With “Retro” and “Industrial” being the trend when it comes to interior design for quite some time already I loved the looks of the Karlsson Cathode Clock. I first saw this clock in 2017 in a small design shop in Rotterdam here in the Netherlands. Influenced by Nixie tubes its simplicity and design stuck with me for a while and kept floating in-and-out of my mind for a couple of years. In theory the inner workings of such a clock aren’t rocket science and I should be able to design one for myself.

Eventually I took my own design inside the woodworking-workshop, glued some pinewood together and hand soldered the PCB into the first version of my very own LED Cathode Clock which I gifted to my good friend.

Even though my modelling skills suck, I’ve used https://app.sketchup.com/ to create a 3D drawing in order to get a feeling for the dimensions of the clock and what distances (e.g. between the glass domes) are visually the most appealing. I added the drawings to this post, you can find them at the bottom.

I am still in the process of refining the design, certainly when it comes to the Arduino code (which I used as the brain for this clock). But in case you would like to try it out for yourselves, here is what you need:

Materials

Hardware

• 2x front and back, pinewood (262mm * 80mm * 6mm)
• 2x sides, pinewood (90mm * 74mm * 6mm)
• 1x top, pinewood (262mm * 90mm * 6mm)
• 1x bottom, pinewood (250mm * 90mm * 6mm)
• 1x inside beam, to raise the domes (250mm * 50mm * 50mm)
• 4x inside PVC (plumbers) pipe, to raise LEDs above the top (40mm diameter)
• Glass domes (50mm diameter), you can get these at Groene Vingers in Delft

Electronics

• 4x 74HC595 shift registers (see also https://www.arduino.cc/en/Tutorial/Foundations/ShiftOut)
• 4x 7-segment cathode displays (large ones (4,5cm/1.8inch))
• Arduino Uno/Arduino Nano
• 28x resistors 200 ohm (depending on initial brightness of LEDs)
• Real Time Clock (RTC) module
• On/Off switch/button
• Push button (for controls)
• Potentiometer (for minutes/hour-settings)
• 9V/12V adapter
• DC Power Supply Female Jack Socket 5.5 x 2.1mm Barrel-Type
• Lots of wire

Realisation

Casing

For the casing you’ll need to follow these steps:

1. Make sure you cut your wooden parts to the right sizes
2. Cut the holes in the top part for the glass domes to fit through. It doesn’t matter if the fit is too tight, you can always sand the holes down. 
3. Make sure you drill and router the holes for the buttons and adapter-input on the back panel (see the previous picture)
4. Glue the box together (except for the bottom)
5. Glue the beam on the underside
6. drill holes in the PVC pipe so you can run the wires through them.  Make them just wide enough for a large pack of cables (see the last picture)
7. Saw/grind slots in the PVC pipe in which you can stick the bottom part of the PCB of the 7-segment LEDs (see the next picture)

The Clockwork

In order to make the clock tick solder the following electronic parts (I don’t have a scheme available, will follow up)

1. Solder the 7 segments to a PCB that sticks out underneath so it fits in PVC pipe (see previous picture)
2. Add resistors to every single LED on the 7 segment display
3. Take the wires to a board with the shift registers (see also https://www.arduino.cc/en/Tutorial/Foundations/ShiftOut)
4. Solder the Arduino to a PCB (use fitting to make it easy to implement)
5. Make sure to connect the shift registers to their own ports on the Arduino or daisy-chain them.
6. Add the On/Off button and a power socket to power up the Arduino
7. Connect the buttons for setting the clockwork to their ports on the Arduino.
8. Connect the RTC to a port on the Arduino and to the power

Code

For the code, check out my Github. It needs a lot of work still. Right now the clock has the tendency to get ahead of time if it runs for longer periods.

https://github.com/MTheelen/Arduino-Cathode-Clock

Evaluation

• The code needs refactoring. It needs to be cleaner, also the clock gets ahead of time
• Have the LEDs blink when someone is setting the time.
• The casing shows the sides of the pinewood on top, which isn’t esthetically pleasing. More work, but nicer, would be to router/drill/chisel out a large beam.
• The LEDs are not so bright. It would  be nice to control brightness automatically based on surrounding light (by means of a photoresistor) or manually with a potentiometer.