Our project started with a requirement to emulate a switch with plastic keys of different colors and nomenclature lighted with low voltage electric bulbs. This switch would then be mounted on a wall, visible from the other side of the room. Our first consideration was a membrane switch with keys lighted in the corner by an LED backlighting solution. However, this did not solve the lighting of the nomenclature, so using it was out of the question. We then considered using a rubber switch with an LED on a PCB membrane switch to solve the problem. The idea was to use clear rubber with painted keys of different color; then we’d over-spray them with black paint. Lastly we’d laser-etch the black to reveal the copy. At first, the copy seemed fine, but upon closer examination a few imperfections in the spray and molded rubber were noticed.
Using this process yielded batches that looked acceptable, however, this process created undesirable variations in the led lighting. In addition, the customer’s existing parts were powered at a higher voltage than was required for our switch so this caused the LED to burn out or shine at a different intensity. To solve this problem, we started replacing the LED on the boards, but not all of them showed the same intensity. At this point we started to wonder whether the root of the problem was with the rubber or the LED. Then, we discovered that the LED had been powered at a higher voltage than expected.
We asked the customer to lower the voltage to 2.5 V – 3V. Once that was established, we tried using some other designs of the rubber with taller fonts; it was better but not good enough. We asked the customer to choose a white pigmented rubber, but the light showed inconsistency through the transparency of the keys because it was too bright as it passed through the rubber. We were almost out of ideas at this point but we felt that we should keep experimenting. Our idea was to put more white pigment in the rubber.
We were willing to try one more time. Our experiment proved that our idea to add more white pigment into the rubber was correct; it ended up being the solution that satisfied the customer. Having the rubber colored cloudier eliminated the unevenness and gave the keys the same brightness and consistency.
Do you have any applications that you’ve been experimenting with to include backlighting? What challenges have you come across, if any? We’d love to hear your feedback and thoughts onbacklighting rubber keypad applications.