Rain sensor



  • It is fall season, time to move inside and build sensors. So far a wind/temp/humidity and a rain gauge have been added to the collection and this post describes the latter.

    This first edition is based on the Misol type and I decided to write a simple piece of code to support that. Because it had to be a battery powered node which sleeps most of the time the natural choice was to base the code around the MySensors sleep function. The combined functionality of event triggered and time triggered interrupt is ideal for the purpose.

    To simplify things I am only interested in the daily amount of rain why I reset to zero mm rain at midnight. I prefer to keep the code as simple as possible at the sensor level because other values are much easier to calculate at the controller level, at least when using OpenHab and its rule engine. I have no idea about the capabilities of other controllers.

    Unfortunately I also wanted to receive messages from the controller. Those are for remaining runtime to midnight as well as being able to send zero or last known reading at any start/restart. This of course is a complication because of a mainly sleeping sensor.

    So the sensor has to function without the normal timer functions. The first approach is instead to use the wake up events from sleep as a timer and keep track of time for remaining cycles to midnight. Also at any sending of amount of rain to the controller there is a short wait to enable incoming messages before going to sleep again.

    The challenge (at least for me :-)) was to establish when midnight occurs without using any standard timer functions and without asking the controller at every end of the cycle. Because of how the rrd4j persistance works in Openhab the cycle time need to be max 60 seconds so there are quite a few cycles each day.

    After several versions the approach became kind of an iteration where the sensor checks the remaining time at half the estimated runtime, then asks again at half of the newly found remaining time and so on. Most likely there are much more elegant ways to accomplish this but it is working.

    The sensor itself is "homemade" based on the 328P-PU which runs at 1 Mhz. The only hardware complication was that I found the signal from the Miso unit to be very bouncing why I added an inverted Schmitt trigger to get a nice signal, simply because I had a few lying around and the use of capacitors only still gave a peculiar reading.

    The sensor box is a standard "waterproof" IP65 junction box and the mounting console was made with my 3D printer. A few pictures can be found below.

    One peculiarity with this sensor was that when using the MySensors library v2 it runs fine at 1MHz but, with everything else identical, modifying the chip to 8 MHz causes the radio connect process to fail. When going back to 1 MHz all runs well again. I have not digged any further into why this is happening.

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    PS. I have also printed the stl's designed by BulldogLowell as well (@BulldogLowell - Thank's a lot for sharing) but not yet finished that assembly. There are a few modifications I wish to make and not yet had time to finish but this is an issue for another thread.


  • Hardware Contributor

    @mbj - Great work and thanks for sharing!
    What is the IC beside the atmega? Ftdi?



  • @sundberg84 Thanks. The item is an inverted Schmitt trigger used to get a clean signal. What came from the reed relay was awfully bouncy so readings were not consistent.


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