RPi weather station: the construction

The original material used by Alecto was plastic and light-weight aluminium tubing of 20 mm diameter. Because I needed to mount both a wind velocity meter and a wind direction sensor, I needed some more material and I decided for the robust zinc-coated iron tubing as used for central heating systems. These come in 22 and 15 mm diameter versions with actually quite elegantly looking junctions. I chose for the main pole of the thicker diameter tubing with a parallel side branch of the thinner tubing. With this construction, I could also mount the rain gauge and the Stevenson screen which held the rest of the measurement system: temperature, pressure, humidity and solar radiation.

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The rain gauge is of the self-emptying tipping bucket type. Here the bucket is assumed to be measuring 0.3 mm rain per bucket. Every time a bucket is full, it empties  and causes one momentary reed contact closure which can be recorded by subsequent equipment. The Alecto way of mounting the rain gauge is by a single plastic bar. That makes it extremely sensitive to vibrations, it almost does so by itself!  Vibrations would make the inside balance wiggle and cause erroneous contact closures; a known problem of these devices. Therefore, two extra bars are attached to the central rod as shown below.

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Furthermore, the rain gauge is mounted relatively low so as to further prevent vibrations due to the wind. The connection to the control box is done by an RJ-11 terminated cable. The reed switch is connected to the two center conductors of the RJ11 plug, see the table below.

The anemometer and wind vane measure wind speed and direction respectively. These are mounted on top of the side and central  rod. The anemometer is the one that comes from the Alecto kit and the wind vane was obtained through Ebay, advertized as spare part number WH-SP-WD for a Misol weather station. The vane is internally moving a little magnet that actives one or two reed relays thus selecting the resistance encoding the wind direction. The connection to the control box is done by an RJ-11 terminated cable. The reed switch is connected to the outer center conductors of the RJ11 plug, see the table below. The wind vane housing also has a socket to receive the anemometer signal and passes it on to the two center conductors of the plug. The cup type anemometer measures the wind speed by closing a contact as a magnet moves past a read switch. A wind speed of 2.4 km/h causes the switch to close once per second.

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The remainder of the sensors are placed in the originial electronics box inside the Stevenson screen. The humidity sensor and the barometer – both equipped with a temperature sensor – are placed on a printed circuit board inside the box. The original RJ11-sockets are now used for the I2C interface.  One to be connected to the control box and one to the solar radiation sensor. The latter is mounted below the top cover that has a tiny lens allowing sunlight to fall on top of the sensor. The stevenson screen is oriented to the South so that no part of the construction interferes with the incoming sun light.

From the various weather station parts there are three RJ11 terminated cables. The advantage of these connectors is their low price and high versatility. The disadvantage obviously is the potential interchange of cables.

RJ11 connections

Stimulated by the existing definition of RJ11 connections as used for many weather stations, the interface wiring was selected such that no interference is to be expected when connectors are accidentally misplaced.

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