$10 chrono

[homedepotpro]

A while ago i built a one of these http://www.spudfiles.com/forums/the-hom ... t6662.html
and one of these http://www.instructables.com/id/HOW-TO% ... -of-Light/

so today i had an epiphany, why not combine them. I ended up with a cheap and easy to use chronograph for projectiles from 1/2'' to 1.5''. I've tested it and it work very well

Here is a supplies list:
2 yellow LEDs http://www.radioshack.com/product/index ... age=search
2 photo resistors http://www.radioshack.com/product/index ... age=search
4 "AA" holder w/ switch http://www.radioshack.com/product/index ... age=search
some wire
Audacity
http://audacity.sourceforge.net/
some pipe
Edit:
Audio connector http://www.radioshack.com/product/index ... age=search
pics:





Basically the LEDs lights up the photo transistors and when the projectile passes it, it interrupts the light beam, this shows up as a negative spike on the recording. take the distance between the sensors and divide it by the time it took to cover the distance (time between the spikes), and boo ya velocity.

[singularity]

wow that looks a lot more ghetto than mine, any reason why you spaced the photo transistors so far apart? its a lot easier to calculate fps when they are one foot from each other. do you have any idea how accurate yours is? i would say mine is accurate to 10fps or less.

[homedepotpro]

no clue how accurate but i put the sensors farther apart for timing accuraccy. do you think i should make it shorter.

[silverdooty]

might just be the old fart in me,

you don't really use it in the house with your desktop?

and what size pvc is it?

[homedepotpro]

i've been testing it with nerf darts and it is 1.5'' thin walled pipe

[paaiyan]

That's actually an incredibly brilliant idea if you ask me. The implementation isn't very clean-looking, but that can be improved upon later. I commend you.

[Novacastrian]

Not being a smart ass but how do you know that it is accurate? I'll make one if you can "prove" it's accuracy.

Edit: What would be the smallest round that it could detect?

[silverdooty]

with out knowing the actual ID, i would guess anything less than 1/2 the ID would not consistently trigger the sensors.

so would you add more sensors or use a smaller diameter pipe?

keeping several various diameters of pipe and not permanently affixing the leds and sensors would be my choice.

any chance of seeing a wiring diagram?

i also don't think this could be done for less than a twenty.
radioshack parts, then add pipe,wire and tape, plus tax or shipping.

[clide]

Pretty much the same idea as http://home.earthlink.net/~jimsluka/Jims_chrono.html isn't it? Except jimmy's uses ambient light

[singularity]

well i have one very similar chrono to this except it used IR leds and IR photo transistors. the smallest round i have successfully chrono'ed is a 6mm airsoft BB (have not tried metal bbs). mine appears to be very accurate plus or minus 10 fps (airsoft gun professionally chrono'ed at 330 fps ghetto chrono says 333, 333, 322)

ps - all the parts can be got a lot cheaper at digikey.com

[iknowmy3tables]

so uhh why do you use the leds?
sorry but I suck at electronics
are you measuring the interference of the light

[paaiyan]

Look people, speed is distance divided by time, i.e. Miles/hour. If you place the two led's two feet apart, you hook them up to the input device, I assume you're using a mic input on the computer. As long as the light reaches the sendor uninterrupted, there is input into the mic jack, when the beam is broken, it registers as a flatline in the audio program. You measure the time between the beginnings of the flatlines in the program. Do a couple calculations, and you have fps. It's pretty simple guys.

[Novacastrian]

mmmm, sounds a LOT better than shelling out bucket loads of hard earned,
i am shite with anything other than basic electronics but i think i may just try this.

[paaiyan]

I occurs to me that this may only work up to a certain speed. At some point that light will not be blocked long enough to register, but I couldn't tell you how fast that would be. Just have to experiment with that.

[jimmy101]

I like mine better (I know, I'm a jerk ) Like Clide linked to.

It uses fewer parts. (No LEDs and no batteries.)
It can be moved from gun to gun.

The smallest thing I've chrono'd are standard steel BBs. Spuds are easy because they are so large.

The methods that use sound directly also work well.