A hand held instrument for carrying out some function on a work piece. The instrument is a handpiece that is held by the user and includes a working element at the distal end of the handpiece for working on the work piece. There is a source of power that activates the working element by providing power thereto in various forms. There is also a pressure sensor that senses the pressure between the working element and the handpiece. In one embodiment that pressure sensor senses a predetermined pressure in order to couple the power source to the working element for activating the working element. In another embodiment, the pressure sensor senses a reduction in pressure between the working element and the handpiece to illuminate an indicator light.