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[on:]

Hi,
I am just beginning with Arduino and simple electronics. I am looking into ideas for moisture sensor. Here are two schematics but i have some elementary queries:

-http://www.botanicalls.com/archived_kits/twitter/
How does the transistor help in this circuit and why is it used here?

-http://gardenbot.org/howTo/soilMoisture/
Why is a capacitor (noise filter) used here and what would it accomplish here?

I was thinking of simply using the one at http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1255471714/

[Reply #1 on:]

1.
the transistor can be used to amplify a little electrical current...
that way u get a bigger reaction on changes...

2.
the capacitor would work like a low pass...
it might help when there is a lot of EMF, because u use low current due to the >50kOhm resistor...

3.
i wouldnt send current through a flower...
u could try to insert an isolated electrode (or electrode pair), that will change its electrical capacitance with the moisture...
http://www.arduino.cc/playground/Main/CapSense

[Reply #2 on:]

I like the second circuit - very simple, takes advantage of the high-input impedance of the Arduino inputs.  That capacitor is an issue though, much larger than needed (and large value capacitors can have significant leakage currents) - something like 1uF ceramic or tantalum would be fine, perhaps even 0.1uF (the break frequency for the original circuit is something like 0.05Hz, the kind of noise interference you'd worry about would be far higher in frequency (50 or 60Hz mains and radio-frequencies).

One problem with these DC circuits is that the +ve soil probe will be electrochemically dissolved over time (over several years) and need replacing.  Using an AC resistance measuring circuit is a superior technique with much less electrochemical erosion over time - but not as simple.  As a compromise if the 5V supply to the circuit is provided from an Arduino pin then you can turn it off between measurements which will reduce the average current flow a lot.