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Old 21st July 2015, 09:21 AM   #1
diyAudio Member
Join Date: Jul 2015
Location: Bordeaux, France
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Hi everyone!

I'm working on a DIY synth controller, sort of drum pad on which I could strike, but also rub, play with a violin bow...

The first idea is to use an arduino to capture data from piezos. But arduino isn't fast enough to capture "simultaneously" 8 audio signals and track the envelope. And I don't really anything else than the envelope for my purpose. So I prefer save arduino ressources to compite antrhing else than high sampling rate data.

So, what I'd like is to build an envelope follower between the piezo and the arduino ADC.

So I would make one from this basic one :
Click the image to open in full size.

But I would like to improve it a bit if it keeps beeing quite simple.
What I'd like to add is a gain pot in the input of this follower, and a decay pot to be able to adjust the decay time of the envelope follower.

So I imagined this circuit. But I miss some skills to well understand everything. So I guess this could be better.
Click the image to open in full size.
I've added an inverting amp at the input with a pot to control the gain. I guess this is OK. I've let the unity gain amp the same, with this 10Kohms resistor (R3) which I just don't know what it is for? And I replaced the 2Mohms restistor and 0.1F cap by a standart 100Kohms pot (R4) and a 4.7F cap (C1) to control the decay time.
But my question is : is that modifications still OK to enter the arduino ADC? Is the impedance OK? WOuld there be something better to do? Simpler?

Thank you very much for your help!

Last edited by fredcazaux; 21st July 2015 at 10:04 AM. Reason: Invalid Image URL
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Old 21st July 2015, 09:43 AM   #2
DF96 is offline DF96  England
diyAudio Member
Join Date: May 2007
I get an 'invalid attachment specified' when I click the image link.
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Old 21st July 2015, 10:09 AM   #3
diyAudio Member
Join Date: Jul 2015
Location: Bordeaux, France
Originally Posted by DF96 View Post
I get an 'invalid attachment specified' when I click the image link.
Thank you! Seems the attachment didn't work correctly. I uploaded the file again and updated so you should be able to see and open it.
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Old 22nd July 2015, 09:36 AM   #4
DF96 is offline DF96  England
diyAudio Member
Join Date: May 2007
Why have you drawn a zener diode instead of a normal signal diode?

You may need to review the bias arrangements for a single-supply-rail opamp. I don't know what the 10k resistor is for, but I suspect it is an attempt at input bias.

Bear in mind that no envelope detector can follow an arbitrary envelope perfectly, so you need to think about attack time and decay time. The capacitor cannot charge up instantaneously, as the previous stage can only supply a finite current. I would use a smaller cap than 4.7uF, and if necessary add a buffer after it.
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