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 Forum index » Instruments and Equipment » Theremin
USB Guitar Theremin
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Inventor
Stream Operator


Joined: Oct 13, 2007
Posts: 5978
Location: San Antonio, Tx, USA
Audio files: 258

PostPosted: Mon Sep 22, 2008 3:45 am    Post subject: USB Guitar Theremin
Subject description: Help me design one for my strat
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Hi,

I've got a guitar-mounted USB sensor that detects gravity and acceleration plus it has an expansion port suitable for measuring a Theremin's output. I would like to develop a suitable Theremin so I can wave my hand over the bridge of my guitar and make sounds.

The current project accomplishes audio effects by sensing the guitar's motion, and you can read all about that by cliking on my www button below. But what would be really cool would be to add a Theremin to this device.

The USB interface can provide a +5V output and has both analog and digital input capabilites. I guess I'm a little confused about Theremin theory and operation. I have a master's degree in electronics plus experience doing digital and analog design and software programming, so I'm sure i could handle the technical details if only given the right theoretical understanding of a Theremin's operation.

I once designed a Theremin with a capacitance multiplier circuit, perhaps that is where I should start, I dunno. Actually I forgot about that but it was an NCSU university lab for some years... At any rate I'm looking for a good simple solution that the average guitarist can build or at least buy and install him/her self.

The circuit was capacitance sensor => relaxation oscillator => speaker or something simple like that. What is your suggestion, i.e. how would you build this device?

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Inventor
Stream Operator


Joined: Oct 13, 2007
Posts: 5978
Location: San Antonio, Tx, USA
Audio files: 258

PostPosted: Mon Sep 22, 2008 4:41 am    Post subject: Reply with quote  Mark this post and the followings unread

Attached you'll find a schematic of the Theremin that I recall from my NCSU lab days that I put together back then. It consists of a capacitance multiplier and a relaxation oscillator.

The capacitance multiplier multiplies the body capacitance that is being sensed and the relaxation oscillator varies it's frequency of oscillation according to this capacitance.

It is a nifty little Theremin in that it can be constructed with a dual opamp and five resistors, but will it really work? I think so. I guess now that I've posted it I better build it, eh?


Schematic.jpg
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My Theremin, will it work?
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Schematic.jpg



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Inventor
Stream Operator


Joined: Oct 13, 2007
Posts: 5978
Location: San Antonio, Tx, USA
Audio files: 258

PostPosted: Wed Sep 24, 2008 10:32 am    Post subject: Reply with quote  Mark this post and the followings unread

OK, I tested the circuit and had a lot of trouble with the capacitance multiplier. I believe this is because I am using a single-ended supply. So I simplified things by just using the relaxation oscillator itself. See the diagram below. It works at something lke 40 kHz which is too fast to be read by my interface, so I'll have to slow it down somehow. Anyway, it's a start and it seems to vary frequency sufficiently when a small spool of wire is used as an antenna.


Schematic2.jpg
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My Theremin, tested and working
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Schematic2.jpg



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Inventor
Stream Operator


Joined: Oct 13, 2007
Posts: 5978
Location: San Antonio, Tx, USA
Audio files: 258

PostPosted: Fri Sep 26, 2008 6:59 am    Post subject: Reply with quote  Mark this post and the followings unread

I have now prototyped and tested both versions and some variants as well. I was only able to get a capacitance multiplication of about five times out of the capacitance multiplier in practice. The circuit's slowest operation with the multiplier is about 5 kHz nominal and without it is about 25 kHz nominal. Either version works fine.

The 5 kHz version would suffice as a super-minimal Theremin because you could listen to the signal as-is. For use in my project, however, I need an output signal that is either a lot slower or is an analog level, one or the other. To slow it down I could use a CMOS counter chip, and to create an analog value I could create a frequency to voltage converter (but that would involve a lot of design and test). Right now I'm thinking the divider application would work best, though it does require a chip that I don't have on hand...

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