Holographic 7-Segment Clock

This is the real deal! It is not based on POV, cheoptics, Pepper's ghost or similar pseudoholographic methods. This 7-segment display uses real transmission holograms based on holographic plates from Litiholo. The segments appear to float in midair about 1cm behind the holographic plates.

After surfacing from my deep dive into scratch holograms I wanted to move on to real hologram making. Thanks to a colleague from work who is a major hologram fan I ordered the hologram kit by Litiholo and started experimenting. The goal was always to make a clock but at the beginning I was not sure at all how to create changing numbers. The final design is a 7-segment display where each segment is a transmission hologram illumated from the back by a white LED from different angles.

Supplies

1. Litiholo Hologram Kit
2. white self-adhesive lampshade foil
3. 30pcs - 3mm white LEDs
4. Adafruit ESP32 Feather V2
5. Adafruit 4-Digit 7-Segment LED Matrix Display FeatherWing
6. custom PCBs (see below)
7. lots of "Dupont" cables
8. M2 screws
9. black matte PLA filament

Tools

1. 3D printer
2. soldering iron
3. vinyl cutter (optional but recommended)

It is always good to know some basics, like

1. What is a hologram?
2. What is the difference between transmission and reflection holograms?
3. What are rainbow holograms?

One of the best explanation of holograms I came across is this video by 3blue1brown. I can also recommend this university course summary and the Holographyforum and Holowiki websites.

I will not go into further details here, just check out the links above.

Before making custom 7-segments holograms I recommend to make the test hologram that comes with the Litiholo Kit. This ensures that your setup is working properly and what to pay attention to when making holograms, like avoiding vibrations or having a stable laser that does not drift.

Using the purchased kit I got a nice transmission hologram of the included toy car on my first try. This is kind of surprising taking into account that the kit comes with a cheap diode laser that uses just a metal spring as heatsink. Also the mounts for the laser and hologram plates are rather flimsy and just made from lasercut acrylic. This shows you that you do not necessarily need an expensive single frequency laser, professional optics mounts and an vibration isolated optical table to make holograms.

In order to make my holograms I replaced the laser from the Litiholo kit with a special red single frequency diode laser that I luckily had access to because of my work. I mounted my laser diode onto a small aluminum heatsink and attached a constant current LED driver board hooked up to my lab power supply. Nevertheless everything should work the same with the laser included in the kit.

I also designed my own holographic setup that uses a vertical configuration where the holographic film is illuminated from the bottom at an 45° angle. Below the holoplate a glass plate with the lampshade foil is placed. The back-illuminated foil is the object of which the hologram is recorded.

Each segment is recorded separately in a different region on the same holographic film and an aperture is used to block the light in other regions of the film. Between the recording of each segment the object plate and holoplate are rotated in plane so that later each segment can be illuminated from a different angle.

The whole setup and also the mount for my diode laser was 3D printed from matte black PLA. A segment-shaped piece of lamshade foil was made using may vinyl cutter and then attached to a glass plate. The latter was just a holoplate from the kit where I removed the holographic film.

You can find the stl and dxf files for the hologram setup on my github.

The schematic above shows in which order the segments are recorded and how the holographic and object plates have to be oriented. The holographic film should face downwards towards the light source. The illumation was done in complete darkness. For alignment before illumnation I used a blue LED (the holographic film is less sensitive in the blue) which was places in the corner of the room facing the wall. I recorded my holograms usually after midnight in order to avoid disturbing vibrations from people running in the house or cars on nearby street. Each illumation step took 5 minutes i.e. prepare for about 40 minutes of quiet but concentrated work. Here are the steps in detail

1. Turn on the laser and let it run for at least 5 minutes to avoid thermal drift
2. Align the optical setup without the holographic film
3. Place a piece of black cardboard in front of the laser to block the light
4. Turn off all lights except for the dim blue LED
5. Take the holographic plate out of the packaging and place it in the right orientation
6. Turn off the blue LED
7. Remove the cardboard bloking the laser beam in a single swift motion
8. Sit very still for 5 minutes
9. Block the laser beam with cardboard and turn on the blue LED
10. Rotate the holographic and object plate and repeat steps 6.-10. until finished

In total I recorded four identical 7-segment holographic plates to build the clock

The clock was designed in Fusion360 and also printed from matte black PLA. It consist of a base plate and holder plates for the holographic plates and the LEDs. Everything is separated into two halfs which each contain two digits. The base plates are then joined together with a dove tail.

I later also added a holder for two LEDs that make up the colon. This part was printed from black and transparent PLA changing the filament during printing.

All stl files can be found on my github.

Each segment is illuminated by a single LED, i.e. 28 LEDs in total. To avoid a wiring mess, I designed a custom PCB where the LEDs can be soldered two. Each PCB contains 7 LEDs for a single digit and the PCBs are designed so that two neighbouring digits can be interconnected without wires. On the PCB the LEDs are multiplexed with a common cathode configuration using the same wiring as in the commercially available 4-digit 7-segment displays.

The PCBs on the picture have a design mistake so that I neede to cut a hole into some of them but the files on my github have already been corrected.

The brain of the clock is an Adafruit feather ESP32 based board. The ESP32 allows to build an NTP based clock that fetches the time from the internet. For this board a 4-digit 7-segment LED backpack is available which is based on the HT16K33 matrix driver chip. Using these two components there is no need for another custom PCB to connect all the parts.

I solder female and male pin headers onto the MCU and the backpack, repectively, in order to connect them. On the backpack I added female headers for the connection of the 7-segment display.

The PCBs are interconnected by a row of pin headers which have to be soldered first. After that I removed the plastic part of the pin headers which later allows to place the other PCB directly on top without space in between.

Then, I placed all LEDs into the 3D printed holder taking care of the correct orientation regarding polarity. After that I attached the custom PCB on the back and soldered all LEDs in place. The PCB can be attached to the back plate using M2 screws. Finally, I repeated the same for the second digit and connected both PCBs by soldering the pin headers.

This step needs to be repeated for the other two digits. Later both halfs are interconnected by female-female Dupont cables.

I also inserted two LEDs into the holder for the colon and soldered both anodes and both cathodes together. Male Dupont cables were then also soldered to the anode and cathode.

At first both base plates can be snapped together using the dovetail machnism. The holder plates for the holograpic plates and the LEDs with the PCBs attached are then simply inserted into the baseplate.

The Adafruit Feather Board with the LED backpack attached is placed into the holder on the base plate and the 3D printed cover is placed on top and attached with M2 screws.

The 3D printed colon with the LEDs can be snapped in the center between both LED holder plates.

In order to figure out which LED needs to be connected to which pin of the LED backpack you can refer to the schematic above copied from datasheet of the 4-digit 7-segment displays. Note that there is a mistake in the shown pin position as the module has 14 and not just 12 pins.

For connecting the PCBs of both halfs I used female-female Dupont cables. Each LED was connected to the driver board using a male-female Dupont cable.

In the end you still end up with a bit of a wiring mess but I tried to keep everything as organized as possbible by attaching the cables to the PCBs with small cable ties.

The code was written with the Arduino IDE and uses the WifiManager and NTP client libraries as well as the Adafruit LEDBackpack library.

In order to adapt the code to your timezone you have to enter the GMT offset of your location. After uploading the clock opens a WifiAP where you can enter your Wifi credentails. I then restarts connects to your Wifi and fetches the time from an NTP server. There is also a demo mode where it just acts as a counter to test if the digits are working.

Again this can be found on my github.

I am happy with how this project turned out but as always there is room for improvement.

The segments appear much more bright and crisp if you use red LEDs instead of white ones. I still decied for white LEDs because of the rainbow coloring that is commonly associated with holograms.

As acan be seen in the pictures, in particular the vertical segments are a bit tilted/distorted. This is because the position and divergence of the LEDs does not coincide with that of the original laser during recording.

In addition due to aperture that was used to limit the illumination area the viewing angle of each segment is limited.

I am currently working on a transmission hologram where I would like to have the segments floating in front of the holoplates instead of behind. This is possbile if use an additional lens that collimates or even focuses the beam during recording of the hologram, so stay tuned.