Initially I imagined the probe as a sleek device with a wire coming out that one would stick in the soil, if necessary completely. This would mean that the circuit pcb would be an integral part of the PCB that formed the plates, but as always, things go different in practice. For one thing , the piece of PCB I intended to use just wasnt long enough. Second, my PCB etching possibilites are temporarily impeded. Third, I wanted to add an LDR, meaning that I had to have some sort of translucent cover.
I constructed the circuit on a small piece of stripboard:
With regard to the LDR, sure that is nonsense. If I want to add an LDR I could just as well add it to my base station. But I am not adding it because I have to, I am adding it because I can and because I wanted to get some experience in programming an I2C slave. An LDR might not be so usefull but in future I may want another sensor, e.g.a sensor that reads if there is really water flowing from the irrigation tube.
Also I had decided on putting the capacitor plates back to back, but as I did not have double sided PCB I just used two pieces glued and soldered(!) together.
So My BOM was rather simple:
- 2 equal size pieces of PCB Size depends on what you have, but do not make them too small. I used 12×3 cm.
- 1 piece of 0.5-1.0 cm plastic for a baseboard. I used an old cutting board.
- 1 clear/translucent cover, I used the lid of a whipped cream spray can.
- 1 piece of thin 4 wire cable, length depending on your need
I glued the two piecesof pcb back to back. drilled a hole in all 4 corners and through soldered a piece of wire through each hole, thus anchoring the plates together. I removed some copper around the solder so it would become an island isolated from the rest of the plate. (See picture).
Soldering the plates together in the corners may not be necessary if you decide to electrically isolate them from the soil with e.g. shrink tube.
Eventually I will place an NTC on the plate as well after it is covered with Shrink tube. Esthetically it might be better to put the NTC under the shrink tube, but that could create an air pocket.
I made the base plate from a o.5 cm thick piece of soft plastic. Cut out a round shape with a 5.5 cm diameter to fit the base of my clear dome and made a slit 3 x0.3 cm into that in which the PCB fits snug. made a round hole for the connecting cable.
I soldered two wires on the top of the PCB, one on each side. Soldered wires on an NTC, insulated those, attached the NTC to the bottom of the PCB with the wires leading to the top and then covered the PCB with shrink wrap.
Andreas Spiess, discusses Capacitive sensors vs Resistive sensors in a youtube video.