There are several different standards when it comes to indicator lights on membrane switches and printed circuit board assemblies (PCBA). Here are some standard color designators.
- Red: closed/off
- Green: open/on
- Yellow: transitional/starting
The lights can also be flashing, which typically indicates a transition from one state to another.
The placement of indicator lights is also important. On a membrane switch they are usually offset from the key to allow for tactile feel. The tactile feel is the result of using a metal dome under the graphic layer. Of course, the metal does not transmit light, but metal domes featuring an open center for the LED to emit light are available. This is an option, but they are more expensive than standard domes.
If illuminating the entire key is required for indication, light guide film technology (LGF) is the best choice. With LGF the LED emits light horizontally across the plane of the switch, and the light is captured by variable depths that are laser etched in the plane. LGF may be a good option for keypads that require illumination in dark environments.