A sensor guide wire assembly having a button-cell type battery includes a transmitter unit configured to be connected to a sensor guide wire that is insertable into a patient to measure a parameter, the transmitter unit being configured to wirelessly transfer data representing the measured parameter to a communication unit. The transmitter unit includes a housing, a printed circuit board holder disposed in the housing, and a printed circuit board disposed in the housing and held by the printed circuit board holder. The printed circuit board includes a plurality of electrical contacts. A slider is slidable from a location outside the housing, the slider including a protrusion extending into an area inside the housing. A space within the housing is configured to hold one or more batteries. The slider is slidable from a first position, at which a micro-switch is not activated, to a second position, at which the micro-switch is activated by the protrusion of the slider, which allows power to be provided to the printed circuit board.