LED clock

A clock made of glowing ping-pong balls. Each ball hides an addressable LED, and the time lights up in big, easy-to-read digits, with a knob for brightness and a button that cycles through themed color modes.

Status: Finished, and on my wall
Based on: The Instructables "Ping Pong Ball LED Clock", heavily modified
Controller: Arduino Nano
Display: 128 WS2812B LEDs, one behind each ping-pong ball
Timekeeping: DS3231 real-time clock module
Controls: Five buttons and a brightness dial
Size: About 80 cm (31.5 in) wide, frameless
Programmed in: C++ (Arduino) with the Adafruit NeoPixel library
Skills: Arduino and C++, addressable LEDs (WS2812B), real-time clock (I2C), soldering, debugging and refactoring code, AI-assisted development, CAD and 3D printing

A wide, frameless wall clock made of white ping-pong balls in rows, glowing in a rainbow from blue to pink, with the time picked out in white digits. — The clock on my wall, in its color-fade mode.

The goal

I first built this during Covid, from an Instructables guide, the Ping Pong Ball LED Clock. I set out to follow the directions, and by the end I'd made it my own.

How it works

Behind 128 ping-pong balls sits a WS2812B LED strip, cut into short runs that snake back and forth behind the rows, one LED per ball. Each digit is drawn with 12 balls, with four digits and a colon across the middle. I left mine frameless. An Arduino Nano drives the LEDs, and a DS3231 real-time clock module keeps the time. A 5 V supply powers the LEDs directly, rather than through the Arduino.

The back of the clock's hexagonal hardboard panel, with a metal mesh power supply in the middle, two blue circuit boards in a black printed box at right, a black printed box and rocker switch at left, and red, white and black wires looping around the edges. — The back: the 5 V power supply in the middle, the two boards in their printed box at right, and my cable box and power switch at left.

The electronics live on two 5 × 7 cm boards: one for the Nano and the DS3231, and one for the buttons, joined by a pluggable cable. Both sit in an open-top box I designed and 3D printed, and a printed surface-mount box glued to the back holds the power plug.

Two blue perfboards in a black printed box: the top one holds an Arduino Nano and a real-time clock module with a coin cell, the bottom one five push buttons and a small blue dial, joined by a bundle of colored jumper wires. — The two boards: the Nano and DS3231 on top, the five buttons and brightness dial below, joined by a pluggable cable.

A black 3D-printed box glued to the back panel, with a power cord running out of it to an inline black rocker switch. — The box I designed to hold the spliced power cables, with the on/off switch on the cord.

The clock's top edge seen from above, with white ping-pong balls on the front of the board and short red, white and black wires looping around its edge. — From above: the wires joining each run of LEDs loop around the edge of the board.

The modes

One button cycles through the themes:

A fade through every color, at three different speeds
Red and green, for Christmas
Orange, for Halloween or as a late-night lamp
Plain white, for daytime office use

The color fade, in motion.

Christmas mode: red and green.

The clock with only the digits lit in orange against unlit white balls. — Orange, for Halloween or as a late-night lamp.

The clock with only the digits lit in soft white against unlit balls. — Plain white, for daytime.

This was my first project coding with AI: I used Claude to add the modes, the buttons and the brightness control.

Key decisions

Easier-to-read digits. I restyled the numbers to make them easier to read.
Controls on the clock itself. Five buttons and a dial handle the time, the brightness and the modes, so I never have to take it off the wall again (see below).
Printed mounts. A box I designed holds both circuit boards, and another keeps the power plug neat on the back.

Problems and fixes

I wired the LED strips backward. The guide runs the first strip down, the next one up and so on, alternating. I wasn't paying attention and started with one going up, then down. Because the strips aren't all the same length, there was no quick and easy fix, so I rewrote most of the code to map each digit onto my wiring.
A clock I couldn't adjust. Even after the digits worked, any change meant taking the whole thing off the wall and plugging it into my computer, even just for daylight saving time. It was also always too bright in the evening or too dim during the day. So it came down for a couple of years. When I came back to it, I added buttons to set the time and a dial for the brightness, and it's been on the wall ever since.

Results

It's finished and has been hanging on my wall for months. Daylight saving time is now a few button presses instead of a trip to my computer.

What's next

Save the clock battery. The on/off switch sits on the power line before the power supply, so whenever the clock is off, the DS3231 runs on its backup battery. I've replaced several batteries so far, so I plan to move the switch to cut power to the LED strips instead.
What I'd do differently: if I'd known from the start that I wanted five buttons and a dial, I'd have put everything on a single 7 × 9 cm board and built a case around it, instead of two boards and a cable.