Summary: Everything you need to build a 868 MHz wireless sensor network with collecting data in rrd databases and presenting graphical results. Please visit my blog http://glaskugelsehen.wordpress.com/ to get details and photos of the project.
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A sensor based on TinyTX (http://nathan.chantrell.net/tinytx-wireless-sensor/) sends data on 868 MHz with RFM12B. The sensor in use is a DHT22 and the processor is a ATTiny84. The software of Nathan Chantrell's TinyTX was modified slightly to send also informations on uptime. For more information on the TinyTX visit the above mentioned webpage.
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A board similar to the RFM12Pi board of OpenEnergyMonitor (http://wiki.openenergymonitor.org/index.php?title=RFM12Pi_V2) but with a Arduino mini instead of the ATMega chip. This leaves all other pins of the mini still available.
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RRD database on the Raspberry Pi was installed and the temperature, humidity and batterie voltage of the sensor is filled in
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Graphical presentation off rrd data The live data and historical data is presented with a apache/php webserver running on the RaspberryPi. The graphics are dynamically generated to prevent too many write cycles to sd flash.