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DIY Chronograph, am I doing it right?

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DIY Chronograph, am I doing it right?

Unread postAuthor: potatoflinger » Wed Jun 18, 2008 8:33 pm

A while ago I decided that it would be cool to make my own chrono, so I started researching other chronographs that people have built, but there are a few questions that I have about building one:

1. Do I need a power source on the circuit with the photoresistors in it?

2. Do I hook up the photoresistors in series? If not, how do I hook them up?

I made a very crappy diagram of my design, but I think you should be able to get the gist of it. I do realize that I left a few things out of the circuit with the LEDs in it, but I know how to hook them up, I just need help with the photoresistors.
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Unread postAuthor: TurboSuper » Wed Jun 18, 2008 9:21 pm

Photoresistors should be in series with a constant-value resistor. That way, when the resistance changes, you'll get a voltage spike (or drop, depending on how the photoresistors respond to light) across the photoresistor. You'll want one of these circuits for each photoresistor.
Or mabye you could just wire both photoresistors in series, since ideally one will always be constant. But the first way will produce much clearer results.

Of course, that's just how I see it, I'm sure there are other ways.

And DO NOT hook LEDs up directly to anything more than a watch battery, they'll be fried instantly. You need to put a resistor in series with them to keep the current to a proper level.
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Unread postAuthor: Ragnarok » Thu Jun 19, 2008 5:10 am

You'll get better results with phototransistors, as they have a faster response time. They're what I used for my own DIY chrony, although it was a little stumped by my air rifle, probably because the pellets are unusually small and fast - it has worked on faster objects, although they were considerably larger as well.

I'm looking at upgrading it to see if I can get it to work on the pellets. If it can track an ~800 fps .177 pellet, then that's going to be as much as I'll ever need.
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Unread postAuthor: TurboSuper » Thu Jun 19, 2008 6:52 am

Wouldn't the fastest response of them all just be putting microphones near two pieces of paper?
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Unread postAuthor: jackssmirkingrevenge » Thu Jun 19, 2008 6:59 am

This method by ant is accurate as far as I can tell and easy to set up.
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Unread postAuthor: Ragnarok » Thu Jun 19, 2008 7:03 am

TurboSuper wrote:Wouldn't the fastest response of them all just be putting microphones near two pieces of paper?

You get other problems doing that - for starters, you have to compete with background noise and to some extent, uncertainty about how the waveform should be interpreted.

You really for best results need the two "gates" moderately close together, and if you tried a mic/paper arrangement at those short distances, both sounds would merge into one.

I have tried the method JSR has linked to with my air rifle, and the results seem decent enough, although I am slightly concerned about drag losses incurred between the muzzle and target.
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Unread postAuthor: jackssmirkingrevenge » Thu Jun 19, 2008 7:05 am

Ragnarok wrote:I am slightly concerned about drag losses incurred between the muzzle and target.


In practical terms, you get the average velocity over the distance fired, which is a more usable assesment of your launcher's power than what you get at the muzzle - unless of course you're content with shooting at point blank range :)
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Unread postAuthor: Ragnarok » Thu Jun 19, 2008 7:45 am

@JSR: Certainly, but for attempting to determine whether my Air Arms sits within the tight 12 ft-lb UK power limits, the important figure is muzzle energy.

I know it's a bit silly worrying about that when I've got spudguns that make a mockery of my air rifle's power, but equally, should a policeman ever turn up at my door asking about "We've had a report of a high powered airgun being used", being able to show them a completely UK-legal air rifle instead would be useful... :twisted:
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Unread postAuthor: TurboSuper » Thu Jun 19, 2008 8:52 am

You get other problems doing that - for starters, you have to compete with background noise and to some extent, uncertainty about how the waveform should be interpreted.

You really for best results need the two "gates" moderately close together, and if you tried a mic/paper arrangement at those short distances, both sounds would merge into one.

I have tried the method JSR has linked to with my air rifle, and the results seem decent enough, although I am slightly concerned about drag losses incurred between the muzzle and target.


Very true, but any simple DIY chrony is going to be far from perfect. At lest the paper method will give you a ballpark for almost any projectile size.
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Unread postAuthor: jackssmirkingrevenge » Thu Jun 19, 2008 9:03 am

Ragnarok wrote:I know it's a bit silly worrying about that when I've got spudguns that make a mockery of my air rifle's power, but equally, should a policeman ever turn up at my door asking about "We've had a report of a high powered airgun being used", being able to show them a completely UK-legal air rifle instead would be useful... :twisted:


Sounds like a plan :)

Since manufacturers have gotten all paranoid about UK legislation however, I doubt your AA's doing much more than 10 ft/lbs.
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Unread postAuthor: potatoflinger » Thu Jun 19, 2008 9:06 am

TurboSuper wrote:And DO NOT hook LEDs up directly to anything more than a watch battery, they'll be fried instantly. You need to put a resistor in series with them to keep the current to a proper level.

Yeah, I did know that, I said in the post that I left a few things out of the diagram. I am going to add a switch to the LED circuit too.

If I use phototransistors, will I need to have a power source connected to them?

Also, would response time really matter since both of the resistors would have a similar response time? There would be the same lag for each of them, so wouldn't the lag cancel itself out?
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Unread postAuthor: jimmy101 » Thu Jun 19, 2008 12:07 pm

http://www.inpharmix.com/jps/Jims_chrono.html has just about everything you need to know about making a homebrew optical (shadow) shooting chronometer.

The photoresistors (CDS cells) in the OP might work, they are very slow devices but you don't need 100% response, a couple percent may well be enough.

Phototransistors or photodiodes respond much quicker and cost about the same. You should be able to get what you need at RadioShack. You really don't need a light source for the detectors, ambient light should be more than enough since you probably won't be trying to chrony the gun at night.

With phototransitors you can wire them as shown in the OP and plug it into the MIC input of a sound card. No external power required since the MIC input is powered. (Need to check if the tip or ring of the stereo plug supplies power.)

To use a line input (which is unpowered and typicall expects a 1~2 volt peak to peak signal) to a soundcard just put a 1.5V AA battery, and maybe a 100~1000 ohm resistor, in series with the detectors.

Don't build the detector into pipe. Slice up a piece of pipe and make a clip-on detector. The detectors can just be mounted in holes in the chopped up pipe. When clipped to the barrel, the detectors are offset from the projectiles path by the thickness of the pipe.

Chopping up a piece of 2" PVC pipe to make the detector holder.
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Completed detector clipped onto a 2" barrel (the green pipe). The detectors can be mounted directly in the holes in the modified 2" pipe, no need for the light tubes. This particular setup lets you move the detectors from one clip on mount to another.
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That should work with just about any ammo. The clip-on design is probably the easiest way to get chrony readings for small ammos like 0.177" BBs since it is much easier to get the detectors alligned with the barrel. Just need to make a new holder for each barrel diameter.

One thing about this design, the clip-on setup makes alignment with the ammo's line of flight easy, but the speed you measure is relative to the gun. If the gun recoils then the measured speed is a bit low since the detector is moving in the opposite direction from the ammo. Usually this isn't much of a problem, the momentum of the ammo and the gun are the same but in opposite directions. So the velocity of the gun (and the detector) is the velocity of the ammo times the mass of the ammo divided by the mass of the gun. Since the gun is usually much heavier than the ammo, the velocity of the gun is much less than the ammo. If you are really concerned about the gun moving just put the breach on the ground and fire the gun straight up, the gun can't recoil much when fired like this. Shooting chronometers usually measure velocity relative to the earth, not to the gun.
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Unread postAuthor: potatoflinger » Thu Jun 19, 2008 3:52 pm

Thanks, that really helped a lot. I just bought all of the parts from radioshack and I started building it.
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Unread postAuthor: judgment_arms » Thu Jun 19, 2008 4:27 pm

jimmy101 wrote:The photoresistors (CDS cells) in the OP might work, they are very slow devices but you don't need 100% response, a couple percent may well be enough...

Actually CDS cells don’t work, I tried them ‘cause radio shack didn’t have phototransistors, all it does is give you a crap load of static...
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Unread postAuthor: jimmy101 » Fri Jun 20, 2008 12:56 am

judgment_arms wrote:
jimmy101 wrote:The photoresistors (CDS cells) in the OP might work, they are very slow devices but you don't need 100% response, a couple percent may well be enough...

Actually CDS cells don’t work, I tried them ‘cause radio shack didn’t have phototransistors, all it does is give you a crap load of static...

Do you have any details on the CDS cells, like their resistance? Did you use the ~2.5V power from the MIC input or did you add a battery? Did you measure the voltage drop across the cells in the light and dark?

I've never tried the CDS cells even though I've got a whole box of'm. If it was done correctly and they really don't work I'll add it to my dyi chony page. To bad, they are cheap and RadioShack carries them.

A typical MIC input has an impedence of a thousand ohms or so. The CDS cells will have to have similar resistance. A 20K ohm light / 100K ohm dark CDS cell probably won't work since the resistance is so much bigger than the input resistance to the sound card.
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