A system having first and second different communication systems can include a plurality of illumination devices having modulatable optical output signals. A plurality of building control units are in wireless communication with one another. Representative units could include ambient condition detectors, intrusion detectors, output devices, or actuators. At least some of the units include optical sensors responsive to the modulatable optical output signals, wherein, in response to received, modulated optical output signals, a respective control unit carries out a predetermined function.