↓ Skip to main content
steeman.be
  1. Posts/

Building a Woordklok — A Dutch Word Clock

· David Steeman · Electronics, DIY
Building a Woordklok — A Dutch Word Clock

My wife wanted a word clock for the house for ages. There’s something deeply satisfying about a clock that doesn’t show digits at all — it just lights up the words: het is half drie. So when I came across a complete Dutch design for one, I finally had no more excuses.

What’s a word clock?
#

A word clock tells the time by illuminating groups of letters on a grid, so that the lit-up letters spell the time in words (rounded to five minutes). The face is a matrix of letters, and at any moment only the letters that form the current time are on. It’s a lovely bit of functional typography.

The design I built isn’t mine. It comes from DJO Amersfoort, who published a full “Maak een woordklok” tutorial — the laser-cut frame, the letter layout, the whole build. The firmware is a fork of vdham/wordclock on Bitbucket, written for a Wemos D1 mini (ESP8266) driving a WS2812B LED strip. So most of the heavy lifting was already done. My job was mostly assembling it — and then, as it turned out, fixing it.

The finished woordklok
The assembled woordklok — a 9×10 letter grid with a Wemos D1 mini behind it.

The build
#

Cutting the case
#

The front panel and frame are laser-cut from the DJO SVGs. My K40 laser cutter’s bed is only about 300×200 mm, which isn’t quite big enough to do the full panel in one pass. So I split each panel design into two halves, cut them separately, and joined the halves back together afterwards. A bit of extra fiddling, but it beats finding a bigger laser.

The face is a 9×10 grid of letters cut into the front panel, with a WS2812B LED behind each cell. The clever part is the wiring: instead of ninety separate wires, the LED strip is one continuous chain snaked across the rows in a zigzag (an “S-vorm”, as the DJO instructions call it). Data goes in at the bottom-right corner — at the letter R — runs left along the bottom row, jumps up and runs right along the next row, and so on all the way to the top.

LED strips on the wooden base
The vertical LED strips zig-zagging across the wooden base — one continuous data chain.

A Wemos D1 mini sits inside the frame. The design includes an LDR on the analog pin for automatic brightness — the firmware supports it — but I haven’t populated the LDR yet. Maybe later. NTP keeps the time with automatic daylight-saving, and there’s an OTA update port so I can flash new firmware over WiFi without having to open the case. The full strip pulls more current than USB can deliver, so it runs off a dedicated 5 V supply.

The controller, wired up
The Wemos D1 mini in the frame, with the LED strip’s data line on D2.

A few changes of my own
#

On top of the original firmware, I made a handful of modifications:

  • Minute LEDs. A word clock only resolves to five minutes, which always feels a bit coarse. So I added a short 4-LED strip at the bottom that shows the minutes within the current five-minute block — one dot per minute past the five.
  • Stability. The original would occasionally crash and reboot the ESP8266. That came from three places: the NTP code blocked the main loop for over a second without yielding, the brightness sampling blocked it for another hundred milliseconds, and the LED refresh fought with OTA flash writes. A few targeted fixes — yielding in the right places, shorter sampling, and a guard that pauses the display during an OTA update — sorted it.

I published all of it on GitHub: steemandavid/woordklok .

And then it displayed the wrong time
#

I flashed the firmware, and the clock lit up — but the words were gibberish. Letters that shouldn’t be on were on; letters that should have been on were off. My first instinct was that I’d broken something with my changes, so I diffed my version against the original code straight from the archive. Nothing in the display logic had changed — the word-to-LED mapping was byte-for-byte identical. The original map was simply wrong for my strip.

The thing about a word clock is that the word→LED mapping assumes one very specific physical wiring, and mine — built from the same DJO instructions — was wired differently. Rather than keep guessing, I added a tiny calibration mode that lights five specific LEDs in five different colours, and read off where each one physically landed. That gave the whole topology away in one look: my minute strip was the first thing in the data chain rather than the last, and the grid was offset and snaked in a way the original code didn’t expect. I rebuilt the map, pushed the fix over the air, and the clock has shown the right time ever since.

It was a good reminder: when hardware and software disagree, no amount of reading the code will tell you which one is right. You have to go and look at the hardware.

Lessons learned
#

  • Charge-only USB cables are the bane of embedded work. Two of the three cables I tried were power-only; the board simply didn’t appear. Check with lsusb before you assume your code is broken.
  • Calibration beats reverse-engineering. I spent a while trying to deduce the wiring from lit letters. Five coloured LEDs told me in seconds what an hour of guessing couldn’t.
  • OTA is worth the setup. Once the firewall was told to allow the upload’s back-connection, flashing new firmware over WiFi became a one-liner — invaluable while debugging.

Resources
#