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n00b Raspberry Pi + ADC questions.
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author n00b Raspberry Pi + ADC questions.
ayruos
Hi!

So I've been recently thinking of integrating my Raspberry Pi with my Eurorack as a video synthesizer and had a few n00b questions. I've worked a bit with the GPIO pins on the pi using openFrameworks and ofxGPIO but always in a standalone manner and not tried it with external gear and I don't want to fry anything!

1. Power. Can I power the pi with the Euro busboards? With 5V/GND or 12V/GND?

2. I want to use CV controls with the Pi and of course I need an ADC for it. What's the best ADC for this? Can it handle bipolar signals? If not, what's the best way around it? Diodes to dump the negative signals to GND?

Any help welcome, willing to do my own research, comfortable with building basic circuits on the breadboards and even designing PCBs for fabrication, just don't want to fry my Pi at the get go! Any tips, or links that might be helpful, let me know! smile
ayruos
Research so far suggests converting the bipolar signal to a unipolar one and using the MCP0008 ADC with the Pi, and keeping the Pi at +5/GND.

This is what I'm referring to: https://masteringelectronicsdesign.com/design-a-bipolar-to-unipolar-co nverter/
Morphology
A Raspberry Pi 3 with all 4 cores running but nothing else connected will draw around 750mA from a 5v supply.

This is, for example, a lot more than the 170mA that something like a uZeus can supply on its 5v rail so, unless you have a lot of power available on your 5v rail, it is probably better to use a separate USB power supply.

The recommended supply for a Pi 3 is 2.5A, but that is mainly to ensure it has sufficient power to supply any peripherals plugged into its USB ports.

The 'normal' way of powering a Pi is via the micro USB connector, which is regulated and fuse-protected. It is certainly possible to power it via the GPIO header (Pin 2), though this bypasses the regulator and fuse protection.

I think most ADCs built specifically to work with the Pi are probably single-ended, and take 0-5v on their inputs, but I haven't done much research on what is commercially available and would work with the Software you are intending to use, so I can't help much with that.

Edit: Yes, that Bipolar to Unipolar conversion method will certainly work fine to feed a Unipolar ADC

Morph
ayruos
Thanks about the tip about the uZeus, that's the exact power supply I have so I'm better of running it with USB I guess smile

The MCP3008 ADC chip seems to be out of stock everywhere :( but i'll build the bipolar to unipolar circuit soon.
infovore
Re: the 'bipolar to unipolar' scaling via a differential amp - have a look at the (open source) schematics to any of the Mutable Instruments digital modules - say, Rings - to see how Olivier scales and offsets inputs down to 0-3.3V - he's picked some sensible ranges for input. (IIRC - I don't have them to hand right now - Rings takes -1.5-5V ish on V/Oct, and +/- 8 on the rest).
MalteSteiner
I also had problems to get the MCP 3008 for a recent project, but did you tried to get MCP 3208? Its 12bit instead of 10 and seems to be more available. You only have to adapt your calculations on the softwareside to the better resolution
ayruos
infovore wrote:
Re: the 'bipolar to unipolar' scaling via a differential amp - have a look at the (open source) schematics to any of the Mutable Instruments digital modules - say, Rings - to see how Olivier scales and offsets inputs down to 0-3.3V - he's picked some sensible ranges for input. (IIRC - I don't have them to hand right now - Rings takes -1.5-5V ish on V/Oct, and +/- 8 on the rest).


Thanks @infovore. Will do. I was thinking about this as well, as -/+12 seems to be a bit overkill as most stuff outputs -/+5V and 0-8V for envelops (at least according to Doepfer specs).
ayruos
Just wanted to say I got this working smile

Now that this hurdle is cleared, the real works starts!

In case anyone is wondering, using the MCP3008 as the ADC w/ Schottky diodes for over voltage and negative voltage protection.

The 3.3V is going through a voltage divider to become 1.65V which goes through a unity gain buffer as reference voltage to scale -/+5 V to 0-3.3V. If anyone wants schematics, let me know smile

I'm so happy I got this working today without frying he Pi razz
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