I haven't the skills to design the circuitry for this, but believe it could be done. Anyone care to weigh in?

Ultra-simple electromagnetic sensors instead of optical

Chronographs today require sensors in front of the muzzle, either downrange (like most optical or infrared sensors) or just in front of the muzzle like the magnetospeed. The Labradar is the sole exception, but is not available yet and is projected to cost over $500. The magnetospeed is not much cheaper at $400 and requires mounting on the barrel (not possible on some firearms and likely to change barrel harmonics -point of impact- on most).

I envision a chronograph whose electronics would set on the shooting bench with sensors of simple wire loops that would be only a couple of feet in front of the shooter (picture a couple of 2 foot diameter loops of wire mounted on a yardstick) that could be positioned without having to call the firing line cold.

The device would detect the passage of the bullet through the two loops by the fact that a magnetic field generated by current through the two loops would be affected by the passage of the metal bullet and the time difference would allow the electronics in the chronograph to calculate speed just as today's chronographs do with optical sensors.

The advantage would be that 1) if somehow a bullet did hit the sensor, repair is no more difficult than getting some wire and making a new loop and 2) sticking a 3' long yardstick out in front of the firing position would not require calling the line cold. (The muzzle blast would not disrupt the magnetic field, but the passage of the bullet would.)

The question I have for the electronically savvy reader is: When you pass current through a 2' diameter single conductor loop, to create a magnetic field and then pass a smallish object through that field, how sharp is the magnetic (induced/inductive) feedback on that current? Is it great and sharp enough to detect easily? If the current-carrying loop could not detect the induction, a secondary loop behind each of the primary ones might be required, but this would still be an easily repaired and conveniently positioned set of sensors, which is the main advantage I seek to gain over current optical chronographs.

Any ideas?

Thanks for reading.

Lost Sheep

Check this one.

http://www.midwayusa.com/produ...cm_vc=ProductFinding

quote:

Originally posted by larrys01:
Check this one.

http://www.midwayusa.com/produ...cm_vc=ProductFinding

Thanks, but no. As I mentioned in my original post, it cannot be mounted on certain firearms and likely changes barrel harmonics, affecting point of impact. Having it further away from the barrel is not viable because the sensors have such a limited range. That is why I am thinking of the wire loop.

It also fails for convenience factor (mounting and dismounting) and price ($400).

But thanks for the suggestion.

Lost Sheep

One fly in the ointment!
The thing passing through a coil has to be freeous in order to generate an electric field.

So unless you shoot iron clad bullets it is a no go.

The magnetospeed thingy generates a magnetic field and then measures disturbanses in that to detect a bullet passing.

So far it has worked with Barnes, Speer, Hornady, and Sierra without missing a shot. Also Group size and POI has not been affected.

Sorry Lost, it won't work. To detect non-ferrous metals magnetically you have to rely on eddy currents. There are a couple of companies that do such things, TI has a chip that will detect objects by eddy currents but the coil has to be approximately the size of the object being detected. Keyence also does eddy current detection, you can get an idea of the size of coil versus object to be detected.

quote:

Originally posted by The Dane:
One fly in the ointment!
The thing passing through a coil has to be freeous in order to generate an electric field.

So unless you shoot iron clad bullets it is a no go.

The magnetospeed thingy generates a magnetic field and then measures disturbanses in that to detect a bullet passing.

Note that the primary coil does have current passing through it. This should then create "a magnetic field generated by current through the two loops" (the front loop and the rear loop). The conductive bullet (copper, lead, whatever, but not plastic and such) should create an abrupt change in the permeability of the space within the loop.

My question is, how much current would have to be in the primary loop of what loop size to detect the passage of a small (20 to 400 grain non-ferrous pellet) pellet at 200 to 4,000 fps

Lost Sheep

what is the magnetic permeability of muzzle
blast with it's plasma like gasses ?.

quote:

Originally posted by hawkins:
what is the magnetic permeability of muzzle
blast with it's plasma like gasses ?.

I though of that, but was hoping (with no basis for the optimism) to luck out on it.

If Magnetospeed gets away with it, why can't I?

Thanks for bringing it up. At least I know y'all are thinking.

Lost Sheep

I really don't know exactly how magneto speed has set there system up, but lets say that it is 2 small coils that act as sensors inside there platform. In order to detect the bullet going over them, a small magnetic field is generated by the coils, that the bullet would break through in order to measure the speed, so the "gap", distance from coils to bullet is important, and would be determined by the strength of the magnetic field generated by the coils.

Now you wish to have the "gap" roughly 1 foot (assuming your 2 foot coil diameter). You would have to generate a large magnetic field that would extend out that far in order to detect an object going through it.

So, yes this can be done, but it will probably cost more than $400 in just the materials by the time all the bugs are worked out.

quote:

Originally posted by mgoodrich:
I really don't know exactly how magneto speed has set there system up, but lets say that it is 2 small coils that act as sensors inside there platform. In order to detect the bullet going over them, a small magnetic field is generated by the coils, that the bullet would break through in order to measure the speed, so the "gap", distance from coils to bullet is important, and would be determined by the strength of the magnetic field generated by the coils.

Now you wish to have the "gap" roughly 1 foot (assuming your 2 foot coil diameter). You would have to generate a large magnetic field that would extend out that far in order to detect an object going through it.

So, yes this can be done, but it will probably cost more than $400 in just the materials by the time all the bugs are worked out.

Thank you. I am hoping that since, in my design, the bullet passes THROUGH, instead of OVER the coil generating the field as well as through the detecting coil that the sensitivity would be compatible with (fingers crossed) current chronograph electronics. Simply replacing the light sensors with the magnetic ones might be possible.

Mgoodrich, another thought.

Of course, another problem is focus. Magnetospeed's sensors have to be close to the bullet's flight path to get a sharp spike in sensing. Since the bullet passes within the loop, I expect, while my spike may not be sharp, the two spikes in sensing will be similar. Also, since the distance between sensors could conceivably be feet instead of inches, the sharpness of the spike would not be so critical for accuracy.

Lost Sheep