November 1, 2014

Homemade Chronograph Writeup!

I apologize for the lack of in-progress pictures, but hopefully some decent CAD illustrations can fill in the blanks!

Seeing as a chronograph is the easiest method for objectively comparing blaster power, I set out to build one that didn't cost an arm and a leg. In addition, all the parts had to be bought locally, so as to allow immediate assembly. Thanks to (enter preferred hardware store here) and Radio Shack, I've accomplished my task.

For the full parts list and guide, keep reading!

Parts List:
(2) Infrared Emitter and Detector, Radio Shack (Model 276-142) - $3.99 each
(1) 1/8 Mono Phone Plug, Radio Shack (Model 274-286) - $3.49 for a two-pack
(1) AA battery holder, Radio Shack (Model 270-401) - $1.49 each
(2) 1 1/4" PVC crosses, Menards - $2.34 each
(4) 1 1/4" x 1/2" PVC slip bushings, Menards - $0.84 each
(4) 1/2" PVC slip plugs, Menards - $0.45 each
(1) 1 1/4" PVC pipe, Menards - $3.99 for 10' length

In addition, you'll need a drill, small gauge insulated wire, soldering tools, electrical tape, heat shrink tubing, and related paraphernalia. I did not list the prices for these; as I consider these staples of a normal hardware/tool collection. If you don't have them, that's extra money, but for tools you use often, it's a worthy investment.

First, take a drill and make a 1/4" diameter hole in the middle of each PVC plug. This is where you'll insert and glue your emitters and detectors. The recessed positions will limit the usable infrared path to just between the emitter and detector. You know, one of those key things to making the device work properly.

Here's a nice schematic showing how everything fits together! I used a little over a foot of PVC pipe. Be sure that, after assembly, you measure the distance between emitter/detector pairs.

Now for the wiring. As you can see, there are two separate systems - one for the emitters, and one for the detectors. Please remember to read the LED/phototransistor package, so that you see the correct orientation/which prongs go where.

This pic was mainly for the UPC, but you can see there's stuff to read when you have the pack in hand.

The detectors are wired in series, and soldered to a 1/8" mono plug, for use in a computer's mic input.

Here's a picture I found online, demonstrating the setup for the plug (a larger one, but it works the same). The "positive" side is the tip of the plug, where a small voltage is applied by the computer's sound card. The "negative" is the larger sheath, which has a large prong extending all the way to the rear of the plug assembly.

For the infrared LEDs, I have them set up in parallel, powered by a single AA battery. I actually ended up buying a 2-AA battery holder - one with a 9V-type snap connector - that I could simply unsnap for use in other projects.

I also made a battery blank out of copper tubing and plastic tubing I had laying around, just to complete the circuit.

Seen here without the plastic covering the copper tubing.

By the way, if you can't see the LEDs on, due to variations in what wavelength light they're making, just take a peek with your phone - it'll show up on a digital camera just fine.

Have everything assembled? Good. Now plug this into your computer and turn on Audacity. Make sure you select the mic input before recording, then shoot some darts through. After recording, zoom in your pairs of spikes, from where the dart passes each sensor pair.

Highlight from the first peak in each blip, and read the length at the bottom. 

If Audacity lists the highlighted length in milliseconds, click the dropdown arrow and change to samples. Then, divide samples by the listed project rate in Hz to get a more exact time in seconds.

Then, divide the measured distance between sensors by the time, and you have the speed of the dart! Obviously, this part takes more time than a standard chronograph, but it allows you to see what goes into the final calculation. It also allows you to see and discard errant shots that hit the inside of the pipe and screw up readings - the readings will be visibly different on-screen.

I have heard of other programs that will do all of this automatically, but have yet to experiment with them.

So far, I've been testing many different dart types with my stock Elite Strongarm, and comparing the results to BlasterLab's Dart Comparison results. So far, I've been within 1 fps of all the results.

I'll call this a success!

I'll list a bunch of sources in a bit, down here at the end of the post. There have been many variations of this concept used in airsoft and spud launching, and this is just my take on the idea.

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