| 摘要 |
PURPOSE:To prevent mis-operation of a key by providing a sensor whose output changes,with a lightness of the environment to the telephone set and using the sensor so as to sense the surrounding lightness thereby controlling the lighting of a back-light. CONSTITUTION:When a power switch of a portable telephone set is turned on, a back-light 3 is lighted. Then the telephone set reaches an input wait state from a key pad 1 and the back-light 3 is turned off when no key entry comes within a preset time. When key entry is given, a sensor 4 discriminates whether the surrounding environment is light or dark and the back-light 3 is lighted when key entry comes while the environment is dark. When the back-light 3 is lighted or the environment is light and when key entry comes, the information of the entered key and its reply or the like is displayed on a display means 2 according to the key entry and each processing is executed. Thus, mis- operation caused by depression of a wrong key due to insufficient lighting on a key to be depressed under the dark environment is prevented. |
