An illuminated keyboard includes optical sensors and units each comprising an optically transmissive key member; a scissors-shaped structure having a top secured to the key member; a light guide diaphragm under the scissors-shaped structure; an insulating diaphragm under the light guide diaphragm and including a light source on a top surface, a positive conductor connected to a positive terminal of the light source and on the top surface, a negative conductor connected to a negative terminal of the light source and on a bottom surface, and a bus having one end interconnecting the positive and negative conductors, and the other end connected to the sensor; a film circuit board under the insulating diaphragm and including a holed rubber dome with the key member positioned therein; and a base board with the scissors-shaped structure retained thereon. Brightness is adjusted in response to light condition as sensed by the sensor.