Get a sensor that supports lower voltages or get a step-up. Earlier discussions on the same topic: https://forum.mysensors.org/topic/3801/witch-temp-hum-sensor-is-best-to-use-for-battery-powered-sensors/ https://forum.mysensors.org/topic/3203/cheap-good-boost-converter-and-good-temperature-sensors/ https://forum.mysensors.org/topic/2027/test-of-step-up-modules-sparkfun-pololu-china-module-any-other/ https://forum.mysensors.org/topic/444/battery-powered-motion-sensor https://forum.mysensors.org/topic/171/efficiency-of-voltage-boosters

Oha, you are right. The capacitor was the solution. I never thought that a battery-powered notebook could be a bad power source. Sorry for asking before having tried it ... Thank you!! Marcus.

What if my neighbour also discovers this site and starts to build a network? Like @tbowmo indicated you can select another channel. If you are worried about your neighbor controlling your lamps or listening to your messages, have a look at http://forum.mysensors.org/topic/9/security