Hi,
this is about monitoring soil moisture for plants, perhaps with the aim of controlling an irrigation pump for my next tomatoes.
Last year I invested a lot of time with capacitive moisture sensors, which you can buy cheaply. - It turned out, that this method is not reliable. The reason is, that this capacitive type of sensor is extremely sensitive to water in the very nearest distance <0.1mm. If there is a drop, then the whole measurement is dominated by this single drop.
Due to corrosion effects a resistive sensor is not usable too.
(Best method would be tensiometers but they are not easy to build.)
Now I am trying a sensor, which actually measures heat flow. A transistor in the soil is used as a heater and used as a temperature sensor as well.
First a reference temperature is noted. Then the heater is switched on and after some time the heater is switched off and the temperature is measured again. The temperature difference is (hopefully) dependant from the moisture content of the soil (and of course from the type of soil too).
My experiments are based on:
https://www.researchgate.net/public...a_novel_heat_dissipation_soil_moisture_sensor
http://s.campbellsci.com/documents/cr/manuals/229.pdf
and:
https://www.best-microcontroller-projects.com/transistor-anemometer.html
The schematic shows two states of the circuit. Heater off and heater on. The adc input is at the emitter of Q1. I do not intend to measure with heater on. A heating time of 60sec seems to be in the right range. The transistor has TO-92 package and is inside a shrink tube. (No porous material up to now.)
Perhaps I will add some amplifier to the circuit.
1000 adc-values are added and divided by 100.
After 60sec heating I get.
36.5/4096 counts in water
92.3/4096 counts in air
71.0/4096 counts in some soil (garden mould) with unknown water contend.
The repeatability of consecutive cycles without moving the sensor seems to be quite good. ~1/4096.
I am using a Teensy 4.1 running ATLAST Forth for this experiments.
Comments, suggestions,... welcome!
Regards Christof
this is about monitoring soil moisture for plants, perhaps with the aim of controlling an irrigation pump for my next tomatoes.
Last year I invested a lot of time with capacitive moisture sensors, which you can buy cheaply. - It turned out, that this method is not reliable. The reason is, that this capacitive type of sensor is extremely sensitive to water in the very nearest distance <0.1mm. If there is a drop, then the whole measurement is dominated by this single drop.
Due to corrosion effects a resistive sensor is not usable too.
(Best method would be tensiometers but they are not easy to build.)
Now I am trying a sensor, which actually measures heat flow. A transistor in the soil is used as a heater and used as a temperature sensor as well.
First a reference temperature is noted. Then the heater is switched on and after some time the heater is switched off and the temperature is measured again. The temperature difference is (hopefully) dependant from the moisture content of the soil (and of course from the type of soil too).
My experiments are based on:
https://www.researchgate.net/public...a_novel_heat_dissipation_soil_moisture_sensor
http://s.campbellsci.com/documents/cr/manuals/229.pdf
and:
https://www.best-microcontroller-projects.com/transistor-anemometer.html
The schematic shows two states of the circuit. Heater off and heater on. The adc input is at the emitter of Q1. I do not intend to measure with heater on. A heating time of 60sec seems to be in the right range. The transistor has TO-92 package and is inside a shrink tube. (No porous material up to now.)
Perhaps I will add some amplifier to the circuit.
1000 adc-values are added and divided by 100.
After 60sec heating I get.
36.5/4096 counts in water
92.3/4096 counts in air
71.0/4096 counts in some soil (garden mould) with unknown water contend.
The repeatability of consecutive cycles without moving the sensor seems to be quite good. ~1/4096.
I am using a Teensy 4.1 running ATLAST Forth for this experiments.
Comments, suggestions,... welcome!
Regards Christof