A coded wireless sensor unit has a slot sized to receive a key card. When a key card is inserted into the slot, the act of insertion generates a energy-harvested power pulse, which powers circuitry that reads electrical device control information coded on the key card and creates a first information packet signal that is transmitted by an on-board transmitter. The signal can be read by a receiver and relayed to a control unit which controls certain electrical devices which consume line power in response to received information packet signals. The control unit decodes received information packet signals and either turns on or activates the electrical devices in accordance with the decoded device control information. Removal of a key card can also be used to generate a power pulse, which creates a second information packet signal that is used to turn off or deactivate the electrical devices.