I want to build a chronograph for measuring velocities of rifle projectiles. I initially wanted to use optical sensors, but these are fragile to light disturbances etc so I want to go for Hall effect sensors instead.
To detect bullets moving at 900 m/s (~3000 ft/sec) you would obviously need a sensor with high sampling frequency. Some expensive ones I have seen have a measuring frequency of 100kHz and most cheap ones I have seen lies at 10kHz. Assuming a 0.03 m long bullet velocity of 900 m/s you'll need a minimum of 30kHz measuring frequency to be sure you detect the bullet, right?
The Arduino has it's limitations as far as sampling frequency goes, but if I use i digital hall effect sensor, I can read values quicker if I am not mistaken. A digital hall effect sensor uses a pre set hysteresis to detemine the values for low and high state? How can you change the "sensitivity" of such sensor?
I hope this project is doable with the arduino, but I dont have much knowledge about sampling rates and the capabilities of the Arduino. I have tried to learn by using the search function and there are many similar projects but they are using optical sensors.
According to my understanding of a hall effect sensor:
The bullet travels over a hall effect sensor, the electromagnetic field is disturbed causing a change in voltage across the hall element.
Maybe there are hall effect sensors with a build in magnet and ones without. The ones without will only detect magnets in close proximity.
From the ad, it seems pretty clear that there are two light sensors in the base unit, perhaps detecting laser light reflected from spinning mirrors. See the slots in the housing and note the comment: "Extra wide sun screens for added shading in full sun operation".
To detect bullets moving at 900 m/s (~3000 ft/sec) you would obviously need a sensor with high sampling frequency.
You want two detectors a known distance apart. The advert for an optical system is measuring the flight time of the bullet between the two sensors. Optical sounds a better bet than Hall Effect to me. The optical system will probably work with the bullet passing further from the sensor and also does not depend on material type.
magnetically detecting a very fast moving non-magnetic object (lead bullet), maybe they can do this with inductive sensors? Still not trivial to get the exact moment the bullet is at an exact location, light is much easier for this.
survive the muzzle blast of firearms. That's a pretty tough one, too.
Probably on top of that the whole contraption is pretty rugged as well. At least it looks the part.
You don't have to worry about sample rates, because it's sufficient to determine the time when the bullet reaches the first and second sensor. Your sensors only should have a digital output, so that they can trigger a timer. Next the sensors must be fast enough to trace a bullet flying by.
True, can use interrupts for that.
The sensor is probably one of the hardest parts. It probably has to be inductive, looking at changes in frequency as the bullet passes by. Reacting to that nearly instantly (at 900 m/s a bullet passes a 1 cm wide sensor in about 10-30 µs depending on the size of the bullet) and reliably doesn't sound trivial to me.
@sjarls did you finally get to building your DIY chrony? I am thinking of doing the same. Assumed it would not be too difficult. But looks like hall effect sensor might not work? How about ultrasonic sensors?
d-tinkerer: @sjarls did you finally get to building your DIY chrony? I am thinking of doing the same. Assumed it would not be too difficult. But looks like hall effect sensor might not work? How about ultrasonic sensors?
d-tinkerer,
Don't hold your breath waiting for a reply from sjarls. He made 5 posts back in 2017 (the last 2 being in this thread) and hasn't been heard from since.
Assumed it would not be too difficult.
If it was not difficult, then they would be on sale for a few £/€/$.
How about ultrasonic sensors?
Another non-starter:
Using sound to measure something that could be supersonic isn't going to work.
