Optical Keyboard Gets Ruggedized for Harsh Environments
Ben Koning’s keyboard uses several layers of different materials to keep it sealed and a reflected IR beam to determine finger placement.
Keyboards are still widely used as the nominal input device for typing and can be found in one form or another on phones, desktops, laptops, and other mobile devices. While most use physical keys or touchscreens, others take advantage of light, making them suitable for underwater and harsh environments. Ben Koning’s optical keyboard can do just that and can function in areas where hermetic sealing is a must. The board utilizes the reflection of an infrared beam that can detect finger placement and tap inputs to denote which key has been actuated.
Koning designed his optical keyboard using several layers of material, with the first being an outside medium such as glass or plastic depending on the environment. Layer two is a transparent acrylic light pipe layer with keycap legend etchings illuminated using white LEDs. Layer three is an IR passing but visually black filter, allowing users to see the keys in full sunlight and total darkness. Layer four is a translucent filter that prevents false inputs by diffusing incoming and outgoing IR beams unless a finger is in close contact. Layer five is a hard plastic 4mm shadow-mask layer that guides the beams up and down only, preventing lateral crosstalk between LED and photosensor at each key site at measurement time. Layer six is the board’s PCB, which hosts all LEDs, photosensors, and other necessary hardware for the board to function.
“Each key site has an infrared LED and an infrared phototransistor (PT). Both LED, and PT are capable of switching on/off in the dozens of nanosecond range. The LED is a high-intensity type with 940nm output, and a 15-degree beam angle (Vishay VSMB294008G) and the PT is closely matched to the LED and has a 24-degree sense angle (ON Semiconductor QSB363GR),” Koning states in his project blog. “The PT is connected via a low-loss MUX switch (Texas Instruments TMUX1108) to an op-amp used as a voltage follower buffer (Texas Instruments OPA350) and then on to a 10-bit A/D converter inside the MCU (Microchip PIC16F18855).”
Koning has uploaded a detailed walkthrough of his optical keyboard on his Instructable page for anyone who would like to recreate his build.