Wheatstones Bridge question

kxspxr · Joined: Nov 20, 2008 Posts: 35 Location: Denmark

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.

wb.png

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My Wheatstones Bridge (untested)

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blue hell · Posted: Tue Dec 30, 2008 2:58 pm Post subject:

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.

seraph · Posted: Tue Dec 30, 2008 4:39 pm Post subject:

sonobotanics

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

fond memories Wink

kxspxr · Joined: Nov 20, 2008 Posts: 35 Location: Denmark

frijitz · Posted: Thu Jan 01, 2009 12:06 pm Post subject:

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

kxspxr · Joined: Nov 20, 2008 Posts: 35 Location: Denmark

frijitz · Posted: Fri Jan 02, 2009 9:50 am Post subject:

Interesting project! Please keep us posted on your progress.

Very Happy

Ian

kxspxr · Joined: Nov 20, 2008 Posts: 35 Location: Denmark

kxspxr · Joined: Nov 20, 2008 Posts: 35 Location: Denmark

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

Sensor Amp V1.png

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Single Supply Instrumentation Amplifier

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