The Weather Station 2007-11-16 to 2015-07-31


From 2003-02-13 to 2007-11-16, the external thermometer sensor sat on the outside wall, in a white-painted block of wood. On 2007-11-16, the external, free-standing, "tower", weather station began operating with its first version of electronics. This is a 2 meter tall steel structure carrying a ventilated box, where the air temperature, humidity, light, and rainfall sensors are located. On the ground below the tower is the ground temperature sensor.

The first version, operating from 2007-11-16 to 2008-10-31, used LM35 sensors for the air and ground temperatures, and analog signals on several coaxial cables to the receiving instrument with a multiplexer and ADC0804. This had limitations on cable lengths.

Since 2008-10-31, the temperature sensors have been MCP9801 integrated circuits, connected to the main unit via a long-haul i2c bus. The light and humidity sensors analog-to-digital conversion takes place in electronics on the tower, the ADCs are in a PCF8591, and the rainfall counter is a 4040, read by a PCF8574. The humidity sensor remains the same Honeywell HIH-400(?) as has been throughout.


The temperature errors are attempted minimized by the free-standing design. Heating by the sun on bright days is still evident; the daily curves change depending on cloudiness. The analog electronics in both versions use 1% resistors and low power circuits so as to avoid self-heating. Accuracy of temperature is now limited to what the MCP9081s can provide, accuracy of rainfall is limited by the tipping-spoon design, which tends to under-report rainfall of partial mms -- the water evaporates before it becomes recorded.

The old wall-mounted units would be heated by the sun via the wall, and show too high temperatures. It may be expected that the readings from 2003-2007 are higher; the ones since 2007 should be more accurate.

Tower Electronics

This is mounted in a box underneath the ventilated box, and two ducts carry cables for the sensors into the box. Inside are the rectifier, voltage regulator, PCF8591 A-D converter, a 4040 counter/PCF8574 combination and op-amps and resistors. There are two marine-style connectors on the back, one is 2-way for the input power, the other is a 4-way one with the i2c bus. A 82B715 is used here for the long-haul connection from the instrument.


This unit contains the 8V transformer, the Picotux and associated automated start-up circuit, and the 82B715 i2c long-haul buffer circuit for the cable connecting to the tower.

Weather Station Instrument at location

Inside the case are the voltage regulators, the Picotux and associated circuits used to make the system self-starting.

Weather Station Instrument inside view Details.

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