Help with variable Range Selection for Voltage Divider to measure wood moisture

Thread Starter

kyotee89

Joined Sep 11, 2019
1
Hi there,

Thanks for taking the time to read my post.
I'm trying to design a wood moisture monitoring system, that is measuring many places using one controller (arduino), using a voltage divider to measure the resistance through a given piece of wood, with 2 probes about an inch apart. There will be many of these measuring points around a structure and I want to be able to measure each one maybe 5 times a day from one central controller.
I have schematics of some moisture meters and many of them use a voltage divider with a range selection switch, as the resistance of wood can vary from millions of ohms when dry to tens of thousands of ohms when damp. The resistances selectable are 1000Mohms, 9Mohms and 10Kohms.
I want to be abl e to change the selected resistance with a microcontroller to get more accurate readings. Iv been looking into using transistors to select the resistance scale but I think I run into a problem that if I am using 5v through 1000Mohm then I have a 5nA current which is possibly not enough to saturate any transistor. I could just use a relay but im trying to avoid high current mechanical systems if I can. Is there a type of transistor that can work in this application? Any advice appreciated.
See my crude drawings attached to hopefully explain the idea. Each probe would have its own set of transistors and analog channel on the arduino, probes would share channels A, B, C to select range.

Cheers
C
IMG_20190912_162413894.jpg
 

Sensacell

Joined Jun 19, 2012
2,449
Leakage currents are your biggest enemy when designing something like this.
Transistors are ill-suited, most have leakage current levels that will be problematic.

Latching signal type relays can be a good low-power solution, with super low leakage currents.

The input current to the "analog read" is going to be problematic, have you devised a solution for this?

Most "meggers" - high resistance measuring gadgets use higher voltages to create measurable currents in the test circuits.
 
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