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Read resistance with timer555

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tablet

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Hello,
I would like to know the functioning of this circuit:
**broken link removed**

It's working in astable mode and is re-fed with pin 3. My question is what are the equations that can provide me the value of the sensor resistance from frequency. Do the equations change to be fed back?

Sorry for my English.
Thanks,
Regards
 
If you read this link entirely it becomes apparent that while they give you an overview of circuit operation they do not provide any guidelines or formula that gets you from conductivity to the 555 frequency out. The way I have seen this done in the past is to use known Conductivity Calibration Solutions. Also, keep in mind that this simple little circuit would never be used in an actual lab environment to provide credible numbers as to a solutions true conductivity numbers. Anyway, the only way to calibrate this is to place the sensor in some known solutions like those I linked to. That would be my guess and I may well be wrong.

Ron
 
If you read this link entirely it becomes apparent that while they give you an overview of circuit operation they do not provide any guidelines or formula that gets you from conductivity to the 555 frequency out. The way I have seen this done in the past is to use known Conductivity Calibration Solutions. Also, keep in mind that this simple little circuit would never be used in an actual lab environment to provide credible numbers as to a solutions true conductivity numbers. Anyway, the only way to calibrate this is to place the sensor in some known solutions like those I linked to. That would be my guess and I may well be wrong.

Ron
How did you get to that link? I could not this morning. Maybe I was just to tired.
 
How did you get to that link? I could not this morning. Maybe I was just to tired.

The TK1530 part led to the link. :)

I cheated. :)

Ron
 
Hello,
I found a document to calibrate the watermark 200ss **broken link removed**. The calibration of this sensor utilize a linear relationship between resistance and Hydric Potential. Then, I need to know the resistance and temperature to measure de hydric potential.
I simulated the circuit in Pspice with the values of resistances on this paper(page 6) , http://emesystems.com/pdfs/SMX.pdf, but the output was different.
Any idea?
Thanks,
Regards.
 
Note that one value in the circuit has to be trimmed. So, you should really do a double Y plot of resistance vs output.

Which output was different? Frequency requires a pull-up and, I think, would likely be the BEST signal to look at.

That part LMC555 is also a CMOS version of the 555 timer and is somewhat different than the standard 555 and it might be a significant contributer.

Datasheet LMC555
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CCAQFjAA&url=https://www.ti.com/lit/ds/symlink/lmc555.pdf&ei=EEyhVMvwOcWegwScu4HQCw&usg=AFQjCNFeoShMMJWgtUUSsELXOofl4gRR5Q&bvm=bv.82001339,d.eXY

One BIG difference is minimum operating voltage 1.5 v for the CMOS part and 4.5 V for the bi-polar part. At 3.3 V the LM555 won't work. You may have missed the difference.
 
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