Now its time to look at the rain gauge and wind speed sensors.
As far as I can tell, these are just single-bit inputs on the GPIO connector. I don’t know which pins and before I wade through the supplied Python, I’m going to use the gpio program with the watch program and see which input bits change when I move them…
watch -n1 -d gpio readall
is what I’m running …
and nothing happened. Suspecting that the inputs need the pull-ups activating (mostly because there is nothing obvious on the PCB), I turned them on for (wiringPi) pins 0 through 6 (7 is used by the 1-wire system)
for i in `seq 0 6`; do gpio mode $i up ; done
and tried again. Success. I found that the wind speed sensor is on pin 0 and the rain gauge is on pin 2. I also checked this by taking the board out and looking… Maybe I should have done that first… (Update to note that the production board inputs are on different pins)
The rain gauge is a clever little device – basically 2 buckets on a see-saw mechanism. When one bucket fills, it tips the unit over, emptying it and sending a pulse down the wire. The other bucket then fills, it tips out and sends another pulse. The pulses are generated by a reed switch and a magnet. The reed switch is normally open and closes momentarily as the see saw passes the mid-point. The Pi foundation have a good write-up on it here. With the internal pull-up enabled, the input normally reads high, but as the device tilts over there is a pulse low then high again.
Note: Going through the Pi Foundation pages now on the rain gauge and wind speed sensor I find that they are using different pins on the production boards, so I’ll make a note of that in the code I write.
According to the manual, Each little bucket holds the equivalent of 0.011 inches of rain or 0.2794mm. We’ll stick to metric units for our code here as no-one uses imperial any more.
So each tip of the see saw gives us another 0.279mm of rain. This will be easy to code in a little interrupt driven function using a falling-edge trigger. (and maybe some debounce – need to check on this)
The wind speed thingy is the usual rotating 3-cups device. There is one magnet and one reed switch, but the way it’s positioned you get 2 pulses per rotation. The pulses are high to low transitions. The Pi Foundation have the calculations on their page about the anemometer, so I’ll probably just use those than working it out myself (lazy programmer principle). Like the rain gauge I’ll use an interrupt driven function to record this.
So I think that’s that with the sensors. I’ve written some new wiringPi drivers for the devices on the weather station – I’ve not really gone into detail about the C code for these as I don’t consider it quite entry-level C. This was more to do with jotting down the discovery process – a little more than I’d log in a Black & Red, but the same sort of thing.
The next thing to consider is putting it all together.