Switch device

A switch device mounts a radiation source 7 and a radiation sensor 9 so that when the device is placed against a screen 12 transparent to the radiation the presence or absence of a finger F or other operating element at a predetermined location on the other side of the screen determines whether or not the sensor 9 receives sufficient radiation to provide an electrical output. The finger or operating element may itself create the path for the radiation between the source and the sensor directly or it may move a reflective element (Fig 3) to create or destroy such path. Fixed reflective elements on the other side of the screen (Fig 4) may provide a portion of a radiation path which can be interrupted by the finger or operating element; and an array of such path portions may be provided and the associated sensors repetitively scanned so that a symbol traced within the array can be recognised.