Power is delivered from a power source in a head-mounted device to a separate accessory that is coupled to the head-mounted device. Power and signals are delivered on a common conductor that couples the head-mounted device to an accessory. One or both of the head-mounted device and the accessory are...http://www.google.de/patents/US8031878?utm_source=gb-gplus-sharePatent US8031878 - Electronic interfacing with a head-mounted device
