Simple/cheap CV-gate input on axoloti - SOLVED

modular
#1

Doing some digging, until now I did not find a simple solution for digital input (gate on/off) without risking the axo to get fried.

I want to connect my axo to my eurorack drum, I figured an optocoupler would do

4 channels working at the moment, looking to expand to eight.

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I'm using a CNY74-4 optocoupler
using a resistor to lower the input Voltage to something acceptable (trial, fry optocoupler, try again, i bet I should have read the data sheet) this works really well
I also put a LED in parallel over the signal cable > Xmas tree!

The panel is rather conductive. Risky business I know, I do not recommend this approach :slight_smile:
however the cost of this solution is very low. Some Resistors a prototype panel and an optocoupler

Also, I can use them as CV and gate. CV resolution is somewhat dissappointing, I bet someone with a bit more electronics background can do better, but for triggers this works really great!

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Needless to say this is still a prototype, but looking to scale it down to fit my modular.
Tips on improving this very welcome...

especially, is an optocoupler a good or a bad idea?

Patching is really straightforward using GPIO/analog-in and a math/<c object for triggering

Connecting Eurorack VCO to Axoloti line in
+/-12 Converter for CV Input
#2

Even though this seems simple, a schematic would be helpful.

Can I make cv inputs with vactrols?
#3

you're absolutely right. I'm not good at this, but I copied my breadboard setup.
Hope this helps. I could not find a mono jack component, but the TIP should be connected to the RED wire
The bended side of the LEDs is the negative side

Screen Shot 2019-03-13 at 21.00.10.png673x585 140 KB

In an earlier version I swapped resistors, this is fixed now.
The LEDs had to be flipped as well...

#4

@tele_player I made proper schematics, it wasn't that hard :slight_smile:

Screen Shot 2019-03-13 at 21.55.06.png910x614 63.3 KB

#5

Now I see what you built.

I really hate Fritzing drawings, though they are definitely useful for helping with assembly.

#6

So this can be used to trigger drums with cv for example? What else could it be used for? Modulating sounds?

#7

It will not work in the negative voltage ranges, so modulation from -5V/+5V will not work in this setup.

If you have a 0V/+10V LFO it will respond nicely. I'll try it with a scope later today

#8

Hey I fed a sawtooth LFO in the optocoupler
Like I suspected, it responds quite nicely but no reaction on negative voltages.

There is also clipping, the LFO in the screenshot is 0v/+7v
it should give you an idea of the current range.
pretty sure this could be improved with an extra pot in between the 560 Ohm resistor and the tip of the patch cable to attenuate the signal. This won't help with negative voltages of course

Screen Shot 2019-03-14 at 08.34.07.png810x564 22.5 KB

#9

with a 10K pot in between I get this signal

Screen Shot 2019-03-14 at 08.59.59.png904x788 23.7 KB

so I was able to tweak it to a reasonable range by turning the knob!

#10

has there been anymore progress on this? i've got a couple of optocouplers and have been itching to find away to give my axoloti control from eurorack and the other way around.

#11

I still use it as designed above. doesn't miss a pulse. Planning a new approach but that's still gonna be a while...