I bought a 'Soil Moisture Sensor' like this https://www.tiagoespinha.net/wp-content/uploads/2014/05/2014-05-09-12.51.20.jpg from ebay, and it turns out it's really nothing of the sort (just a resistance meter, with an analog output and a manually configurable comparitor circuit to give a higher/lower type digital output).
Does anyone know of anything on ebay that packages up a better solution. I have 4 large beds and a 4 shelf plastic greenhouse, and would like to monitor and hence control the watering with a series of networked Pi's, so want a relatively cheap solution that I can connect multiple instances of to one of 5 Pi's, that can give me a reading on which bits of the beds are drying out etc.
I suspect the device I have, is a good start, but would need some kind of ADC converter, ideally that I could connect to a USB bus, or to one of a number of GPIOs, but I don't really want to buy lots of PiFace boards, when all I need is a cheap ADC/resistance solution.
Answer So following research from @Mark Wagoner's suggestion to use a SHT-10 based sensor, I found a couple of things.
- Firstly that I probably want a gypsom block tensiometer, to sense soil moisture (basically 2 probes in a porus ceramic, such as 2xA2 or Galvanised nails in a block of Plaster of Paris), and
- Secondly I need a cheap ADC circuit to read those resistances into the pi.
So in solution, I found the following:-
- A link to suggest how to build the tensiometer: http://www.cheapvegetablegardener.com/how-to-make-cheap-soil-moisture-sensor-2/
- And found this link https://www.raspberrypi.org/forums/viewtopic.php?f=91&t=96135 which suggests a mcp3008 as an inexpensive (<2GBP per IC) low power (2uA) ADC which can connect straight to the Pi's SPI interface, using the PCB design here:- https://123d.circuits.io/circuits/368938-raspberry-pi-analog-to-digital-converter-breakout-for-mcp3008#pcb
Extra I found this http://www.trickle-l.com/new/sensors/ which is a handy list of options for sensing soil moisture