Howdy folks,
I'm trying to make a chronograph that measures the speed of a projectile like this one:
https://www.amazon.com/Caldwell-720001-Ballistic-Precision-Chronograph/dp/B00HTN5DTEI want mine to work with anything that shoots something with a velocity between maybe 200 and 4000 feet per second with 1 fps precision. I want the physical 'window' I have to shoot my projectile through in order to get a reading to be maybe 5"x5" at the smallest.
Overview of current strategy:
Right now, I plan on using a photodiode, probably in 'photoconductive' mode, along with a transimpedance amplifier as my main sensor. The sensor would detect changes in light caused by the projectile.
There will of course be two of these sensor things a fixed distance apart. I'll call each of the two sensor things a 'sensor plane'. When a projectile crosses the first sensor plane you start a timer, and when it crosses the second sensor plane you stop the timer. You measure the time between when each sensor plane is triggered by the projectile, divide the fixed known distance by that time, and get your speed. Easy.
I understand how to design and implement the electronics part of the project, I've done photodiodes reacting to fast changes in light before in other systems. However, I'm unsure of how to do the 'light' related stuff in terms of placement of the sensor, how many sensors to use per 'sensor plane', and where or if to include a light source. Remember, I want a reliable 5x5" ish target area through which I shoot to get the thing to work. I also want to guarantee that the sensor only senses a projectile starting the moment the projectile crosses the 'sensor plane'. In other words I want the 'sensor plane' to actually be a plane, and not an upwards facing cone or something weird and unpredictable like that.
Specific Questions:
As far as I can tell there are two general optical strategies. Either you detect a decrease in light due to the projectile casting a shadow over the sensor, or you detect an increase in light due to the projectile flashing light back at the sensor. I see pros and con's with each approach, and I need someone with more experience to tell me which approach is more normal / easy to implement / what the commercial units do.
Other questions:
For each 'sensor plane':
If I go with detecting a shadow:
Do I just use the suns light or do I make my own LED array?
Do I bury my sensor in a hole/slit in the body, or do I put it out in the open? How do I make sure the thing is detecting a projectile passing the plane, and not a projectile that has 'almost' reached the ideal sensor plane?
Do I need multiple sensors facing multiple LED's, or just one sensor?
Does it work more like a break beam sensor, or like a high speed high gain light intensity sensor?
If I go with detecting a glint:
Do I need an LED shining in the same parallel direction as the sensor?
What LED / photodiode wavelength can I use so that sunlight doesn't interfere with this strategy too much?
Does this really work? It seems like the signal from a 'glint' is probably pretty weak and could be washed out by the sun?
Do I need to have my thing in a dark enclosure for this strategy to work?
Should my sensor and possible LED source point towards the ground or towards the sky?
Thanks in advance! I'm really looking for someone who owns a chronograph and who can tell me how it was constructed in detail regarding the placement and quantity of optical components, or someone who has built one of these successfully before and has good evidence that it was accurate. Quick sketches would be great if you have an idea. Thanks in advance!