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windywindmill
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Mar 2011
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#1
DIY Chrono
1 July, 2011, 13:46
More than wanting to build a chrono cheap this project was more a, can i build it and can it be made accurate enough to compare with a real chronograph.
This has been on the shelf for a while but after a recent post about homemade chrono's and some amazing ideas bachelarius and from others i thought i'd pop this on and see where it can go.
first off, the idea and basis was not mine, i've just simplified it and hope that with a bit of input from others it can be made better.
The idea is basically:
Two Light sensor in series connected to a microphone jack, these are placed along a tube at set distances (4 inches) with holes in the opposite side of the tube to allow the light to go through. This is then connected to the mic in port of a computer. A pc app running will recordered the sound input from the mic, when a bb travels through the first sensor a peak will appear then another peak when it travels past the second sensor. Then the time it took to travel past (it'll be a a very small figure, something like 0.0365 seconds.) then we work out the speed from this).
Take the distance, 4inches and time this by 3 (this will work it out in feet per second (fps)) which is 12
so speed = distance / time
12 / 0.0365 = 328.767123 rounded up is 329 fps.
The things to watch out for is the distance in the light sensors - you will need a micrometer to measure accurately, and measuring the audio peaks when the bb passes the light sensor.
Also using standard photo transistors (light sensors) makes it too sensitive to external lighting - so Infrared Phototransistors - these can be brought for about a quid from electronics shop.
This was it built up, this does not have the infrared diodes on the opposites sides as they were being used for something else.....
Building the thing....
You''l need a tube about 5" long, ive used an old 3/4" wide bit of metal pipe
a microphone jack
some wire
2 x Infrared Phototransistors
2 x Infrared Diodes (i just used ones from old remote control)
3 x AA batteries
Get the tube and measure as accurately as humanly possible a distance of 4" and mark with a dot.
Drill these dots with a 5mm drill bit through both side of the pipe.
you will need to mount the photo transistors in theses holes, i'd used hot glue or epoxy so that they dont move around when they are in there. and then do the same on the opposite side with the diodes.
So it should be 2 photo transistors one side and 2 diodes the others side of the pipe.
The wires will then need connecting this is the diagram
Also a separate curcuit to power the diodes using the batteries.
Now that its built up you can test it with a multimeter to check that the current flows though the photo transistor when the diodes are on.
Now plug it in to the mic port, used a audio recorder like audacity , record the mic input and shoot through the tube.
You shoot get two peaks, on going through the first and one through the second phototransistor, get the time it took going through the sensors by highlighting the distance from one peak to the second peak (you will have to zoom in a fair bit to identify the peaks) then get this distance, it says at the bottom of audacity.
You can experiment by putting resistors in and having smaller holes to stop so much Infra Red light passing through the tube to get better identifiable peaks.
Going forwards.....
Ideas from Bachelarius
Automated software that will measure the peaks.
Using the usb as a power source.
Using a com port rather than the sound to have the sensors trigger the pc to timestamp and measure the time took and work out the fps.
iPhone input? use the headset mic input and have an app measure?
. .
Bachelarius
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#2
1 July, 2011, 20:59
Re: DIY Chrono
Awesome! Glad you got the thread running :D
Your design also looks far better than my one, which I sent you earlier, but I just need to clarify a few things:
How exactly do the Photo Transistors work? Trying to model the hardware in my head, so it's easy to know what to look for in the coding bit.
Do the transistors basically act like an N-channel fet, with the gate replaced with a light sensor? So, light available = circuit on?
Also, what is a response rate of these things? Lets say that if we use a laser, the beam will be about 1mm across. That means that (depending on the sensitivity of the opto-transistor) only 5-5.5mm of the BB will be fully cutting the bb.
So, if the BB is travelling at 100m/s (328fps), it covers 5mm in 0.00005s
That's a blip at about 20kHz, can be spotted at a sample rate of 40kHz easily. Yay :D
Lets say we want this thing to top out at 200m/s. Now it covers the 5mm in 0.000025s.
This blip will be at 40kHz, meaning that we need a higher sample rate. Any music/audio buffs care to confirm my ramblings?
But yeah, can the transistors keep up with those time frames?
I would personally avoid using LEDs due to their massive spread. You can see how a spread of even 1mm effects the necessary sample rate required to spot this thing. Anything larger would make things worse.
Of course you can counter this by putting the sensor further away from the point of crossing. But whatever works man, I'll try using LEDs first and if the results are crap, try something else.
Also, make sure that reflections from the BB will not reach back to the sensors and give false readings, so put partitions around the sensors, or just stick them in little tubes.
USB as power source is easy btw, the outer-most pins are 5v and GND, so use them drop the voltage to something that the 2 LEDs/lasers can handle and off you go
Finally, can you get stereo, 3 contact Mics? Or rather, does a PC mic jack have all 3 contacts inside, so it can take standard TRS connectors? If so, we can wire each sensor to the Left and Right channels and then we can identify which sensor was tripped, i.e. better error detection.
Anyway, those are my random ramblings for now to kick this thread into action :D
Oh yeah, and as for smartphone compatibility, I see no reason why not, just worried about the sample rate again.
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Bachelarius
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#3
3 July, 2011, 20:02
Re: DIY Chrono
Actually, looking at it, where is the current coming from? Do you have any links to relevant info? May need to hook those photo-transistors up to an external power source of about 100mv, if using the mic port or 1v if using LineIn, otherwise I can't see how you will induce a current in that circuit.
Been looking at how people have been managing to make
Sound Card based Osciliscopes
, which is in essence what we are doing.
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windywindmill
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#4
3 July, 2011, 20:21
Re: DIY Chrono
The mic port provides the current, with the current flowing through the phototransistors the mic does not pick anything up, when the bb passes it breaks and that's where it's picked up as a peak or sound .
Will hook it back up and post a pic of the resulting sound wave when I get chance.
The spread of the light is a problen, laser would give a clear beam but much more expensive , putting them in tubes to channel the light sounds like it'll work best.
Can't find too much about the response rime other than 'fast' which ain't that helpful
. .
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Bachelarius
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#5
4 July, 2011, 12:01
Re: DIY Chrono
Just ordered a
pair of these
to play with in that case. I have a load of 930nM IR leds which should match those nicely. Will see how they work and adapt the circuit if necessary.
According to the
datasheet
, the response time is in the range of 15uS, which is more than fast enough.
End of the day, if it works, it works, I don't need to ask how, hahaha. Oh and lasers can be bought quite cheaply, it's just tricky to build the drivers for them. And USB is a great power source for the LEDs btw, you can calculate what resistors you need using this
calculator
.
Last edited by
Bachelarius
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4 July, 2011, 12:07
.
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windywindmill
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#6
4 July, 2011, 14:33
Re: DIY Chrono
Are they not standard light phototransistors? The problem with normal light is that any ambient light will interfere with the results, the BPV11F ir phototransistors should do the job
. .
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Bachelarius
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#7
4 July, 2011, 22:54
Re: DIY Chrono
Nah, 900nM wavelength is a fair way below (above?) visual light (which ends at 800-850 or so), it's what I am using in my NVD project. My IR leds are ~930nm which is still in that peak area. Even if they do turn out to be normal light sensitive, I can put a section of Floppy disk material, which will filter out most of the visible light or just make it an enclosed unit, with a bb catcher on the end.
Looking forwards to messing around with these now, still can't believe it's that simple, makes no sense why no one else has done it yet :/ I reckon that even at 40kHz, we should be able to spot 600fps or so, will test with my air pistol once I get that far. Apparently it is possible to get as much as a 96khz sample rate, which should be perfect.
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Bachelarius
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#8
11 July, 2011, 01:03
Re: DIY Chrono
Good news!
I've been coding the software (in C#) and I now finished the basic GUI and have the Mic monitor working! Been a bit of a hurdle, but it works and is damn fast
Other stuff done is the splash screen, most of the graphics and a large amount of the structure.
Things left to do:
Make the hardware (parts arrived yesterday)
Make sure everything is properly commented up
Release basic v1.0 on GoogleCode (requires fully functioning FPS mode)
Set up decent version control, so it's not an insane mess of code
Make the Options/Setup screen, to match the software to the hardware.
Pick an input device
Configure single and double Mic channel setups (basically, use all 3 pins, tag a sensor to each one)
Configure the cutoff point between No BB and BB
Make a .ini file to store the settings in
Force of BB mode
Convert what I already have into a working FPS measure
Test accuracy
Develop MPS and Joules settings
Develop Shot Consistency readout
Rate of Fire mode
Develop the RPS mode
Add RPM mode
Work on RoF consistency readout
Work on saving the data and doing something cool with it
Plot graphs etc.
Sounds like a lot to do, but I can only tell you that massive progress has been made this weekend. The only part I was a bit unsure about now works, so the rest is actually quite easy, technically. In short, it's not far from a beta release, so you guys can help me get some of these features going
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Im Dave Angel
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#9
11 July, 2011, 22:25
Re: DIY Chrono
Excellent work guys.
Whilst I have no electrical engineering skills, I do understand the software side of things. So if you want a hand, then just let me know.
I have been working on the sound-based option I was working with, and will be writing this up with more detail later.
Good work guys
#carried away by a Moonlight shadow...#
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Bachelarius
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#10
11 July, 2011, 23:33
Re: DIY Chrono
Appreciated :D
Like I said, once I have the core code working, it'll be up on Google Code and I would hope to have a bit more of a road map in place, than the bullet list above.
Definitely feel free to add, improve or steal the code for your own evil ends, if you are happy to use C#. I may yet change languages if C# proves to be too unreliable, but hopefully it will be ok. In fact, it may not be a bad idea to include your methodology as a possible input device, if you like the platform, once it is out
. Either way, definitely looking forwards to seeing what you kick out, I am fairly sure you know what you're doing, be it as a simple exercise in 'I did that!' or as a fully functional way to measure. Should be awesome!
I've also posted a link to this thread on another forum (Arnies), and so far some suggestions from a lady under the name of 'ParanoidDroid' have included ambient temperature sensing, which would be great for GBB guns. This may be possible with a thermistor in line, between the optical sensor and the mic read pin, then use the amplitude of the spikes to read the temperature as well. Depends how repeatable the voltage spike amplitude is I guess.
She also suggested timestamped logging of every reading, which I was going to do anyway for the sake of saving me having to code in later, when I have to.
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windywindmill
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#11
16 July, 2011, 11:08
Re: DIY Chrono
right, was playing about last night, only time in a while due to damn work and found:
4.5v on the IR LED's just blow them (gotta source some more now!) dropping down to 3v over two in series i think will be a better option, have ordered a variable resistor to test the optimum level over a few test shots.
Using a mates caliper ive found that my 4" distance between the holes are actually o.1" out so i'll have to adjust the calculations on mine.
Amazing work on the software side tho guys!!
. .
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Bachelarius
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#12
16 July, 2011, 15:25
Re: DIY Chrono
For your LEDs, you need to look at the stats of it, so for instance, my LEDs are
5mm 940nM LEDs
. If you plug the LED's Forward current and voltage into
this calculator
you find that you can wire them directly into the USB's Power supply (5v) using only a 33ohm resistor, as long as they are in series (handy, because I already have a load of 33ohm resistors handy).
If you simply want to test your LEDs, usually a coin cell is a good one to use as they kick out such piddly currents. Use the LED's stats and that calculator to prevent blowing them though.
Not had much time to work on software lately (Just got myself a job, so been prepping for interviews and such all week), so hang in there, progress is just a bit slow, but still there
If anyone is trying to get any funky wiring done, (multi channel or in parallel or something like that) send me the details (and an explanation of what sort of signal it should kick out) and i will work support for it into the SW as I go
Cheers, Nikolay.
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