Dear Tyler

Arduino output pin may not be able to sustain the needed current/voltage of your soil sensor. I would advise you using a simple buffer (it provides infinite Input impedance and low output impedance), as an operational amplifier.

See for instance https://en.wikipedia.org/wiki/Buffer_amplifier (fig 3). Or here, with the protoboard picture (http://www.facstaff.bucknell.edu/mastascu/elessonshtml/OpAmps/OpAmp3Note1Buffer.html).

good luck

marcelo

You can use that https://en.wikipedia.org/wiki/Common_collector or "... using a simple buffer (it provides infinite Input impedance and low output impedance), as an operational amplifier." (Marcelo B. Perotoni )

@E.V. Kurbatov, in Figure 1 of the Wikipedia article you cite for a common collector (i.e., unity gain voltage buffer), is the NPN transistor emitter is connected to the capacitance sensor (EC5) in series? In other words: NPN(emitter) -> EC5(Vin) -> EC5(GND) -> Arduino(GND).

Also of note, I'm finding that the expected output voltage from the EC5, which should be between 10%-40% of the input voltage (i.e., between 0.25 V and 1.0 V for 2.5 V supply), covers a small range for the ADC (which goes from 0 to 1023 for analog reads between 0 V and 5.0 V). I moved the AREF to 3.3 V and found a better range of values, but is still pretty small to capture small changes in soil moisture. I get an ADC reading of about 7 in air and about 17 in water with AREF at 3.3 V. This is in comparison to ADC readings from 5 to 10 using the 5 V AREF. 

Arduino is easily capable of delivering 10mA directly, as single pin can draw/sink 20mA. the problem lays in proper voltage level. If you have 3.3V at your board - use it as power to your sensor!

http://tinypic.com/r/8ziweb/8

The other issue is reference voltage. If you use Arduino Mega, you can use internal 2.56V reference with analogReference(INTERNAL2V56) command (please take a look on https://www.arduino.cc/en/Reference/AnalogReference)

This will give you readout values :

0% - ~131

100% - ~530

However, if you are interested in even higher resolution, you may use analogReference(INTERNAL1V1), which will clip your measurement range down to 76%, but you will get resolution like:

0% - ~307

76% - 1023

@Michal J Radziwon: The NPN transistor with the base connected to digital 5 V out, the collector connected to the 3.3 V rail and the emitter connected to the sensor works. Setting the analogReference to INTERNAL (Arduino UNO) provides reasonable resolution (analogRead returns an integer of ~330 in air to ~980 in water; soil seems to be in the 700 range). I will just have to do my own calibration, which I was planning on doing anyways. Thanks for your help!

Your welcome, and good luck with your project!

Hello Tyler,

I have another suggestion. Take an adjustable regulator such as LM317 (may be low current TO-18 package) and design it to give your excitation voltage level or say 3.3V.

Now, use a NPN transistor to connect (shunting one of the resistor) across the grounded resistor from the ADJ pin of LM317. Drive this transistor (base) from your Arduino for the duration (10ms or like) from the digital I/O pin so that the output is 3.3V otherwise it is 0V. Refer the LM317 datasheet for more details.

If you want to drive more of the sensors, then simply change the LM317 package to TO220 to deliver more current.

The solution I have suggested is a robust and well tested one.

Wishing you best luck in your work!

Thanks

-Prasanna Waichal

@Prasanna Waichal, thanks, I'll look into it. I am planning on connecting five EC-5 sensors to the Arduino. Right now I'm testing the limits of how many sensors the 3.3 V rail can power (in succession, not simultaneously). I'll try your suggestion if I run into troubles.