Household electrical wiring is used to send an address and a command via a digital signal on 120kHz, during the quiet zero crossings of the 50 or 60Hz AC alternating current waveform. More advanced versions can also query devices to respond with their status, such as "off" or "on" – or even with the temperature or another sensor reading. Devices usually plug into the wall, where a lamp, television, or other household appliance plugs in; however some built-in controllers are also available for wall switches and ceiling fixtures.
The relatively high-frequency signal cannot pass through a transformer or across phases of power, so a repeater is sometimes used with a system. In order to block signals from getting in or out, an attenuator may also be used.