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Here is mine:
Your waveform is very messy, you need better gates, or raise them.
Oh hell yeah, now you're talking. This is so much easier for electronic handicaps like myself! Brilliant and thanks for sharing this design you came across with us. Having a laptop, this is something I can easily see myself actually making and bringing to the field. I am SO glad I didn't chuck $100.00 at a chrony!
Maybe you could make it more accurate (and make the curves less messy) by putting a piece of cardboard with a hole in it in front of the endcaps.
There is now a very large region in which the led and the transistor can see each other, which is not very accurate.
You might also want to look into lining up the caps, leds and transistors more precisely.
Nice build and very understandable howto. If I didnt already have a real chrony, I would certainly build one of these.
Till the day I'm dieing, I'll keep them spuddies flying, 'cause I can!
Spudfiles steam group, join!
Same thing I was thinking. I may even build one of these anyway.
My biggest concern is the accuracy. I know you said it was accurate but is there a way this can be verified? Say, chronoing a shot from this, then from another chrono?
This doesn't look right, "20,000x3=60,000 samples/second".
On the Audacity screen shots it says the sample rate is 44100 Hz. So there are 44100 samples each second and the time between each sample is 1/44100 = 22.68uSec (0.00002268 seconds).
Looks like the peaks are ~4.4mSec apart, for 1' gates the average velocity in the trap is 1'/44mSec = 227 FPS. So your 238 FPS is correct. (The difference is just that I can't get an accurate time from just the Audacity screen shot.)
A handy capability of Audacity is the zoom function. Click and drag from a little before your fist peak to a little past your second peak then hit the zoom button. Now click and drag from the top of the first peak to the top of the second peak. The info bar at the bottom of the Audacity window will tell you the time at both ends of the selection and the delta time. (In the view menu there is an option to change how Audacity displays times in the info bar.)
You can estimate the accuracy of that number by looking at how many samples you have between the two peaks. The accuracy can be no better than 1/(number of samples)%. You probably have about 200 samples between the peaks in the recordings so the sampling error is;
1/200 = 0.5%. Or about 1.1 FPS at 227 FPS. Not bad for probably less than $10.
Overall your setup looks good. Nice use of the PVC caps to reduce the amount of ambient light hitting the detectors. The setup should work even under fluorescent lights (which blink on and off 120 times a second).
Like Tech said, this setup (actually any setup using a sound card) will have problems with very small projectiles at very high speeds. You can estimate what that speed and size is pretty easily. The smallest peak the sound card can record is probably about one sample width wide. At 44.1KHz that's 22.7uSec. Lets say the ammo is an 0.177" BB. The fastest velocity the BB can have and still be expected to give a strong peak in the recording is 0.177"/22.7uSec = 650 FPS. You might get a useable speed at higher velocities but the peak is going to get smaller and smaller. For long ammo like spuds this isn't an issue. At 650 FPS the accuracy will also be less. There will be about 70 data points between the two peaks so the sampling error is 1/70=1.4% (+/- 9 FPS), which is still pretty good.
You might consider using red LEDs instead of the IR ones. The detectors aren't as sensitive to red light as they are to near IR but with red LEDs you can actually see where the light beams are. That makes alignment a bit easier. Another handy thing to have is a place you can hookup a volt meter across one of the detectors. The voltage will change as you block either detector with your hand even though the soundcard can't record a signal that changes that slowly. Very handy for getting things aligned.
Your circuit diagram needs some work. I don't think I've ever seen that symbol for an LED. The phototransistor symbol is kind of odd as well and it looks to me like the wires are connected at the wrong places. A phototransistor is exactly like a standard transistor except it has a clear case and might not have an electrical connection to the transistor's base. (The RadioShack phototransistors definitely don't have a base connection.) The symbol used is usually just that of an NPN transistor (for an NPN phototransistor). No electrical connection to the base and the squiggly line with an arrow head pointing at the base. That's how these transistors work, light supplies the base current to the transistor which turns it on allowing current to flow between the emitter and collector.
You have no current limiting resistors on any of the LED's or photo transistors!
It may be advisable to use them.
@jimmy: Man, you know what your talking about!
44100 samples each second make a lot more seance.
The symbols, i found on google... not sure if they are right... ill fix it when i write up a good how to today.
As for the RED leds.... i am going to use infrared to reduce and interference... and also... this whole thing is hot glued to a 2x6... when its done,... it will be encased in a chunk of PVC and have small slots for the gates... not just open 1/2'' caps..
Thank you for all that useful info though!!
@Dewey-1 I dont need one... the forward voltage of the LED is 1.7v a AA battery is 1.5v... so there is no need for a resistor. I am no stranger to electronics... and would not have made that mistake. Thanks for checking though, it was a good observation.
Check out Express SCH/PCB for creating schematics if you are familiar with Electronics.
Free and simple to use!
http://www.expresspcb.com/ExpressPCBHtm ... ftware.htm
Last edited by dewey-1 on Thu Sep 10, 2009 5:49 pm, edited 1 time in total.
I mentioned the sample rate as often the sound recorder is windows is set up to record speech and uses a low sample rate to save file size. Speech is defined for the telephone company as 300 hz to 3Khz. This low fidelity recording would have the short duration events filtered out so small high speed projectiles will be invisible.
Don't use the Windows application "Voice Recorder" for this. It's fidelity is speech grade. I prefer to record directly in Audacity.
Be aware that Sound Blaster sound cards have a fixed internal sample rate that is translated out to the application by the driver. You may be able to select from faster sample rates in your recording application but the sound card won't provide it. More important is Sound Blaster cards and compatibles often employ low pass filters in the input to attenuate high frequencies that will cause aliasing with the sample rate. A typical frequency roll off can be seen here;
The filtering on a mic input is even worse.
Just because you sample at 44.1 or 48 KHZ doesn't mean that events with frequency components near 20Khz will even be detected.
If you pick up a USB pro sound capture box, sample rates of 96K and 192 K are possible. I have an M Audio with the 96K sample rate as well as 44.1 and 48K sample rates.
I must post pone my HOW to for A few hours... while i run a few more tests....It would appear that i have a bouncy ball traveling at 1200+ Ft/s
is it possible that i have hit mach1?? with a bouncy ball???
This is why i need more tests... i am a little bit, skeptical of that data... although.... no matter how many times a shoot... i get a Ft/s reading right around 1200 give or take 200... if i got a ball to go faster than 1,125... i have broken the sound barrier... it would be AWESOME if that was true...but... like i said... to the testing range!!!!!!!!
PS: 1200-ish Ft/s
ammo: 1" bouncy ball
Chronograph: the one pictured, encased in 2'' PVC pipe... with smaller "light holes"
It is possible. I'm studying how to do it right now. I'm hoping to get a marshmallow at close to 1.5 and a golfball close to 1.
I've gotten true chrony readings in excess of mach 1, but that is still debated, BUT, I was using an efficient piston valve and several hundred psi to achieve it.... And even at that, I won't say beyond a shadow of a doubt that I achieved Mach... I believe I did, but science says otherwise unless I got lucky with my chamber barrel transition...
My Cannons can be found by clicking the following link.
http://www.spudfiles.com/forums/viewtop ... tml#256896
I'm hoping to use an oversize valve for subsonic flow into a transition into a smaller barrel with Mach 1 or close to it in the transition and then supersonic in the barrel with expansion of the pressure.
What I am finding from NASA shows this is possible for light projectiles and gas flow. My early high speed video seems to indicate I have already had some supersonic flow, but haven't clocked a projectile there yet. I seem to have reached about 1.3 to 1.4 in flow from frame to frame mist advance at 1,000 framse/sec.
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.
Tech is right about things that can be hidden in what the sound card is doing. Unfortunately, it is nearly impossible to find the specs for the sound card in most laptops. In addition to the input filtering some sound cards digitally filter the data stream. If someone wants a bit more reliable, if only because it is actually documented, then a decent after market sound card in an old PC would be the way to go. Especially if you're pushing 1000 FPS or more.
One thing though, if you really are sampling at 40+ KHz you will still get a signal for events at greater than 20 KHz. The signal will be attenuated but it'll still be there. The oft quoted "need to sample at at least twice the data's frequency" refers to getting a reasonably accurate recording at that frequency. If amplitude accuracy (and frequency response) isn't all that important, and with OK peak shapes, you can get peaks up to, and sometimes beyond, the sample rate. However, the accuracy in the time domain will always be limited to (1/sample rate).
For a chrono there are really two time domains you are concerned with. The first time domain is the length of time the round takes to cross one gate. This is where you might run into problems with very high velocity small ammo giving a peak that is too narrow for the sound card to record.
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.
For example, an 0.177 cal BB at 1000 fps (i.e., a top end BB gun) will have a peak width comparable to about 66 KHz. I've never tried it but it would be very interesting to see if a phototransistor + sound card can get a signal from a peak that narrow. (I figured the travel distance is 0.2", 0.177" from the BB plus 0.013 as the width of the PT's element.)
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.
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.
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