Ok after a few requests I sat down and made a schematic of my DIY chronograph.
As you can see it's very easy to build it and most of the stuff needed are usually available at any home.
I used a piece of heavy plastic tube - the broomstick of an old broom that lost most of it's bristles long ago...
- two small 10x4x4cm wooden blocks.
Then you need a power drill and a 5mm bit to drill a hole through the wood and across the pipe's center. A wood screw inserted from the top keeps the wooden blocks on the pipe.
Two pairs of IR LEDs/Phototransistors are inserted in the holes, opposite each other.
I have used this IR LED
And this matching IR Phototransistor
both working at 950nm wavelength, but any matching (in wavelength) pair will do the job.
The IR LEDs are working on 1.2 Volts and can be either run in parallel from a single 1.2 Volt NiMH AA battery, or - like I did - use a higher voltage source and drop down the voltage with an in-line resistor.
I figured out that since this contraption is going to be used always next to a computer, I might as well "steal" some power from the USB port. In that case you need a 25 Ohm resistor - any wattage will do.
Observe the LEDs polarity. The kathode (shorter leg) goes always on the negative pole, the anode (longer one) on the positive.
Then you take a 3.5 stereo plug (I used one and the cable from an old cheap headphone set) and wire the two phototransistors in series with the tip of the plug going to the collector of the first transistor and the ground (barrel) of the plug to the emitter of the second (see schematic). The ring pole in the middle of the plug is not used.
This is the prototype (before cutting the excess tube)
And here is next to the laptop running the SoftChrono software
The software can be downloaded for free here:
This project - as everything else - has advantages and disadvantages:
-You have to waste about 30 mins of your life to build it.
-It's working like a charm.
-It's cheap! The parts/components don't cost more than a buck and the software is free.
-Unlike the commercial chronos is self-contained and is not influenced by external factors like, fluorescent lamps, AC flickering, clouds, sun, rain, etc... You don't need to build light boxes.
-Since all the data is collected by the computer and displayed on the screen, they are easily saved, analysed, compared, and manipulated in any possible way. Unlike commercial chronos that need extra accessories in order to be interfaced to PCs.
-You can source the IR LED/Phototransistors for nothing from old TV-sets,VCRs,CD-players etc that used IR remotes. They are matching and free!
-If you can't/don't feel like soldering you can use a Terminal strip like thhttp://www.radioshack.com/product/index.jsp?productId=2103232&CAWELAID=107595253
for all the connections.
-The distance between the sensors (length of the pipe)is not important as it's a value that can be adjusted later in the software. Shorter pipe, more compact chrono - longer pipe, more accurate. Approximately 1.5 feet long is a good start.
-USE AN IRON PIPE instead of plastic IF YOU INTEND TO USE THIS CHRONOGRAPH WITH HIGH POWERED AIRGUNS.
-USE A SUITABLE BACKSTOP AT ALL TIMES!
-Keep the laptop away from the pellet's flight path.
-MAKE SURE YOU SHOOT STRAIGHT IN THE PIPE. You can even place the gun's barrel in it.
-This chronograph is not built or tested for use with firearms. The products of gun powder combustion might foul the sensors or not, but I consider very dangerous to shoot a firearm in a metal pipe.
-This information is provided as it is for experimentation. Use common sense and observe all the rules when using power tools, airguns, guns, and/or working with electricity. I shall not be held responsible for any damage, injury or death caused during the construction or any direct or indirect implications of using this chronograph.