PICAXE-18X & Philips EE: Simple Weather Station

This application shows the development results for a simple PICAXE-18X weather station, using simple passive components for light (LDR) and temperature (NTC) recordings.

Introduction

This was our (me and Joost) first project, after having tried the basic PICAXE commands individually. The set-up was kept extremely simple, using the PICAXE's EEPROM for data storage (motivated by P.H. Anderson's simple 250 byte datalogger). The measurements were performed over two-and-a-half days, with light and temperature measurements taken each half hour. The results clearly show the cooling of the air during the night, revealing that the coolest moment actually occurs approximately one hour after sunset instead of around midnight.

Hardware & Software Methods

The hardware setup uses two inputs connected with voltage dividers containing a LDR and a NTC respectively. Another input is used to read a switch (read low when pressed, high otherwise). The program itself is very straightforward; about two seconds after power-up the switch input is read to see whether new light and temperature data has to be logged (switch pressed) in the PICAXE's eeprom, or previously logged data has to be dumped from the eeprom to the PC using the SERTXD command (switch left open, so unwanted deletion of previously recorded data cannot occur). Some LEDS reflect the program status. If either recording or dumping has finished, the program ends and the PICAXE is put into low-power mode. As the hardware was configured such that high light and temperature conditions resulted in low voltages, the values read were immediately negated before being stored so that high values would correspond to high light and temperature conditions. The Philips EE system is used for prototyping

Code:

• weather.bas
• weather_nl.bas (original Dutch version)

Results

The graph below shows the light and temperature measurements for about two days, with a recording made each half hour. The plot uses separate axes and scaling for the light and temperatures for clarity.

The graph (showing light intensity in pink, and outside temperature level in blue) shows the following details:

• Minimum temperatures occurs approximately one hour after sunrise, not somewhere around or just after midnight
• The fact that the sensors were not shielded or put in an ideal position showes up as fluctuations in data; the temperature sensor heated up quickly in the sun, but sometimes became shadowed by a tree, and the light sensor was put behind the window in a room that was frequently visited in the evenings, with lights put on and off several times. In realistic use, precautions should be taken for proper sensor functioning.
• The LDR is most sensitive in low-light conditions (i.e. largest variation in resistance value for a given change in light intensity), which explains the steep curves at sunrise and sunset.

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