Work in Progress: Chrony alpha successfully tested! + interesting new vs old darts FPS comparison

Our homemade chrony has been confirmed working! Here are some pictures.

As you can see, the chrony has been integrated into a retaliator stock barrel. The barrel has been extensive gutted to fit the 1 inch diameter PVC pipe. However, the barrel's attachment system is untouched and remains fully functional. This allows for easy FPS measuring of all Nerf blasters with barrel extension tips!

More after the jump!

Holes were drilled inside the PVC pipe to house the infrared emitters and detectors (shaped like standard 5 mm LEDs). There are two sets of IR emitters and detectors placed 6 inches apart to determine the dart speed by calculating the known distance traveled over the elapsed time. The micro-controller that is powering this system is the Arduino Pro Mini 328 5V.

Here is a video of the chrony in action:

This is the alpha version of the chronograph and the design is now confirmed working. The beta version of the chrony will be a professionally PCB (printed circuit board) version of the current system. In short, it will house the Arduino's atmega328 chip, FTDI chip (to interface with computer), LCD output display, input buttons and battery holders all on one convenient board!

So what is the accuracy of this chronograph compared to a retail one? We cannot conclude this definitively because we do not own a retail chronograph. However, speculation can be applied with logic. Here is a detailed article on how most retail chronographs work (all you really need to read is the small how it works section and perhaps the accuracy section):

http://www.pilkguns.com/Chrony.shtml

A retail chronograph uses a huge photo-detector to sense a wide area for motion. This is to allow a variety of guns/ammunition to be used for measurement. There are downsides to using a huge detector, such as reduced signal to noise ratio as well as slow signal propagation/detection. Our system uses small IR emitters and detectors housed inside a system designed for Nerf darts. The PVC pipe greatly reduces IR noise and the compact detectors allows for responsive signal detection and minimal signal travel. A retail chronograph typically operates at 0.25 us, while our chrony has a time resolution 1 us. Although this is 4 times greater, our system allows us to use digital signal read as opposed to analog, which is typically 200 times faster. In addition, our system lets the user read data directly from the chrony into their computer, making data analysis quick and simple. It is also possible to program the chrony to read data sequentially, allowing for easy FPS data streaming of automatic blasters such as Stryfes, Rayvens and Rapidstrikes to measure dart speed under maximum rate of fire.

What about cost? The cheapest chronograph I could find was $75. This is why we initiated this project, as we already had the majority of components needed to make this chrony. Here is a complete list of parts used in this alpha version:

• 1x Arduino Pro Mini 328 5V - $9.95 - https://www.sparkfun.com/products/11113
• 1x 16x2 LCD screen - $14.95 - https://www.sparkfun.com/products/791
• 1x OPA2350 - $5.18 - http://www.mouser.com/ProductDetail/Texas-Instruments/OPA2350PA/?qs=sGAEpiMZZMtOXy69nW9rM6CBK5NDKGQzRVkB0SVMSsE%3d
• 2x IR emitter - ~$1.00 - (will post mouser link later)
• 2x IR detector - ~$1.00 - (will post mouser link later)
• 2x 47 ohm resistors - ~$0.20 - (will post mouser link later)
• 2x 560k ohm resistors - ~$0.20 - (will post mouser link later)
• 1x PVC pipe 1"x1' - ~$1.00 - available at any hardware store
• 1x small breadboard - ~$10.00 - available at any electronics store
• 1x solid core wire roll - ~$5.00 - available at any electronics store

So if you are starting from scratch, this would cost you around $50 without tax and shipping. This may seem quite steep, but it is still cheaper than buying a retail model. And personally, I always prefer DIY over pre-made products for the learning experience :)

For the beta version, the cost of parts will be reduced. Here is a rough list of parts:

• 1x atmega328 - $3.05 - http://www.mouser.com/ProductDetail/Atmel/ATMEGA328P-AU/?qs=sGAEpiMZZMtVoztFdqDXO3RbBx7FKCmV
• 1x 16x2 LCD screen - $14.95 - https://www.sparkfun.com/products/791
• 1x OPA2350 - $5.18 - http://www.mouser.com/ProductDetail/Texas-Instruments/OPA2350PA/?qs=sGAEpiMZZMtOXy69nW9rM6CBK5NDKGQzRVkB0SVMSsE%3d
• 2x IR emitter - ~$1.00 - (will post mouser link later)
• 2x IR detector - ~$1.00 - (will post mouser link later)
• 2x 47 ohm resistors - ~$0.20 - (will post mouser link later)
• 2x 560k ohm resistors - ~$0.20 - (will post mouser link later)
• 1x PVC pipe 1"x1' - ~$1.00 - available at any hardware store
• 1x fabricated PCB - ~$15.00

The construction cost should be approximately $42. Most of the savings come from the integration of the Atmel chip instead of buying an entire Arduino and using a fabricated PCB instead of prototyping hardware. Furthermore, you can save an additional $15 if you do not mind using your computer to read data, so no LCD display will be necessary, bringing the cost down to $27.

A complete guide of how to construct your own DIY chronograph as well as the more professional PCB version will be posted once this project is finalized.

Here are two interesting sets of data I gathered while testing this alpha version:

Plot 1 - retaliator with stock spring, stock bolt using new elite darts' FPS data

Plot 2 - retaliator with stock spring, stock bolt using new elite darts' FPS data

For the new darts data set, every measurement was taken with brand new, never used elite darts. For the old darts data set, all of the darts were used, with two of them in great/good condition, two of them in decent/fair condition, and two of them in poor condition. Before I conducted the used darts test, I thought there was a problem with our chrony as the stock retaliator FPS data did not match those of torukmaktor's. Naazrael then suggested me to try some old darts and sure enough, this is the reason behind the data inconsistency. From the averages, the old darts were outputting only 78 % of the new darts' FPS. This is quite a significant degradation in performance! Hope this gets you to rethink about dart care!