AC resistance measurement...

[brutc001]

Hi

I am looking at making a remote soil moisture probe using a pic micro controller. The probes are read by measuring the resistance between two electrodes within the probe. The resistance can vary between approximately 0.5K and 35K. the problem is to prevent the probe from becoming polarized the measurement must be taking using an AC source, around 3Vp-p. Does anyone have any experience in doing something like this? Would I just use a shut resistor, rectify the voltage on each side and read the voltage drop with two A/D channels?

 

[BeeBop]

Hi

I am looking at making a remote soil moisture probe using a pic micro controller. The probes are read by measuring the resistance between two electrodes within the probe. The resistance can vary between approximately 0.5K and 35K. the problem is to prevent the probe from becoming polarized the measurement must be taking using an AC source, around 3Vp-p. Does anyone have any experience in doing something like this? Would I just use a shut resistor, rectify the voltage on each side and read the voltage drop with two A/D channels?

Start with an opAmp configured as an oscillator - wien bridge perhaps, then use another opAmp to measure the drop, finally use a precision rectifier to convert to DC which you can then measure on a micro.

 

[ericgibbs]

Hi

I am looking at making a remote soil moisture probe using a pic micro controller. The probes are read by measuring the resistance between two electrodes within the probe. The resistance can vary between approximately 0.5K and 35K. the problem is to prevent the probe from becoming polarized the measurement must be taking using an AC source, around 3Vp-p. Does anyone have any experience in doing something like this? Would I just use a shut resistor, rectify the voltage on each side and read the voltage drop with two A/D channels?

hi,
Somewhere on this site is LM3914 soil resistance measurement circuit.?

I have been trying to find it, but the search engine is not bringing it up.

 

[brutc001]

Thanks beebop, very helpful. i had a look inside an already existing reader yesterday with an oscilloscope and thats how that worked too, except it just fed halfwave ac to the micro i suppose they just take a lot of reads and find the maximum. i think i'll put a few caps in and smooth it out though.

 

[BeeBop]

Thanks beebop, very helpful. i had a look inside an already existing reader yesterday with an oscilloscope and thats how that worked too, except it just fed halfwave ac to the micro i suppose they just take a lot of reads and find the maximum. i think i'll put a few caps in and smooth it out though.

I guess that would work too, just a bit more complex on the firmware end. I would think the correct caps would make it seem like an average.

Do you recall what frequency the existing reader was operating at?

PS I found a gold plated audio plug worked well as a probe.

 

[brutc001]

the reader was running at 250Hz if i recall correctly. I already have some gypsum block probes sorted.

 

[MikeMl]

It doesn't have to driven with an sinusoidal signal. Just put a DC blocking capacitor in series with the probes. If there is ANY DC through the probes, they will corrode.

 

[brutc001]

i don't need very long to take the reading, do you think i could use DC if it was only for a short period of time, say less than a second?