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In modern buildings the majority of sensors and actuators used for controlling temperature, roller shutters or alarm systems are cable wired. The use of wireless sensor nodes in such a scenario would facilitate the installation and would reduce costs significantly. However the main problem is the energy consumption of the nodes. Real time behavior and lifespans of several years seem to be conflictive. In this work we present a wireless sensor node with attached wake-up radio, that consumes as few as 2.78 μA current in sleep mode. The wake-up signal that triggers the node from sleep to active mode consists of an 125 kHz square signal modulated on an 868 MHz carrier. For a selective wake-up of different nodes the 125 kHz signal is additionally modulated with an 16 bit address information. In a sleeping node the incoming wake-up signal is passively demodulated and fed to a separate 125 kHz wake-up receiver IC. The sensitivity of the node was measured to -50dBm which means a wake-up distance of 40 meters at +10dBm output power in an open space scenario. Inside buildings the signal was able to pass through two concrete walls.