STM32:
The analog inputs are not bipolar, they measure voltage from 0 to 3.3.
So, you’d need to shift the -5to+5 up by 5v, so it becomes 0-10, then scale it down to 0-3.3.
It's not exactly simple. Search this forum, others have done it.
I read the forum the whole day..
could'nt you handle the range in a patch (to scale the values)?, my question was just about the voltage in the hardware itself.
The analog inputs on Axoloti are for 0-3.3v.
I don’t know exactly what CV you’ll be supplying, but if it is not in that range, it won’t work as expected, and might damage Axoloti.
There is no way to change that in a patch.
I know this, that's why I asked. I guess my question was not clear (I want to use some CV Inputs, not to add them), sorry for this. What I meant was to scale down the range of 0-3.3 to the half (-1.65 to 1.65) in a patch but this is obviously possible. The example was about drum gates (on/off), it should work just to reduce the voltage with a resistor (why not?), for what do you could need an optocoupler with it?
I started to write an answer, then realized I still don’t know what you are asking.
Yes you can scale down to read the analogue inputs, the east straight forward way is to use a voltage devider to scale down the voltage.
However there are a few things you need to be aware of.... here are the ones I know of and would consider if I was doing it, maybe others can add the ones I may have missed.
- Do you have a way of protecting the Axo from a voltage spike higher then 3.3 volts hitting the analogue input ? This may not hurt the Axo, but would definately kill the analogue input and make it unusable.
- Where is the voltage source to be scaled down comming from ? If the power from the source and the Axo are connected to the same power source, this would be OK, but if it is a different power source, the voltage going into the axo needs to be isolated from the source origin, allot like how Midi uses an opto coupler to isolate two midi devices, but I am not aware of opto couplers being used for Analogue signals so you may need to find a different way to to get the signal through, for me, so far I have found solutions by sharing the same power supply to cover all my needs which is in line with have a complete seperate standalone system for the Axo.
I know I have only mentioned two things, dealing with analogue signals there may be a few more, but hopefully this will get you in the right direction.
No, you can't do that.
As stated before by tele_player, if you feed a negative voltage to Axoloti (even -1.65V) you will damage the inputs.
You have to stay between 0V and 3.3V.
The safest way is to use a transistor to act as a voltage controlled "valve" and connect it to pass various amounts of a fixed 3.3v DC voltage from the Axo board itself (one of the multiple VDDC or VDDA pins), to the CV input. This way, no matter what the external voltage is controlling the transistor, the most that can get to the CV in on the Axo is the 3.3V reference voltage.
This can also be done with a Vactrol dunning between the VDDC and the CV input you want. There are surplus sites that occasionally sell bags of vactrols pretty cheaply (I got a bad of 50 for $20 from electronic Goldmine once).
These are the safest way to do it because the voltage can never add up and be too much for the CV input on the Axo.
Yes, forgot about vactrols... can even make these yourself... a great solution, could be a bit fiddly if you want very fine tuning of the signal, but should be fine for a basic 7 bit analogue input on the Axo.
thank you. I will think about it.
Here I found an alternative to clip down biploar signals with rail op amps:
(=> "Connecting gear with higher voltage levels, like Eurorack", 2nd circuit)
, have to find out the advantages, maybe transistors are the standard solution but less accurate.
not so difficult: https://laeliaeffects.wordpress.com/2017/01/08/diy-optokoppler-vactrol/
here is a nice imaged explanation about vactrols: http://www.doepfer.de/a100_man/Vactrol.htm
Of course, there are ADCs built into the STM32, which is what this is about.
If you use external ADC, you still need to make sure the input falls into the appropriate range, and additionally, you need to decide how the ADC connects to the MCU, I2C? SPI? something else?