Scale a Resistance?

Discussion in 'General Electronics Chat' started by DanRilley, Oct 16, 2009.

1. DanRilley Thread Starter Senior Member

Jan 13, 2008
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0
Hi I have a fabric that is knitted with conductive thread. When the fabric is stretched the resistance changes. My problem is that the resistance changes very little, for instance 5ohms. Is there a way to amplify the change in resistance so that 0-5ohms will be more like 0-100ohms?

Here is an example of a fabric. I was using a microcontroller to deal with scaling the resistance here, but I'd prefer an all-analog solution to save on cost.

Dan

2. mik3 Senior Member

Feb 4, 2008
4,846
69
Make a simple voltage divider with a fixed resistor or put your fabric into a wheatstone bridge arragement and use an op amp to amplify the voltage.

How you use this resistor in a circuit?

3. MikeML AAC Fanatic!

Oct 2, 2009
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Do you really want an output which is expressed as "OHMS", or do you really want a voltage which is proportional to the fabric's resisitance, such as you would use to feed to a meter or an A/D input on a u-Controller? If so, what voltage out at 0Ω? At 5Ω?

4. DanRilley Thread Starter Senior Member

Jan 13, 2008
107
0
Yeah sorry the reason I expressed it as ohms is that in my mind I am treating this as a potentiometer instead of turning it I am stretching it. That is why I am thinking in terms of ohms. It's the same idea as if I had a 10K pot and wanted it to act like a 100K pot. I'm going to rethink it in voltage and I'm sure I'll find the solution.

5. MikeML AAC Fanatic!

Oct 2, 2009
5,444
1,073
So what voltage out at 0Ω? At 5Ω?

6. DanRilley Thread Starter Senior Member

Jan 13, 2008
107
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OK I kind of thought I could handle this one but I'm going back to it now because I just couldn't figure it out. I've got all my numbers now so maybe someone can help me out:

Power Supply is 5.1V, 750mA

I have a variable resistor that changes between 200ohms and 220ohms

Need to convert that to a voltage range of 4.9V to 5.1V

Using any kind of divider doesn't seem to work because range of 20 ohms just isn't enough.

Any ideas?

7. MikeML AAC Fanatic!

Oct 2, 2009
5,444
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Here is an opamp circuit. Resistors must be 1%. Opamp could be LM358 or any rail-to-rail in and out opamp. I'm using the 274Ω pullup resistor to pull the output high. It pulls almost all the way to 5.1, but not quite.

What is your load? If it is a resistor to ground, and much less than 200K, the circuit will not pull to near 5.1V. It would take a more positive power supply voltage to fix that.

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8. DanRilley Thread Starter Senior Member

Jan 13, 2008
107
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Hey thanks a lot for your time I'm going to try it out and will report back.