A modular junction box receptacle includes parallel high and low voltage circuitry portions separated by a barrier. A base housing of the receptacle receives a termination block that includes high and low voltage connections to respective high and low voltage portions of the base housing. A housing cover is installed over the base housing and termination block; the cover has an opening for passage thereinto of a face portion of the block. The receptacle accommodates a plug and play unit containing smart circuitry, the unit including high and low voltage circuitry connections for respective high and low voltage portions of the termination block. The unit can act as a variety of interchangeable devices in a smart system that incorporates pluralities of receptacles, including electrical outlets and simple electrical toggle switches, each containing low voltage connections. A plurality of such units may be incorporated in, for example, a household circuitry system.