Don't be so wet! ...

.... or so dry!

It's been six months since my last post, a long time but I've been busy.

Continuing my exploration of all things electronic, I've been happily making stuff, making mistakes, making a fool out of myself by asking stupid questions of anyone and everyone, but also learning a lot. I've compounded all the practical work with a couple of free online courses too - both from Coursera, I've stumbled successfully through Linear Circuits and also Introduction to Electronics - not too advanced, but useful nonetheless and certainly as much as I could manage with a full time job!

So, what's with the "wet and dry" theme?

How to explore your own hobby while keeping your "significant other" happy!

My wife is a keen gardener. That's keen with a capital K, and gardener with a capital G. So we have an active and ever evolving garden, plus more houseplants than you can shake a stick at.

So, for my next project, I cunningly suggested that I might make her something that would allow her to monitor the state of her houseplants from the comfort of the sofa - that is to say, a wireless plant moisture monitoring system! No, I didn't mention that you can buy such things for very little money and with much more hope of success, but I got the green light to start building things, which was the main result I wanted ... hehe.

Wireless wetness

If you Google or eBay for nRF24l01p, you'll find loads and loads of people who will sell you these little units very cheaply:

What it is, is a 2.4GHz wireless transceiver that you can talk to with Arduino, Pi, PIC, whatever and generally get simple wireless capability into your projects very easily. So I bought a handful.

They run off 3.3V so rather than using my normal preference of 5V PICs, I chose to use one of the PIC16LF range of low power microcontrollers, in this case the PIC16LF1554, which has much more available than I needed for this project (I just need SPI and ADC really) but it did allow me to test and prototype with a UART talking to my PC via a TTL to USB adapter.

The grand plan

So the grand plan was to have a number of moisture monitoring sensors, each one stuck into a favoured houseplant pot, and each periodically measuring the soil moisture level and pinging off a packet of data to a central monitoring station, which would collect and collate the various wetness reports and allow them to be presented in a suitable manner.

I planned to use simple resistive moisture sensors because a) they are cheap, and b) they are cheap. So a few quid on eBay and a long wait for China Post and I harvested a dozen of these:

To read the moisture level, you simply wire this as part of a voltage divider that you sample through the microcontroller's ADC (analog to digital) pin to get a resistance measurement that you blindly assume is both accurate and varies in a linear relationship with the moisture level. Neither of these are true, but who cares - that's not the point! I did pay homage to some advice though that warned against always sampling with the same polarity - risk of gradual electroplating of one prong from the other - so I always take two ADC readings, one with polarity reversed, to even up the battle a little.

To make things even more interesting (complicated), I also wanted a way to reconfigure the sensor units without having to reprogram their PICs, so I used a simple protocol that allows me to remotely adjust the frequency of wetness sampling for each individual sensor. It goes something like this:

• Sensor wakes from sleep mode.
• Sensor takes two ADC readings and creates a packet of data.
• Sensor sends data to the master unit.
• Sensor waits a reasonable number of milliseconds for a reply.
• If the reply arrives and requests reconfiguration, the sensor adjusts its sleeping time.
• Sensor goes back to sleep.

By using the PIC's watchdog timer to wake it from sleep mode, along with some suitable preset combinations of prescaler and counter, I could manage a sampling period of anything between 15 seconds and 16 hours. The former useful for testing and the latter 8-hour or 16-hour period being used in practice.

The sensor

The sensor circuit went through two revisions. Mainly because I was stupid. Ok, no "mainly", but only because I was stupid!

But then you don't learn anything without making mistakes, so stupidity is a good thing ;-)

So the first version of the sensor PCB looked like this:

The first stupid mistake is the battery on the left. I chose the wrong part in Eagle, so instead of having a battery holder, I had a fixed battery with welded solder tags that you can't remove from the board without desoldering. So obviously, you also can't switch it off. Hmmm .. too keen to get the boards ordered, and I don't mind admitting it. Lesson learnt. Probably.

The second stupid mistake was that I forgot to add the recommended 10uF capacitor across the power terminals of the RF unit. In practice it didn't seem to matter, but as I was going to redesign anyway, the second version attempted to fix these issues. The final schematic for the sensor was like so:

At the top left is the 4x2 header to receive the nRF24l01+ unit, then under that is the voltage divider circuit to the moisture sensor (JP1), and apart from power and bypass cap, that's it. So it now looks like this:

The battery can be removed or replaced as required and so I now have a nice stock of the other, fixed tag, batteries because I found I could buy 80 of them for ~£5 on Amazon. Ho hum.

Anyway, I made six sensor boards, each PIC pre-programmed with a unique identifier that gets put in the data packet when sending to the master. So the master correlates the sensor ID with its - also pre-programmed - list of houseplant names, ready for display. Hooray - nearly done.

Mastering the master

The master unit is actually the more complicated part of the system. Not because it has any inherent complexity, but really because it's difficult to fit all of the code into the 4096 words of flash memory and 256 bytes of RAM. For anyone familiar with MPLAB X, how many times have you seen your resources so completely used as this?

This is because we need the code for talking to the RF unit, the code for talking to the OLED, the code for interacting with the 23LCV1024 SRAM, various text strings, and also font mappings. Quite a lot to fit in!

Anyway, here's the schematic:

I've put a 3.3V LDO there because I'm powering it from a 5V wall-wart even though it's 3.3V throughout - it's much easier to find spare 5V supplies in the rummage boxes. So, nothing too difficult to see there - the external RAM has a fixed battery (I knew they would come in useful!) to keep its content safe during power off and I've also added a jumper in there to clear the RAM if I want to start again. In the code, it first reads the RAM looking for a magic number to see if it was previously initialised or not. If not, it writes six pages of empty data to the SRAM with the plant names coded from a 5x7 font. So the display is really just mapped to the SRAM and the PIC only needs to write the bars for the incoming data and read back a single page at a time to display.

The populated board looks like so:

Power comes in top left, the PIC is bottom left and the SRAM bottom right. The momentary push switch at top right allows one to cycle through the six houseplants to see the current moisture level and a bar chart history over the last 128 sample periods (only 128 because the OLED is 128 pixels wide).

Seeing is believing ...

So, finally (yes I know I'm rushing), I've made a short video of the master unit in action so you can see the principle. As usual, I'll link to the code downloads at the bottom ...

One thing that may be interesting is what happens to the soil moisture sensor after a few months use. This is what my first sensor looks like now, after something like five months in an indoor plant pot (over-watered, as it happens!):

Source code downloads

As promised!

Plant monitor sensor
Plant monitor master