is by far the most accurate means of measuring the velocity of the projectile.
The speed trap consists of two photodiode gates separated by 100mm. The first
gate triggers the flip-flop and second resets it producing a TTL level pulse on
the BNC connector. It is necessary to reset the circuit when it is first switched
on because the flip-flop powers up in a random state. The circuit diagram is shown
gate setup is very sensitive to the IR emitter diode power and the separation
of the emitter and detector so it takes a bit of experimentation with resistor
values to get it working. This circuit uses the reverse breakdown characteristics
of the IR photodiode which allows a low component count however the switching
time of the gate is slow (for a photodiode). This limits the useful range of the
speed trap to about 50 m/s. In the future an updated version of the speed trap
will use the photovoltage signal generated by the diode which has a much faster
1. Internal parts of speed trap.
2. Completed speed trap.