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Old 02-21-2015, 15:05   #1
oneofthose
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Homemade chronograph

http://www.americanrifleman.org/arti...e-chronograph/

In case you're looking for a weekend project........

Check the link for a pictorial diagram. Please note the safety recommendations in bold below.

Quote:
Originally published in February 1940, American Rifleman.

A Simple Chronograph
By Earl Seidlinger

Almost every gunman who loads his own shells gets the urge to find out how fast his bullets travel. But by the time he actually gets under way he is ready to give up. Too many unnecessary complications loom in his way simply because he thinks he must use no end of gadgets and expensive equipment in order to achieve results.

Measuring the speed of a bullet is easy, and the outlay for equipment is almost negligible. To begin with, it is necessary to have a fairly good Model T Ford coil, an electric switch, two pieces of gas pipe from eight to ten feet in length and not more than 1 1/2-inch in diameter—3/4 inch being the best; also paper, wax, and your car battery.

In operation, you shoot the projectile between the two lengths of pipe, as shown in the illustration. The spark from the Ford coil jumps from one pipe to the projectile, and from the projectile through the wax paper to the other pipe, to complete its circuit. Then, knowing the number of sparks the coil emits per second, you can take that number and multiply it by the distance in feet between the holes punctured in the wax paper by the spark, and get the exact number of feet per second the projectile was traveling.

In assembling the apparatus, you first mount the two pipes parallel to each other on a heavy board or 2 x 4, making sure that they are fastened securely and in perfect alignment. To do this it is best to drill holes every foot or so, and screw them down. The distance apart you set the pipes depends entirely upon the size of projectile you are going to test. A Ford coil will throw a spark approximately a half-inch long, so to be on the safe side, allow 1/8-inch on each side of the projectile between the pipes. In this way you will be sure the spark will be there when you want it. This doesn’t give much room for inaccurate shooting, but if you arc any kind of shot you should have no trouble for such a short distance.

This test can be made outside or in your basement—but if you intend doing it in crowded quarters, be sure your backstop is strong enough to do the work. The stop can be made in the form of a target to aid shooting; otherwise it is only a safety feature.

Assuming that you have the pipes securely fastened, and everything lined up, you now take a piece of wax paper and glue it tightly (if you don’t the vacuum created by the projectile may tear it loose) to one of the pipes, so that the sparks will have to jump through it in order to complete the circuit. If you haven’t wax paper at your disposal, use any white paper and paint one side of it with molten wax, using an ordinary paint brush. (To remove the wax from the brush, hold the latter in hot water. The water will melt the wax and let it float to the surface.)

When mounting your gun to shoot between the pipes, it is necessary to keep the muzzle several inches, or even feet, from the beginning of the pipes, the reason being that the burning powder escaping from the barrel will spoil the test by burning holes in the wax paper the same as the spark will do. The distance the two must be separated will depend upon your gun and the type of powder you use. One or two shots will indicate the proper distance.

You should now be ready to try the apparatus. The only essential thing left to do is to determine the number of sparks per second the Ford coil emits. The mere mention of it sounds difficult, but in reality it is the simplest part of the business—if you have an ear for music. Take the coil in near your piano, or your neighbor’s piano, and start it buzzing. Then strike middle C on the piano, and tune the coil's buzzer, by means of the adjusting nut, until it sounds middle C. Middle C has 256 vibrations per second, which means that the coil will now give 256 sparks per second, because it throws a spark every time it vibrates.

You now connect the coil to your apparatus in the manner shown in the wiring diagram. Then turn on the switch and fire the shot. After firing, turn off the switch and measure the distance between the holes punctured in the paper—and start figuring.

For example: suppose the spark punctured a hole every four feet. You then multiply 4 x 256, and get 1,024, which means that your projectile was traveling at the rate of 1,024 feet per second. Or suppose the distance between holes happened to be, say, 4 feet 8 inches, and you had the coil sounding the C three octaves above middle C on the piano, or vibrating at the rate of 1,024 times per second. Four feet eight inches is 4 2/3 feet, so you multiply 4 2/3 x 1,024 and get 4,778 2/3 feet, so the speed of the projectile was 4,778 2/3 feet per second. The 2/3 foot is equal to 8 inches, so this also reads 4,778 feet 8 inches per second. Remember always to read the distance between the holes punctured by the sparks, in feet.

You may think four feet is a long distance between sparks. If so, take a higher note on the piano, and tune the coil to that. The different vibrations per second are given in the table herewith, and you can take any note you wish, and get the exact number of sparks the coil is emitting.

Be careful not to touch any of the apparatus when the coil is buzzing, or you will get a terrific shock. And be sure to use wax paper, for holes punctured by a spark through ordinary paper are invisible. Also, if you haven’t an ear for music, ask someone else to tune your coil. At least every other person you meet can do it.

Vibrations per second of notes on a piano, going up from Middle C:

C—256
d—288
e—320
f—341.3
g—384
a—426.6
b—480
c—5 12
d—576
e—640
f—682.6
g—768
a—853.3
b—960
c—1024
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Last edited by oneofthose; 02-21-2015 at 15:05.. Reason: crappy typng
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Old 02-21-2015, 15:34   #2
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This works because the Model T coil has an internal vibrator, unlike modern coils. Still, it could work with a more modern coil by using a left over distributor (the kind with points and a capacitor) driven by a motor of constant speed.

Google for 'trembler coil'.

Apparently these Model T coils are still available.

Pretty clever idea.

Richard
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Old 02-21-2015, 17:08   #3
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Quote:
Originally Posted by F106 Fan View Post
This works because the Model T coil has an internal vibrator, unlike modern coils. Still, it could work with a more modern coil by using a left over distributor (the kind with points and a capacitor) driven by a motor of constant speed.

Google for 'trembler coil'.

Apparently these Model T coils are still available.

Pretty clever idea.

Richard
Reminded me of "The Bullet's Flight From Powder To Target". Interesting read if you enjoy and appreciate the tedious nature of reloading.

I remember seeing a WWII era video of a chronograph used for quality control in a hidden underground ammunition plant. It had large paper discs mounted on a dowel a certain distance apart. The discs had markings and were perfectly aligned with each other. A motor would turn the dowel at a known RPM and a round would be fired parallel to the dowel, so it would pass through both paper discs near the outside edge. The holes in the two discs were compared with alignment makings on the paper. The faster the bullet, the closer the holes would align with each other. Some simple math would tell you how fast the bullet was traveling.

Different times......
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Old 02-21-2015, 17:27   #4
willie_pete
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It was a simpler time.

WP
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Old 02-21-2015, 17:46   #5
Brian Lee
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Quote:
Originally Posted by F106 Fan View Post
This works because the Model T coil has an internal vibrator, unlike modern coils. Still, it could work with a more modern coil by using a left over distributor (the kind with points and a capacitor) driven by a motor of constant speed.

Google for 'trembler coil'.

Apparently these Model T coils are still available.

Pretty clever idea.

Richard
Nowadays it would be a lot smarter to use electronics to trigger the coil with a timer circuit based on something simple like a 74xx221 monostable multi-vibrator chip and a power transistor instead of using an electric motor & distributor. That way you can alter the sparking rate over a wider range while keeping it running at the set pace with much greater accuracy.

But if you know how to do that, you'd obviously prefer to build your own chrono using LED's & photo transistors & let the bullet break the light beams just like the store bought units do. Actually, truth be told, even if you don't know how to do that already, putting together a kit as described (if you could buy them) would still be easier & cheaper than building the contraption in the article.

A lot of people think modern electronics make things too complicated, but it's actually the opposite. Electronics rules everything today because it's so much simpler than the complicated (and usually hard to calibrate) mechanical systems we used to use to accomplish the same things.

If people are actually interested in building devices like this stone age chrono, maybe someone should start selling kits to build modern ones cheaper. They'd be less work to build.

Last edited by Brian Lee; 02-21-2015 at 17:55..
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Old 02-21-2015, 18:03   #6
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When I was young a friend of mine had a vibrator coil, we hooked some wires to it, hooked an ice pick to it with an alligator clip and were having great fun watching the arc jump pretty wide gaps.

I got my turn and was enjoying myself as I inched my hand unknowingly down the wood handle, when I reached the metal ferrule that joined the pick with the handle I got a jolt I remember vividly to this day.
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Old 02-21-2015, 18:10   #7
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I haven't seen a chronograph that uses the 'break the beam' concept although it could probably be done. But it implies a bullet path that might be difficult to achieve. The normal approach is to detect the glint as the bullet passes overhead and placement isn't a huge issue. It has to be in the approximate area and the lighting has to be right but, clearly, these systems work.

Then there is the MagnetoSpeed which uses inductive pickups. A VERY nice chrono for rifles - useless for pistols.

Ah, electronics... You are correct, a 555 timer and a power transistor would do the job quite nicely for the ignition-coil/wax-paper approach. The problem is, of course, how many average shooters have the skillset? I do and I still just go out and buy a chrono. The sensors are the problem, not the microcomputer and display.

Still, I admire the old approach. People were able to make things that worked using the components of the day. Real ingenuity!

Richard
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Old 02-21-2015, 19:41   #8
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Quote:
Originally Posted by F106 Fan View Post
I haven't seen a chronograph that uses the 'break the beam' concept although it could probably be done. But it implies a bullet path that might be difficult to achieve. The normal approach is to detect the glint as the bullet passes overhead and placement isn't a huge issue. It has to be in the approximate area and the lighting has to be right but, clearly, these systems work.

Then there is the MagnetoSpeed which uses inductive pickups. A VERY nice chrono for rifles - useless for pistols.

Ah, electronics... You are correct, a 555 timer and a power transistor would do the job quite nicely for the ignition-coil/wax-paper approach. The problem is, of course, how many average shooters have the skillset? I do and I still just go out and buy a chrono. The sensors are the problem, not the microcomputer and display.

Still, I admire the old approach. People were able to make things that worked using the components of the day. Real ingenuity!

Richard
I went out and bought one too.

What I was referring to by "break the beram" is that my own chrono uses sunlight as the light emitter and that's the beam of light that gets broken. When it's cloudy, I have a set of LED's that get mounted abve the chrono and they provide the light source, so it's still a matter of breaking a light beam to sense the bullet. It's exactly the same thing you just referred to as "detecting the glint" which is when the light received by the photo transistor gets momentarily reduced. The sensors are nothing but photo transistors, and those are dirt cheap and easy to work with. You can get some at Radio Shack for less than a buck that respond more than fast enough to do it. You can also do it without any microcontroller if you're willing to do some math (as with the mechanical system) after the fact, to calculate what the velocity was.

For people who've studied even just a little electronics, it's less of a skill set than is required than to make a complicated mechanical contraption and actually get it to work right. Back in the sixties & seventies people who knew nothing about electronics used to buy Heathkit TV's and stereo's and build them without really knowing what they were doing, and the stuff worked good.
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