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Home made infared chronograph How To. Very Acurate

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Unread postAuthor: jhalek90 » Sat Sep 12, 2009 10:21 am

jimmy101 wrote:jhalek90: How did you launch the bouncy ball? Mach 1 is pretty unlikely, even anything over 1000 FPS is pretty unlikely without a very serious cannon.

The second time domain is the time from gate to gate. Usually the sound card won't have any problems with this time (and frequency) domain but the accuracy of the chrony is limited by the number of data points between the two gates.

With a 1 foot gate separation it'll take 1 millisecond from gate to gate. That's only 1 KHz and a 44 KHz sample rate would give OK accuracy.

EDIT: the ball was lunched with my piston cannon, at 90-100 PSI
The cannon will be posted latter today!!!! woot.

The Reason im not sure if i hit mach1 is that there was 3-5 samples each time the ball passed a gate.... and only a few more in between the gates....

So, i am reaching the limits of this chrony...THAT is why i said i need more tests.... i refuse to claim that i hit mach1 for sure.... until i can test it more.

The results from my chrony indicate that the ball was traveling at 1200FPS, but i cant be sure that the ball missed the second gate... and that the second "peak" was just interference when you are dealing with a single thousandth of a second.... i wish i had a more acurate instrument... lol

this chrony is GREAT for slower speed or bigger projectiles. but i believe that with a bouncy ball... anything above 1000 FPS must be taken with a grain of salt.
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Unread postAuthor: jimmy101 » Sat Sep 12, 2009 11:50 am

Technician1002 wrote:
One thing though, if you really are sampling at 40+ KHz you will still get a signal for events at greater than 20 KHz.

Only if you bypass the anti-aliasing filter. Sweep test your card. The input filtering often cuts the record response much more than the output A/D anti-aliasing filter. More info is in the article below. Sound cards don't oversample as a rule unless you go to a pro card and sample at 96Khz or higher.

Do you think the MIC input of a typical sound card has any filtering except perhaps a high pass filter? The schematics I've seen (which isn't that many) are just a series cap and a resistor to ground to DC uncouple the MIC input. That also acts as a low pass filter that is rolling off the response once the frequency gets below 100 to 200 Hz or so. These are pretty crappy audio systems so the designers tend to worry more about killing 60 and 120 Hz than they worry about getting decent responce down to 50Hz. There might also be a diode to +5V and another to -5V to protect the AD from high voltages, or that is included within the AD chip.

A sweep test won't quite answer this particular question but it would help. I don't have a suitable generator. Can you thing of a simple way to generate a single pulse that is 6uSec wide? That would correspond to a half wave at 80Khz. I guess I should just wire up a 555 as a one shot and see if the MIC input can get a signal.
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