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A YIPPEE!! day for me.  I received the Ladybug Blue Lite Beta 2 PCBs from OSH Park yesterday.  Today I tested reading pH and EC values.

It worked!  It worked!…well, results were the exact values but they were within an acceptable range.  Particular given the pH probe is an inexpensive probe I got from eBay about a year ago.  Since pH probes “go bad”, I am not surprised results were a bit off from what the model predicts.

It is such a great feeling.  BLE, pH, EC…working on a small PCB!  Off the Arduino and shields!  I plan to use this one to help me grow tomatoes this winter.

Here are the measurements I read from the AINs:

pH

The probe was placed in the pH 4 and then pH 10 calibration solution.

 VGND(mV) AIN(mV) pH (mV) pH pH 4 1455 1620 165 4.2 pH 10 1455 1314 -141 9.4

I discuss calculating the pH in earlier posts.  The first thing is to calculate the mV reading from the AIN.  The AIN is not a differential reading so there are two AINs for pH measurements.  One is the virtual ground (VGND) used to move the rails of the op amp to accommodate the AC waveform of the pH reading.  The other is the pH measurement (above virtual ground).

For pH 4, the pH in mV = 1620 – 1455 = 165 mV. pH =  7 – 165/59.16 = 4.2

For pH 10, the pH = 1314 – 1455 = -141 mV. pH = 7 – (-141/59.16) = 9.4

If I decide to use this probe, I would calibrate readings to be on the slope of the line for the 4.2 and 9.4 readings.

EC

I placed the EC probe into a 1000TDS/2mS calibration solution.

 VGND(mV) EC VIN (mV) EC VOUT (mV) Gain Resistance(Ω) EC (mS) 1455 1650 2085 3.2 448.3 2.2

Gain = Vout/Vin = (2085-1455)/(1650-1455) = 3.2.  Resistance = 1000/(3.2-1) = 448.3Ω.  EC = 1/448.3 = 2.2mS.  Pretty darn close!

Off to finish the firmware and iOS app…