Not sure if I'm oversimplifing this module ...

Correct. The RC filter and peak detector idea could use a PWM /square wave output instead of the fixed frequency 555. The variable frequency idea shown further down is probably better for accuracy. The peak detector idea may be good enough, the temperature coefficient error of the diode drop could be reduced by driving the PWM output steady high during a calibration cycle but that may be gilding a donkey.

piglet

What you wrote is true: A peak detector is not a F-V converter. But the humidity sensor cap in the article is wired as an RC lowpass filter, so the detected voltage does drop with increasing capacitance. Seems a usable low cost way of doing things for uncritical apps?

piglet

Regarding [2]: The design is flawed anyway. An electrode in moist earth should be DC free to prevent corrosion. Using an electrode with a solder mask over it is not a solution, the solder mask is not water proof. It is much better to use the capacitance as the frequency determining element in the 555 oscillator instead of in the low pass filter. Place 100nF in series with the electrode to remove the DC component. Remove the detector and low pass filter completely, and measure (count) the frequency of the 555 digital output. You will get an enormous dynamic range, and less noise in the signal.

Arie

I wonder if that design actually measures capacitance. Or accurately measures soil moisture content.

Interesting project

But given climate change and that it’s 108 degrees in summers here now,

I have watered everything in a pot and all “younger” plants, every day for weeks in the last summer and expect to do so for many weeks to come week. (Usually in summer I only have to water every 2-3 days) It takes me about an hour to water everything and doing just that outside is borderline pass-out activity out there.

mk5000

When lost in wild rain there is nothing you can do When lost in city rain there is something you can do To take refuge wherever’s marked “Admission Free” As we did one night, back in late ’43--black midi – 27 Questions