GPS pendulum timer & beat analyser.

How well does the clock keep time?

After building the tower clock, I wanted to measure how accurately it ran. As the adjustment improves, measuring the remaining error by eye becomes harder. A pendulum timer gave me another way to investigate it.

The project uses an ESP32 and a photointerrupter to timestamp the pendulum’s beats. A GPS module supplies time and a pulse-per-second reference, while environmental sensors record temperature, humidity and pressure. Readings can be logged to an SD card and viewed through a web dashboard.

The prototype hardware

The published parts list includes an ESP32 DevKit, GP-20U7 GPS module, BME280 environmental sensor, photointerrupter, SSD1306 OLED display and microSD module. The original cost estimate was under £30; it is a historical estimate rather than a current shopping quote.

The software explores beat intervals, frequency, drift and asymmetry alongside environmental readings. GPS timing and microsecond timestamps are useful tools, but timestamp resolution alone is not a guarantee of measurement accuracy.

The download below preserves the V17 project archive from the original page. This remains a prototype project. In a later update I explained that I had continued working on the prototype rather than completing a production PCB.