Moisture sensor board design  PSoC4 smarter life challenge

Got some work done on the sensor boards today.

Here is the logic diagram:

As you can see the Moisture sensor end of the design is very simple. Just a CMOS 555 timer set up in astable mode. Resistor values of R1 and R2 were chosen experimentally in my last post to result in a 10kHz to 20kHz output from a 5" probe. Other than that just a LED power indicator and some decoupling caps. I am intentionally trying to make the probe as small and cheap as possible. This will be in direct / indirect contact with moist soil and is the most prone to damage / corrosion.

The PCB layout is relatively straight forward. I chose to go with SMT components to keep the PCB size and therefore cost down. Used 0805 desecrates and could have saved a little space with 0603 / 0402 but not enough to make it worth the pain of placing them.

The 8 holes along the bottom are for the probes. I am set up to support either a single probe using the center 4 holes or dual probes using the left and right quads. I added the extra probe set for short pots that cannot take a full length probe. Instead I can use 2 probes at half the length and get the similar response as a single probe. The dual holes on each pad is so I can wrap the copper wire up though the card and back down for mechanical strength. If supporting the probe by other means only one hole would have been plenty.    

As for the input/output connector I decided to go with direct attach wires. Just did not seem to be a good idea to have a open connector that close to the soil and moisture. My current plan is to use standard 3.5mm headphone patch cables. Will cut them in half and solder the shield to ground, middle ring to out, and tip to V+. Some of the bonuses of using headphone cables is they are normally shielded and they are cheap.

Finishing up a parts and raw card order right now. So hope to have the prototypes back next week. Until I get those back I am going to start working on the PSoC4 end of things. First off is getting the software installed and my first hello world (ie blinking led) program working on this platform.