I was asked to build these starting blocks for a Science of Sport event at Glasgow Science Centre. The idea was to give people a chance to measure their reaction time "off the blocks". At first I thought about some sort of switch under the blocks, but I found that it would be difficult to arrange a switch to cope with users of widely varying weight. The solution was to make the blocks measure the force applied to them. This can be plotted on a digital oscilloscope giving a rather richer measurement than simple reaction time. I included electronics to make the operation of the activity as simple as possible.
The volunteer got into position on the blocks, and the presenter would say "On your marks...set" and press a button. Two seconds later a loud beep sounded, which was the "gun". The volunteer then started as fast as possible, and a few seconds later graphs showing the forces exerted by each foot on the blocks during the take-off were displayed on the lecture theatre screen. These graphs included a mark to show when the starting gun fired, so that users could see how soon their foot pressure on the blocks started to rise.
Transduction of force
To transduce the force applied by the user, I used a rubber bellows sold by RS (intended as part of a foot-operated, air-actuated switch system). This bellows fitted underneath the hinged surface of the starting block. The picture on the right shows the bellows in position, with the top surface of the block raised up. The tube from the bellows led to a pressure transducer, which in turn was connected to the control electronics. The system was not calibrated - the output indicated the rise and fall of force without giving any absolute values.
Electronics
The signals from the pressure transducers were amplified by instrumentation (differential) amplifiers to give ground-referenced signals. These were fed to the two inputs of a Pico PC-based oscilloscope, operated in single-sweep mode. Control was by a PIC microcontroller chip. This chip did a number of jobs:
- It sounded the beeper two seconds after the presenter pressed the start button.
- It provided a trigger signal to the oscilloscope shortly before the beeper sounded.
- It grounded both of the oscilloscope inputs for 30msec immediately the beeper sounded. This provided a visual indication on the oscilloscope trace of when the "gun" had fired.
Mechanical
The blocks were made of plywood. The baseboard had two rows of holes in it. Each block had a pair of dowels underneath it that located into the holes on the baseboard. This made it easy to adjust the starting blocks for different people. Screws and wingnuts held the blocks to the board when it was being carried around. The dummy socket on the baseboard was for storing the power supply.