I was playing around with a couple Nerf guns, and reading some of the posts about people modifying Nerf guns for better performance. One thing led to another and I could not resist the idea of making a chronograph, or velocity sensor, for Nerf darts.
I had all of the hardware I needed already lying around - mainly an Arduino controller, and an LCD display. Add to that a pair of infrared LED light sources and detectors from Radio Shack and I was good to go.
The idea was to place the two sensors in a PVC tube spaced 4 inches apart. Then time how long it takes for a dart to trip the two sensors and compute speed. Since the Arduino can do timing in microseconds it is more than capable of doing accurate speed for a dart moving around 100fps.
The Arduino 5v output is connected to the transmitter led through a 270 ohm resistor and to ground. Two analog input pins are connected to the receivers and then a 270 ohm resistor to ground. Very simple circuitry. The LED's are mounted 4 inches apart in a 1 3/8 inch PVC tube. About the only thing I did wrong was not mounting the LED's to be flush with the inside of the tube. My mounting has them sticking into the tube about 1/8", which means the dart can hit them. That means I have to be slightly more careful in settup up the shot to go down the middle of the tube.
Once the speed is calculated, the display is a Sparkfun LCD-09395 serial LCD with 2 lines of 16 characters. That just takes 5v power, ground and a single digital output pin from the Arduino which makes it very simple to use. After the picture was taken I also added a second line to the display for the average velocity of the previous 5 shots.
It did not take long to get it all working - 30 minutes to get the first breadboard version working and a couple days to put it all together.
It anyone is interested in the code (Arduino sketch), or the circut diagram, let me know and I can make that available.
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