"Invisible” Clock Features a Holographic Dot Matrix Display

Light refraction is an important mechanic behind a variety of technologies. For the same reason that things underwater seem to be in the wrong place from a perspective above the surface, we can produce technology like pseudo-holographic displays. As light enters a medium, it changes speed based on the density of the material. That, combined with the angle at which the light enters the medium, determines how much the light bends and that is refraction. Shiura’s HUD-like “invisible” clock relies on refraction to show the time on a holographic dot matrix display.

This isn’t a true holographic display —such a thing isn’t even possible right now if we’re talking about the types of displays people imagine when they hear the word “holographic.” But it is close enough to look cool and provide some wow factor. The actual LED matrix is hidden out of few, but the light from its LEDs refracts in a sheet of transparent acrylic and bends towards the viewer. This results in readable digits that look as if they’re hovering in the air (or at least inside of the acrylic). It is a neat effect for a cool clock and is surprisingly easy to accomplish.

To replicate this project, you’ll need an ESP32 development board and four 8x8 LED dot matrix display panels that use MAX7219 drivers. The minimalist base enclosure is 3D-printable and the acrylic refraction sheet is easy to obtain from a variety of hardware and craft stores. You’ll want the base to reflect as little light as possible in order to maximize the effect. So you should either choose a very matte black filament or coat the base in something like Black 3.0 paint, which absorbs 99% of visible light.

The provided code is flashable through the Arduino IDE and uses the ESP32’s built-in WiFi adapter to connect to the internet and pull the current time. It then simply displays the time across the dot matrix panels. Their LEDs refract through the acrylic sheet at the proper angle to become visible.