Experiment: Optical Stopwatch

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Optical Stopwatch
Experiment Optical Stopwatch
Category Timers
Used sensors Light sensor

Different timer experiments in phyphox.

The experiment "Optical Stopwatch" reacts to changes of the illuminance of the light sensor and measures the time inbetween. The acuracy depends on the speed of the light sensor in your device, but it typically does not get better than 200ms. The Experiment: Acoustic Stopwatch and Experiment: Motion Stopwatch achieve more acurate results, but react to motion and sound.

Note that while iPhones have a light sensor, the sensor cannot be read from an app, so this experiment is not available on iOS devices.


There are no special requirements.


There is no specific setup.


The analysis is not too complicated, but uses a few if-modules to decide how many events have already been counted. Otherwise, chunks of data are analysed for their maxima and compared to the threshold. If the threshold is exceeded, we use the time of the maximum.

Problems and resolutions

  • The stopwatch does not trigger. Check the data recorded from the light sensor to see if it actually crosses the threshold. Especially when trying to react to a shadow, the human eye often underestimates the change in lighting conditions in different places, so make sure, that the threshold is set accordingly.