I have a Grove soil moisture sensor that works, but I have trouble understanding how the sensor actually works.
I think the circuit starts with the 555 timer, that outputs a 487 kHz wave (R1 = 1.5K, R2 = 2.4K & C = 0.47 nF) to the positieve plate of the capacitor that is in the soil.
The soil acts as a variable dielectric, where the dielectric constant will be around 3 in dry soil and around 19 in wet soil, so the capacitance of the soil capacitor goes up when the soil sets wetter. On the other side is the negative plate of the capacitor that is connected to the GND.
This is the point where I get stuck.
I get that when the timer switches output high the soil capacitor starts to charge slowly through a 10K resistor, and that the soil capacitor discharges when the output is low (although I’m not really sure where this charge is going.) I don’t understand what comes after it.
The positive plate is also connected to a 1N4148 diode, which I suspect works as a rectifier that transforms the AC voltage that somehow comes from the positive plate of the soil capacitor to DC voltage. Is this diode indeed an AC to DC converter? What exactly goes into the diode from the plate of the soil capacitor?
I know that the soil capacitor, with a 5 volt input, should have around 5 volt output in air, 1.67 volt in dry soil, 0.26 volt in wet soil and 0.06 volt in water (based on input volg / dielectric constant = output volt.) Is it the case that when the power to the positive plate is turned on by the timer, all current first goes to the soil capacitor, until it is fully charged, and then when it is getting more and more charged, more current is going into the diode?
After the diode there is a ceramic capacitor and the LMV358 amplifier, that eventually feeds into the output pin of the microcontroller. Does this indeed just amplify the signal, or does it do something more?
Sensor: https://wiki.seeedstudio.com/Grove-Capacitive_Moisture_Sensor-Corrosion-Resistant/
It is the yellow part of the circuit that I’m struggling with:
