Finish Line Sensors Bring Legitimacy to Pinewood Derbies

The pinewood derby is an iconic BSA (Boy Scouts of America) event. It challenges scouts to design and craft a small gravity-powered car, then race that car down a sloped ramp. Participants generally start with a basic kit sold by BSA and their cars must adhere to a handful of criteria covering maximum weight, length, and so on. That keeps the competition fair, but scouts are free to express their creativity within the regulations and try to build the fastest car. To help determine the true winner of each race, Redditor AvalancheJoseki built this pinewood derby finish line sensor.

Most pinewood derby events rely on a judge to determine the winner of each race. But humans are famously fallible, which means that judges can make mistakes. This finish line sensor ensures objective results. The current implementation features a limit switch on the start gate, which starts the timer when the gate releases the cars. As the cars reach the finish line, they cross over optical sensors. The microcontroller records the time it takes for each car to cross the finish line and displays the time for that car's lane on a small LED matrix panel.

The microcontroller is an ESP32 on a development board and the finish line sensors are CNY70 photointerrupter optical sensors. The display is a Hub75 RGB LED matrix panel.

This current setup isn't perfect, as the CNY70 sensors have a hard time detecting dark-colored pinewood car bodies and the start gate is manually operated. The former issue is solvable with different sensors, like infrared beam sensors. The latter issue is more complicated. The problem is that manual release start gates can introduce some variation between races. By switching to a servo-actuated gate, it would be possible to compare times from different races. AvalancheJoseki also wants to increase the precision from tenths of a second to thousandths of a second.

The coolest thing about this project is that it fits in with the general ethos of BSA. Scout troops could construct their own finish line sensors as a group activity, which is the whole point of being a scout.