Soil moisture meter , Time domain Reflectometry

[berger.h]

I dug up and disassembled the old soil moisture meter
These are two concentric rings made of stainless steel wire, one serving as a transmitting antenna and the other as a receiving antenna.
Distance between antennas of the order of 5cm.
The output is the voltage corresponding to the soil moisture
I tried to draw the electronics wiring

if I'm not mistaken
A 2MHz square wave signal goes to the transmitting antenna 100ns Hi and 400ns Lo.
U1A is clock source 4MHz,
U2A is divder/2
U2B change clock form 50/50 to 25/75
U3A does the same

I don't quite understand the function of U3B, but somehow it compares the signal received by the receiving antenna with the signal from U3A
Its outputs Q and Q / are inverted and converted to analog signal R11, R16 C3, C4 by RC elements

4 pcs of U5 amplifiers then probably convert the signal delay from the receiving antenna back to the way it was transmitted on voltage

The amount of water in the soil around the antennas causes a signal delay, the more water the higher the humidity and the signal delay
In fact, this is a form of TDR (Time Domain Reflectometry)
I was surprised by the relative simplicity.

Questions
What does U3B do?
If anyone could explain the function of individual amplifiers, I would be happy


Time has progressed, I am thinking about how to remake it with a processor.
It is not a problem to use PWM with any pulse width to generate a pulse to the transmitting antenna
The received and transmitted signal could again be "compared" to U3B, converted to voltage on RC and then converted to digital information using internal amplifiers in MCU and ADC, but this is the first idea and you may come up with a better solution.

 

[carpenter]

U5 A,C,D is instrumenttion Amplidier with Ag= 66.5 for R=5k to 520 for R5=0
this amplifier amplify diffenrence between U2 - U1, U2 is on C5, U1 is on C4.
U5B and Q3 is probably a 0-20mA current loop converter for 0-5V on R13
I emphasize, probably

 

[betwixt]

It seems a complicated way to do the job but I think the principle is to drive a square wave to the transmit rod and use the soil conductivity to drop the voltage sensed at the receive rod across R1. U3B is configured as an SR flip-flop, set by a short pulse from the master clock and reset by a pulse from the receiver. This means the on to off ratio will depend on the rise time of the received signal compared to the transmitted one. R11/C3 and R16/C4 convert this to an analog voltage which is amplified then summed by U5A. I would agree it seems to have 20mA current control at the output.

I did something similar using an MCU (PIC16F1847 if I remember) but in a different way that avoided electrolytic corrosion of the probes. The present design presents a unipolar voltage between the probes which may reduce their life and contaminate the surrounding soil. The method I used was to alternate the voltage between the probes (driving them out of phase) and the UN-driven probe was connected through a filter to the ADC input. Both probes were driven low after taking the ADC reading before swapping the drive signal and repeating the cycle. I took two ADC readings a fixed time apart and calculated conductivity from the difference in measurements.

Brian.

 

[berger.h]

First of all thnaks.
After kicking and redrawing it a little more clearly, it's clear to me too. Yes instrumenttion amplifier with amplification controlled by R5 asnd Voltage to Current converter controlled by R20. See new pic
With U3B I understand the function, but I don't know how the given type of connection D FF is called correctly and so it is not commented on the picture.

I did something similar using an MCU (PIC16F1847 if

I'm very interested, don't you have a schematic?
I'm also considering how to redo it with the MCU maybe STM32 .
It also occurred to me to convert the pulses directly to the transmitted and received pulses and measure them with an ADC.
Any as ADC connect dirrectli on C3 ,C4 on schematics, but I'm afraid I won't measure anything reasonable.
Why?
Here, the voltage difference between the voltage of the transmitted and received signal is amplified by the order of 100 times.
And ADC in you use a PIC is 10bit on 5V approximately 5mV on LSB.
I would really be interested in your experience and schematics your device with PIC.
Maybe use XOR as phase detector and measure voltge from its output or use any as STM32F with high resolution timer (217ps) and measure dirrectly pulse width? Probably not the timer has a high resolution, but the accuracy / speed of the STM32 input is normal

 

[betwixt]

U3B configured like that is simply an SR (Set Reset) flip-flop. It has two stable states, a low on 'PR' PResets it and a low on 'CLR' CLeaRs it. The difference in time between the signals decides how long it remains in one state. Here the PR signal is fixed and the CLR depends on conductivity. It could be done by using only one of Q or /Q but even more amplification would be needed. I suspect the reason for using this method is to help eliminate other sources of signal the probe might pick up, just measuring the flow of current makes it susceptible to interference being picked up.

I can't be too specific about the exact method I used but I checked my archives and it was actually a PIC16F88 I used (although a '1847 would work just as well). The probe schematic is

Note that in this design the probes are only about 1cm apart but it can be much more. Not shown is a 100nF ceramic capacitor wired directly across the probes. The bottom horizontal line is VSS and the line leaving on the left is VDD, the other lines go directly to the PIC. The charge on the 100nF capacitor is what matters and the sequencing of the signals from the PIC and measuring the returned voltages allows the time constant set by the soil conductivity to be measured.

Brian.

 

[berger.h]

Thansk.
For clarity, I dared to redraw it
So if I understood correctly, for example
Release some PWM1 into the lower probe and sense the ADC1 voltage on C1 on the upper probe
If next measurement, I may swap it.
Yes?
If so I see as the main problem, capacitor C2. This introduces a large capacitive coupling between the probes.
I can't even estimate what effect the permeability of the environment between the probes has in the whole measurement, I come to the conclusion that the probes are more like a parallel capacitor to C2, but it's just an impression.
Did you not measure exactly the accuracy and resolution with which it measures?
My has the stated accuracy of better than 5l of water in m3 of soil in the range of humidity 5-50%, with the fact that the version with circular electrodes 250mm measures the humidity in about 20l volume of soil. Version a probe in the shape of the letter U 20 cm and a central sensing wire in about 15 l of soil

 

[betwixt]

Almost right. It isn't PWM but alternating pulses. The PIC pins are alternately configured as digital outputs then analog inputs. That allows each probe to be driven high or low as transmitters and measure a voltage as receivers.

C2 is actually the measurement tool. Ignoring the diodes which are there to protect against reverse voltage and the 220pF capacitors which are for HF interference suppression, you get a signal path like this:
output --> probe and C2 in parallel --> 22K resistor to ground. Measurement taken across the 22K resistor.
The voltage is measured twice, I can't remember the timing I used but it was only one or two mS. The charge current into C2 is measured across the 22K but the difference between the timed measurements indicates the rate of charge and hence how much current bypassed it through the probes. It has advantage over simple current measurement that the voltage between the probes is AC and C2 makes it very immune to picking up low frequency interference such as AC mains signals.

Brian.

 

[carpenter]

I thought a bit about the original scheme from post # 1 and # 4, more precisely about the physical nature of the whole humidity measurement by this method.

From a physical point of view.
We generate a Dirac unit impulse (her 35ns 5V pulse) and let him into a line of known lengt (her for example 0,5m wire conducted coaxially with GND wire).
If the dielectric around our "coaxial" will form a vacuum or air in general an environment with a relative permittivity around 1.
There will be a pulse delay of approx. 800-900ps through the passage of a pulse through a 0.5 m coaxial.
If we immerse the whole coaxial in water, it will change dielectric constant ( relative permittivity ) from 1 to 80 (RP for water) and this causes the pulse to delay by about 14.8ns on the same 0.5m path.
Heureka. The 0-14ns signal delay will be a directly proportional amount of water around the coaxial (humidity 0-100%)

From a electronics design point of view.
We need a phase comparator that can compare the difference between the sent and received pulse and return something directly proportional delay in the order of fractions and units ns.
Unfortunately, most simple phase comparators suffer from so-called Phase detector dead zone (they do not register small differences in phase)
Therefore the author of the given connection to use the divider 2 and thus obtained two identical signals of rotation by 180 degrees (250ns) , subsequently using U2A and U3A for change dutty cykle and create two 35ns pulse which are rotatable by 180 degrees.
Finally, on U3B compares the inverse values of these two pulses.
If they were rotated by 180 degrees (there was no delay in one) the output Q and Q- would be active at the same time, ie. Voltage at C3 C4 identical theoretical 2,5V (for 5V logic) voltge differnece C3-C4=0
As the signal delay increases, the voltage difference between C3 and C4 also increases.
Thanks to the measurement of 180 degrees of rotated signals, the dead zone is eliminated

Anticipated problems
1. 74HC logic have transition time typ 7ns which is relatively slow and the sensor will probably be saturated before at 100% humidity. may be use 74LVC with transition time typ 2ns , but the inverters in the short circuit would have to remain HC.
If someone comes up with another, simplify the solution ...
2. Relative permittivity of water is temperature dependent, I calculated it and changed the water temperature from 20C to 25C causes shortening the delay by approx. 160ps to 0.5m. The solution is to measure the temperature of the measuring electrode (ground wire), which should correspond to the water temperature, and then perform a mathematical correction in the MCU
3. The propagation speed is affected by dielectric losses in the dielectric,in the case of water, therefore, the amount of salts dissolved in the water. If the rainwater is generally surface soft, it will not have a practical effect, but with hard and especially even slightly salty water, yes. Cheap technical solution probably not max to allow correction if I know it is about salt water.

I'm just playing with LoRaWAN and the Internet of Things, for such sensors as made.I was quite interested, I will try to build a soil moisture meter with an MCU on this physical principle.
Due to a certain frequency dependence of the Relative permittivity of water, it should work regularly with pulses with a frequency of 10MHz.
If anyone has any idea on the issue, I'll be happy.