The Fraunhofer-Institute for Solar Energy Systems has been working in the field of hydrogen technology for nearly ten years. The safe and reliable operation of hydrogen appliances is a very important task and has to be monitored continuously. In the framework of a project a small safety device for hydrogen appliances has been developed. The hydrogen sensitive switch bases on the catalytic conversion of hydrogen resulting in a temperature increase due to the heat release on the catalytic surface. The temperature of the catalyst is compared with a non-catalytic surface. A positive temperature difference indicates the presence of hydrogen in the atmosphere and leads to an alarm signal. Due to the overall design the switch can operate even in ignitable hydrogen/air-mixtures without any risk. As second property the electrical energy demand is very low compared with state of the art sensors.