I've built an NFT type hydroponic garden to grow tomatoes. 25 plants of various varieties.
I'm currently working on the controller which will take into consideration a number of factors when deciding on the frequency/duration of running the pump which wets the roots.
The controller PCB sports a ds1307 RTC, an SHT11 temp and humidity sensor, 4 ADC inputs (CDS for sunlight intensity, pressure sensor for reservoir level, 2 extra for trough moisture sensors)..
There are also 2 relays, one for a 24vac valve for filling and one for the circulation pump, a 2x16 LCD, RF for future upload to network/internet and some Mega32 glue to hold it all together...
Anyhoo, the focus of this post is the 2 extra ADC inputs I have included on the board.
Figuring that while sunlight, time of day, time of month and temperature would be pretty good factors for controlling the pump duration to keep the roots wet, a sensor in the trough where the roots grow would prolly be most ideal in determining when to wet the roots.
So, I have be playing around with the idea of using a cleaning sponge sandwiched between 2 parallel PCB rectangles to form a capacitive moisture sensor.
Somewhat similar to the concept of a stud finder.
The idea is that the sensor will sit in the trough amongst the roots.
When the pump runs, the sensor will saturate and this will be used as a calibration point.
When the pump stops and as the roots absorb the remaining water the sensor will dry out indicating that it's time to run the pump again.
So far I have constructed a prototype that ranges in capacitance from about 0.5nF to 0.8nF (dry & wet).
I have simulated a modified analog capacitance meter, the original sch is from here http://talkingelectronics.com/html/CapMeter.html.
The simulation is giving me about 42% delta on the duty cycle as the capacitance goes from dry thru wet.
I will low pass this pwm and then use the opamp on the controller to full range 0-5v for 10bit resolution.
Attached is the schematic, sensor, controller and garden.
Looking for any suggestions/etc, as I'm just making this up as I go along... lol