Trying to understand capacitive soil moisture sensor output stage

Thread Starter

Ross Satchell

Joined Jan 2, 2017
I am looking at getting a capacitive soil moisture sensor with the following schematic.
I understand the power supply stage (bottom), and I understand the 555 timer circuit set up in astable mode using the following equation:
555AstableEquation.png which gives me a frequency of 867.9kHz.
For the output stage (top of schematic), from what I can tell the resistor and capacitor are there to give a smoothed signal so an ADC can read it (am I correct in this?). Then is the diode just there to prevent the capacitor from trying to discharge into the 555 timer? Or is it doing something else?


Joined Jan 29, 2010
Hi Ross,
I have been looking for more info online.
But as you say the ADC take off point is on pin 3, the diode /cap and resistor form a simple rectifier/filter for the square wave pulse from the 555.
So the ADC sees a smoothed DC level.


Do you have the Full documentation from the web, I do not want to sign up in order to see the doc's


Joined Jun 22, 2012
Does the capacitor sensor not alter the 555 frequency , and then the ADC would read a different voltage based on the moisture content..? this is how humidity sensors work..

Audioguru again

Joined Oct 21, 2019
The resistance of the soil changes the output level. Damp soil with salts in it will measure a low resistance and with pure water will measure infinite resistance. Does the capacitance of soil change with its amount of moisture?

Have you measured the resistance of distilled water and compared it to the resistance of water with fertilizer in it? Measured the resistance of tap water?


Joined Aug 7, 2020
Does the capacitor sensor not alter the 555 frequency , and then the ADC would read a different voltage based on the moisture content..? this is how humidity sensors work..
That’s what I thought as well. Then you don’t need an a/d, just a counter.
You do have to keep the 555 dry, otherwise any moisture around the timing resistor also changes the frequency, and leakage between the probes would also shift the frequency. I wonder if an LC oscillator wouldn’t be better as shunt resistance from moisture will affect the Q but not so much the frequency.


Joined Jun 19, 2012
This sensor uses a pretty sad method to measure the capacitance, color me not impressed.

As others have stated, using a timer/counter to measure the frequency of a simple oscillator whose frequency is determined by the sensing probe is a vastly superior idea.

sparky 1

Joined Nov 3, 2018
The capacitance of a parallel 2 plate capacitor having air as a dielectric between is determined by the area of the plates.
When the plates are not parallel and have some coating the field will contort and curve having density at the edges.

When this arrangement encounters moisture the capacitive value effects the frequency of the circuit.
It is a nice study because helps to understand the small details and parasitics to trouble shoot and improve circuit trace design.

Some other configurations using capacitive probes do not use 555 timer instead
the higher end capacitive soil moisture probes use a microcontroller that can convert Farads to % moisture or I2C
Some of the $12 ESR component testers have capacitance accuracy to about 3pF others (0.25-0.5 pF resolution of a high end capacitance meter)
The 555 timer based capacitance meter is ok but the capacitance range is a bit higher but the working concept is less intimidating.

I really liked this modification which used crystal timed pic chip and a comparator. It does 0.5pF if you zero out and fiddle with the connections.
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Audioguru again

Joined Oct 21, 2019
The capacitance of the sensors does not effect the frequency of the 555 oscillator. The capacitance is in an RC lowpass circuit and more capacitance reduces the AC output level of the 555 which is rectified into a DC level.