I did a six hour test run of the evapotron control circuit today, with mostly positive results.

This graph shows the bus voltage, the depth in the reservoir, the current drawn by the pump (in mA), and the voltage in the circuit being charged. It all makes sense, but it might benefit from a walk-through.

The bus voltage declines over time, of course, because this is the battery being drawn on to run the system. Periodically it drops about 0.8v — which is when the pump switches on. The depth stays roughly constant at about 25cm of water in the reservoir, which I’m measuring by using a load sensor. The pump can push water up faster than it returns, so occasionally the pump doesn’t finish it’s cycle before it runs out of water; those are the interrupted cycles. The current in the third panel is the inverse of the bus voltage and depth curves: it’s high when the pump’s running, and otherwise just noise and leakage (this is a little troubling but may just be sensor noise).

Then the charging circuit reminds us how three-stage lead-acid battery charging works. I’m watching it for when it drops back to 13.75v, which is when it hits stage 3. At that point, the battery on the bus could hand off to the newly charged battery, and charge itself.

This cycle doesn’t work the pump hard enough. I’m off until Monday, but I’ve got a couple of days to tune the work-charging cycle to find the max time the pump can run matched to the battery charge time.

Oh, and the second evapotron? Not done, but it’s close. The data logging shield for the Arduino doesn’t work (we’re debugging on the Adafruit forums); the sensor cables aren’t made, and the whole thing isn’t tested. I wish I were substantially farther along, but it’s all going to have to be enough.

(I have a long blog rant coming about how hard it is to measure liquid depth — the eTape failed; the Ping))) failed; and various attempts to test resistance of the water all failed)