The interval timer switched off: black 3D-printed case with a carry handle, four green buttons, a switch, a knob, a 14-segment display and an LED matrix The timer on, showing MOD3 on the 14-segment display and SELECT on the LED matrix The timer mid-round, showing RD01 and a countdown of 00:45 The timer from the front right, counting down 00:26 The back of the timer, with the power switch and a USB-C cable plugged in
Alex OliveriInterval Timer

Interval Timer

A round timer for martial arts training, in service at ABK Sansára Dojos. It plays Bluetooth music and pauses it for every rest.

Scroll to switch it on

Alex OliveriInterval Timer · design files

The project

Why it exists

Why I built it

Our martial arts club wanted a round timer that could play Bluetooth music and pause it during rest periods. The pause marks each round clearly and gives the instructor a quiet moment to talk.

What was hard

Assembly. There are a lot of short wires between the board and the parts on the faceplate, so it took a lot of crimping and soldering connectors, and fitting the faceplate was difficult.

What I learnt

Design for assembly. If this ever had to be mass produced, the current wiring would make that impossible. Next time: fewer cables, fewer crimped connectors, and a faceplate that goes in easily.

Four modes

Built for the classes it runs

  1. 01Nidan grading2-minute rounds. At halfway the music pauses for 3 seconds as the signal to swap sides. 15-second rest between rounds. The 14-segment display shows SxRy: section x, round y.
  2. 02Class1 minute on, 30 seconds off.
  3. 03Muay Thai class2-minute rounds with a 30-second rest.
  4. 04WiFiJoins the local WiFi so rounds can be customised from a phone, with no reflashing needed.
Design files to finished product

As designed, as built

Layout, from the Gerbers
The finished timer from the front left, counting down 00:32
Finished, in service
Controller
ESP32 dev board, U1Runs the timer, WiFi set-up and Bluetooth music.
Displays
14-segment and LED matrixRound and mode on the 14-segment, time on four matrix panels.
Audio
MAX98357 I2S amplifierDrives two speakers in parallel. A buzzer on J7 as well.
Controls
4 lit buttons, encoder, switchEach button has a switch line with a 10 kΩ pull-up and an LED line.
Power
USB-C, 5 VThrough the power switch on J16, on 1.2 mm tracks.
Board
95.5 × 98.5 mm, 2 layersAll through-hole. Ground is a pour across the back.
In service
ABK Sansára DojosRuns classes and gradings.