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Wheatstones Bridge question
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kxspxr



Joined: Nov 20, 2008
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Location: Denmark

PostPosted: Tue Dec 30, 2008 5:16 am    Post subject: Wheatstones Bridge question
Subject description: Wheatstones Bridge for measuring bioelectric signals.
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I am building a plant synthesizer. Not to synthesize plants of course, but to synthesize sound from the bioelectrical signals in plants. In order to register the minute changes in resistance (to measure voltage and eventually generate CV) I am using a wheatstones bridge circuit to which the electrodes connect. The bridge is then connected to an opamp section.

My question is, to avoid loading of the bridge and cause inaccurate readings, will I need to place a buffer before the bridge and after the electrodes, or should I buffer the output after the bridge and before the opamp section? Or should I consider a completely different approach?

I included my somewhat rough schematic of the wheatstone bridge I plan to use. The 10K pot is for controlling the power supplied to the bridge and the 100K is to null a meter on the opamp section (which I didn't draw yet). I am sorry if my post is too confusing or lacking information to answer it properly.

I am using (or plan to) homemade electrodes made from gold foil. More about this later.

EDIT: Corrected a few errors in my post.


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My Wheatstones Bridge (untested)
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blue hell
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PostPosted: Tue Dec 30, 2008 2:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

I'm not sure ... just some thoughts ...

A Wheatstone bridge setup measures resistance in a very sensitive way, and as such it would not make sense to put a buffer before it (erm .. I don't see it, I mean Wink ).

But I'm wondering if measuring resistance is the thing to do, or that maybe some activation potential would be more interesting ... in the latter case it would make sense to not have a bridge but go into a instrumentation amplifier directly (an opamp with good common mode rejection).

I've no idea what plants do ... electrically I mean, I know you can smoke 'm of course.

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also .. could someone please turn down the thermostat a bit.
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seraph
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PostPosted: Tue Dec 30, 2008 4:39 pm    Post subject: Reply with quote  Mark this post and the followings unread

arrow sonobotanics

arrow http://electro-music.com/forum/topic-25740.html

arrow fond memories Wink

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kxspxr



Joined: Nov 20, 2008
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Location: Denmark

PostPosted: Wed Dec 31, 2008 1:20 am    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:
A Wheatstone bridge setup measures resistance in a very sensitive way, and as such it would not make sense to put a buffer before it (erm .. I don't see it, I mean Wink ).


Oh yes of course. Dang! My mind must have flicked or something. Thanks.

Blue Hell wrote:
But I'm wondering if measuring resistance is the thing to do, or that maybe some activation potential would be more interesting ... in the latter case it would make sense to not have a bridge but go into a instrumentation amplifier directly (an opamp with good common mode rejection).


I did amplify the action potential in plants some years ago and that was exactly the way I did it Smile

EDIT: Ooops - My memory played a trick on me. I used a very sensitive meter to measure it. I didn't build the unit myself though.

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frijitz



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PostPosted: Thu Jan 01, 2009 12:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

Shocked Ummm ... that's not a Wheatstone bridge.

The lower 1k resistor should be connected to "top of leaf", rather than ground.

But to answer your question, the usual approach would be to feed the differential signal into an instrumentation amp. You can buy these as an IC, or make one out of three opamps.

Very Happy

Ian
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kxspxr



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PostPosted: Fri Jan 02, 2009 3:07 am    Post subject: Reply with quote  Mark this post and the followings unread

frijitz wrote:
Shocked Ummm ... that's not a Wheatstone bridge.

The lower 1k resistor should be connected to "top of leaf", rather than ground.

But to answer your question, the usual approach would be to feed the differential signal into an instrumentation amp. You can buy these as an IC, or make one out of three opamps.

Very Happy

Ian


Thanks Ian! Hmm... I see what you mean about the bridge. Good thing I posted here first. I have a bunch of quad opamps so I'll just make an instrumentation amp from those.

-Kasper

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frijitz



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PostPosted: Fri Jan 02, 2009 9:50 am    Post subject: Reply with quote  Mark this post and the followings unread

Interesting project! Please keep us posted on your progress.

Very Happy

Ian
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kxspxr



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PostPosted: Fri Jan 02, 2009 4:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

frijitz wrote:
Interesting project! Please keep us posted on your progress.

Very Happy

Ian


I will keep you posted. I'm thinking about this simple design and use a quad opamp. I may need some filtering of the signal as well to avoid the plant acting as an antenna, picking up to much of the mains hum.

-Kasper


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kxspxr



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PostPosted: Sat Jan 03, 2009 11:38 am    Post subject: Reply with quote  Mark this post and the followings unread

I will solder this thing tomorrow Smile

EDIT: The differential amp at Vin2, Vout2 will be used to amplify the action potential - stem (Vin2), soil (gnd). V1 and V2 is for electrodes placed on top and bottom of a leaf.
EDIT 2: There's a typo in the equation for the notch filter. It should say: F=1/(2*pi*R*C)

-Kasper


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