A card edge connector assembly comprises an elongate housing mounted on a mother board, and a switch key for engaging with the housing and activating inner circuitry formed in the mother board. The housing comprises an elongate slot formed between opposite side walls thereof for receiving a daughter board, a plurality of receiving passageways defined in each side wall for retaining contacts therein, a pair of cutouts transversely defined in a mating face of the housing, and a projection transversely formed between the side walls. The daughter board defines a pair of notches corresponding to one of the cutouts and the projection of the card edge connector. The switch key can transmit information to the mother board by electrical contact between a conductive membrane disposed on the switch key and the corresponding contacts received in the passageways below the corresponding cutout of the housing. Thus, a daughter board having a certain operational voltage can be properly inserted into the slot of the card edge connector. The switch key may also transmit information to the mother board by separating an engagement between the contacts received in the passageways below the corresponding cutout of the housing.