Go Down

Topic: soil moisture sensor (Read 307 times) previous topic - next topic

Amulsharma999

I am designing a project in which i need a soil moisture sensor. during my research about soil sensor, i go through two types of sensor :- (1)resistive soil moisture sensor and (2) capacitive soil moisture sensor.

firstly, please can anyone tell me the difference between these two sensors?,  and secondly can anyone provide me PCB design of these two sensor as i find that both have similar PCB tracks and with 1 trigger IC which takes input from sensor probes and fed its output to micro-controller.

so i am confused in the portion of PCB design and IC part - how it takes input from probes and what types of output it generates and how a micro-controller reads IC's output for measuring the soil moisture content???

pylon

The resistive version doesn't need a trigger IC, you simply put some voltage to one pin (might be a nail) you put into the soil and measure the voltage you get on the other pin (nail) a few millimeters away, which is connected to GND via a resistor to build a voltage divider.

This kind of sensor is known to have problems with corrosion so don't use it in a productive environment.

The other type of sensor uses some high frequency signal to measure the moisture in the soil but for that type I don't know the details.

Amulsharma999

Thanks for your response. i have done further reading on other sensor, and i got to know that the Trigger IC used to generate the square pulse corresponding to the moisture content of soil, but i have one more confusion now that how to calculate the capacitance using these square pulse?

pylon

I guess the measurement is based on the capacitance acting almost like a resistor starting at some minimum frequency. And probably the higher the moisture in the soil the higher the capacitance of the element.

wvmarle

Capacitance is usually measured as part of an RC circuit.
So either you use it in an oscillator and measure the frequency (higher frequency = lower capacity = dryer soil), or by measuring the discharge time of the capacitor (shorter time = lower capacity = dryer soil).
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

tigger

Resistive/conductive soil moisture sensors work well if fed with an AC rather than DC voltage. The rapidly changing polarisation considerably reduces any corrosion. There used to be (now obsolete) a nice little chip from National Semiconductors called LM1830 that was designed to do just that and was used in continous industrial applications. If you use a datasheet archive service (with a good anti-virus as some are suspect) you could get an idea how they operated.
Commercial conductivity meters use as many as 4 electrodes and the electrodes are often carbon composition.
If you just a "is it wet or is it dry" indication, a BJT transistor or almost any CMOS logic gate will do - the input impedance of the CMOS inputs is so high that corrosion would be negligable.
Remember that any soil sample needs to be in a condition to actually allow conduction. If it consists of large separate lumps of damp soil, it probably won't work, then nor would capacitance

Amulsharma999

Thanx to all of you, its helps me alot. i start working on pcb circuitry.hope so,everything results positive.

Go Up