measuring speed of object

[MrDEB]

just doing some studing on air cannons and find an artical on using a shooting chronograph to measure the speed of the projectile.
Artical talks about using a laptop computer and a soundcard to measure with.
basically its two phototransistors, mounted 1 foot apart in the cannon barrel then measure the time it takes for projectile to travel the 1 foot.
artical mentions that the photo transistors he used had too slow of a rise time to be accurate.
**broken link removed**
they are using 1.7ms rise time photo transistors.
Well the clock on the soundcard they are using is 44.1khz.
any thoughts on using a PIC for this app??
doable?

 

[be80be]

They show you how to make speed gun at EPE when i find it i'll post what month it was in

 

[Sceadwian]

For an object traveling through air the rise time of the photo transistor is effectively irrelevant, you just need to measure the exact moment when the signal hit's it's peak, if both sensors are the same then you measure the distance between the two peaks and you're all set. Not sure how you would interface this to a sound card, but if the projectile is small and you simple feed the transitor output into an or gate you'll see ticks on the sounds wave form corresponding to the interuption occuring and then ending. OR'ing both inputs together and you should see the two peaks you need to measure the distance. At 44.1khz you count the number of samples between peaks, multiply by the reciprocal of 44.1 khz and you have a very accurate time stamp. If you have really slow photo transistors increasing the distance between the two points to perhaps 5 feet will help separate the two peaks. Should be pretty easy to test.

 

[MrDEB]

the link I posted describes how the author did it using a sound card.
He mentions about the photo transistors have a slow rise time.
I have some SMD photo transistors that I may solder up and experiment with using a PIC

 

[Sceadwian]

The link you posted says how the author thinks it should be interfaced with a sound card, and mentions that a link will be provided next week for the circuit to do so. He also provided no details about the photo transistor or how he determined the rise time was too slow.

All you need to do to remove the rise time of the sensors from the picture is to increase the distance between the sensors so you can determine the two individual peaks.

 

[MrDEB]

link for PIC but its hex

this link tells how to build but the code is in hex and it uses a DVM with frequency measurement for display.
can hex be converted to editable code (swordfish or ?)
would like to use the 18F1320(have on hand but need to find out if it will run at 20mhz?link uses 16F676
most on web use a PC and a sound card using a prg called audicity.
I want self contained w/ LCD readout
http://www.geocities.com/pest3125/chrono/chrono.htm
another method of building but PC based
Jim's Homemade Chrono
any ideas or suggestions on where to start writing code or ?
getting my books out but they are in picbasic and assembly. don't want to get to confused.

 

[Sceadwian]

Conversion from .hex to .asm is bad enough from .hex to C code would be completely unreadable. Contact the author and request the original form of the code then you have something decent to go with.

 

[MrDEB]

Think I am right track

over at **broken link removed**
I posted what I want to do.
came up with a code for the 18F452 using an LCD module.Have several 18F1320's on hand. not sure i would handle? then need an LCD module.
question now is photo sensors-speed of detection?
looking at a Fairchild QSE157
optiolodgic??
70ns rise time, fast enought for my application?
is ns faster than us?
all confusing
any suggestions

 

[atferrari]

Questions to be answered by yourself.

is ns faster than us?

Too basic to be asked. You should be able to answer that question after searching and reading about units.

Have you tried?