SpudFiles

Seeing multiple versions of a homemade chronograph I might as well post my idea. The usually homemade chrono uses two microphones and paper or metal targets, the targets have to be spaced correctly usually a 6 to 20 feet part and the microphones might pick noices for valves opening and you have to pick out the highest point on the out put graph.

Why not use switches? namely the small roller arm switches, with a coupler a piece of pipe the size of your barrle is put over the end or your barrle its about 16-18" long. With a rotarty tool or drill to holes are made in the side a foot apart with some scrap PVC the switches are attached so the roller is just inside the bore of the barrel. As the projectile comes down the barrel it will hit each switch sending a spike to the usually audiacy program with a foot between spikes on the graph I think it would be easier to calculate the speed of the spud being its only the time it takes to go one foot. In the diagram I have the two switches hooked up as N.O. with a 1.5volt battery(AA) between the switches and the audio jack, now that I think of it I that even might not be needed. Just closing the contacts should probably make a spike on the graph.
The soft ware:
http://audacity.sourceforge.net/latest/ ... -1.2.6.exe

The math I think would be, 1 second divided by the time between the spikes in audiocy,
example the projectile takes 0.01 seconds to go one foot so
1second / 0.01sec = 100fps
or
1sec / 0.002 = 500fps
or
1sec / 0.0035 = 285.7fps

So think it might work?

DeathBlade wrote:Why not use switches? namely the small roller arm switches, with a coupler a piece of pipe the size of your barrle is put over the end or your barrle its about 16-18" long.

Just mechanical wear and some small drag on the projectile really. Your biggest challenge may be to get the switches mounted where they will actually switch with a shot, yet not be up too far into the barrel.

I would probably setup a little voltage divider and a capacitor on the output so as to just catch the AC signal and not send the large dc offsets into the sound card.

One problem with this setup is that there is a good chance that the first switch will not have opened back up before the second switch closes.

Lets see, smallish spudgun firing at 300 FPS. Switches 1 foot apart. Time between switches 0.033 seconds. I doubt the spring in the switch will open it that fast. At 1000 FPS the time interval would be 0.001 second. No way the first switch can open that fast. So, chances are that you won't be able to see the second switch close because the first switch will not have had time to open.

You could get a bit fancier and do as starman suggested, use a pair of capacitors and a pair of resistors so that the switch closures cause a voltage spike of much shorter duration than the actual time the switch is closed.

Or, you could use the two stereo channels of the sound card, one channel for each of the switches. That way it doesn't matter if the first switch is still closed when the second switch closes.

A couple of phototransistors or CdS photocells seems a lot easier, just as cheap, and much more flexible. (It would be tough to measure a very small projectile, like a BB, with mechanical switches.)

BTW, the signal you get with mechanical switches won't be a nice square wave, the contacts will bounce and the signal will have a fair amount of noise in it. Not a big deal really since you only care about the first rise (or fall) in voltage.

BTW2, you are right (if I'm understanding what you said), the battery would not be needed if you use the MIC input to your soundcard. The MIC input is mono and powered but usually uses a stereo (3 conductor) plug. You would have to check if the power is on the plug's tip or ring connector. If power is on the tip then the switches should connect the tip to ground. If the power is on the ring then the switches should probably connect the ring to the tip. (Usually only the line input is stereo, and uses a stereo plug. The line input is unpowered so it'll need the battery.)

BTW3, "usually homemade chrono uses two microphones and paper or metal targets" Around here photodetectors are a bit more common than the audio methods though both are used. Commercial shooting chronometers are generally photodetectors. For an audio based spud chronometer the paper is generally omitted and the round is just bounced off a hard surface like a board. The muzzle noise of most spudgun is generally so loud, and there is rarely any sound before the muzzle noise, that paper at the muzzle is not needed. Given the size of most spudgun projectiles, the paper at the end of the range isn't needed either, just a hard surface.

Hey! The switch idea was mine first

(granted mine involved hair-thin bits of wire)

I say try it, I'd love to see how it works out. There's no reason why it shouldn't, although you'll want to throw a switch debounce circuit in there.