Tubeless chronograph ideas wanted

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Hi guys

Ok, I've already made an ATMega88-based chronograph for measuring the speed of the BBs from my airsoft guns.
The programming and the circuit around the processor was easily enough made, but, as always for me, the mechanical stuff ended up terrible.

By "mechanical stuff" I'm referring to the tube which the user must shoot through, which has two points where the BB must break an IR light beam which is sensed by a group of phototransistors... Long story short, this arrangement isn't very stable in the long run and really hard to "manufacture" easily. So...
It looks like this:
http://imgur.com/n9ETu

What I was imagining was to replace the tube with a vero-board with two parallel lines of light sensing components (phototransistors or photo diodes).
The idea would then be that once every .5 seconds or so, the processor reads the light level from both and use it as a reference value. When the BB is then shot, the shadow it casts on the photosensors as it passes each of the lines would be compared to the reference value and trigger the timer to start/stop.

My question is: Based on the collective experiences of people on this forum, do you guys think this idea is viable? Are the photo sensing components sensitive enough, and is it possible to perform A/D-conversions fast enough to trigger on the passing BB with any decent accuracy?

Also, as a bonus question, do any of you have suggestions to what the photo sensing and amplifying circuit could look like?
I imagine the two parallel lines of sensors to each consist of 3-10 transistors/diodes and some sort of amplification before the signal enters the A/D circuit in the AVR...

Note: The BBs are expected to move 50-250 m/s (that's 150-850 fps, roughly).

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Hmmmm....

http://nutsvolts.texterity.com/n...

It looks like all of the detection in their setup is done in the analog circuitry. The avr microcontroller is used purely for timing.

- S

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Very interesting article and generally along the lines of what I want to do.
Except... I want to get rid of the IR LEDs and let the photosensors use only ambient light levels instead.
I am, of course assuming either daylight or indoor use.

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Malmkvist,
Forget the garden variety photosensors, they're too slow. You need something really fast like they use in fiber-optic work. Or....

Magnetize the BB's and shoot them thru wire coils. The coils will produce a current pulse as the BB's pass thru. The faster the BB, the bigger the current pulse.

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I wonder if you could make two metal detector type coil arrangements to shoot them through?

#1 This forum helps those that help themselves

#2 All grounds are not created equal

#3 How have you proved that your chip is running at xxMHz?

#4 "If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand." - Heater's ex-boss

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Lol, good trolling :D

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I thought airsoft bbs were plastic.

The largest known prime number: 282589933-1

It's easy to stop breaking the 10th commandment! Break the 8th instead. 

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They are...
I didn't think the other guys were quite serious. Sorry about the trolling remark if you were.
But yes, they're plastic, so I think we're stuck with light rather than magnetics.

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I'm not sure why illuminating the photosensor is a problem.

I'm guessing it is easy to have the first photosensor adjacient the end of the barrel, so that one can make sure the pellet does interrupt the light beam.

I'm guessing it is harder to insure that the pellet interrupts the second beam, hence the use, possibly, of an array of photo detectors to insure that the beam is interrupted at the far end.

I might suggest an alternative, (which I've not tried!).

Use an illuminated photosensor for the first trigger, as you are now using.

Instead of using a photosensor for the second sensor, why not have the pellet just strike a small plate, to which is attached a Mems microphone?

Now the exact aiming and accuracy doesn't matter, as long as the gun barrel's allignment is good enough to break the first beam.

Remember where you got the idea when you are rich and famous!

JC

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Sorry, I didn't realize that "airsoft" meant plastic BBs. But I was serious about the mag-BBs (most have a steel core & copper plating - in my experience).

The MEMS microphone, indeed any contact microphone, accelerometer or motion sensor, is a brilliant idea! Very forgiving of aiming and set-up errors.

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Doc, that's a great idea, actually.
Chuck, pardon my previous remark, then. I know that the original meaning of BB is a steel or lead pellet for a "real" air gun so there might be some confusion. Still, good feedback.

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Ohler balistic chronographs use photo diodes. They are the standard that other chronographs are measured against. Check out Ohler Skyscreens. They have the optics and the photo diodes available as an assembly. look here http://www.oehler-research.com/c...

Roger

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pin photo didode is the type of opto sesnor You need to use to measure velocity.

Photo diodes and photo transistors are simply too slow ( junction capacitance too high )

This also gives You a clue.

If circuit too slow to discern the pellet at speed try to improve the bias voltage to reduce junction capacitance.

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Hey, nice link to the Oehler-spares. Definitely worth considering for mech-noobs like me :)

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I hear that bullets only move a few feet when the bullet travels through water. Maybe build a small tank, fill it with water, and measure where the BB lands, and do some math to figure out the velocity.

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A Photo transistor may not be the fastest thing..

But Air-Soft is Low velocity any way.

4 or 5 photo transistors in a Parallel line will work OK. (for your start and stop sets)

Use a "Comparator" if to square wave them

And use your "INT0" and "INT1" pins to do the start and stop triggers (If you aren't already)

Nice Project

"We look for things.. Things that make us go."