DIY Chronograph

[luke213]

Well guys I've been planning on picking up a chrono again since I started fixing my own guns. I borrowed one but I wasn't convinced in the accuracy of the results since it went against what I expected to see by around 30 FPS. More less guns all seemed to register around 30 FPS lower than expected across the board. So since that is a known issue with allot of airsoft chrono's I started looking at better units that would hopefully be more accurate. Well the price starts to go up and honestly I'm not convinced they are going to be any more accurate than the cheaper ones.

So I started looking at real steel chrono's like I used to use. I was pretty happy with mine overall but I'd rather not spend $100 on one. I could but I'd rather not. So I started looking at DIY stuff and ran through a bunch of different options and finally decided to just buck up and build one myself.

So I ordered up the parts yesterday they are coming from China so it will take a little time to get here but I'm curious if anyone else has done this before here? I've read about it on other forums etc, and it seems fairly easy to do hooking to a laptop MIC input and recording the shot times and then converting to FPS or run a soft chrono app to do that for me(more likely option). One of the major reasons I'm thinking about building it rather than buying is I've got an idea to make it adjustable. IE adjust the length between the sensors to be able to actually tune the chrono to a known accurate chrono. So instead of the stock solutions where if it's off 30fps your just stuck dealing with it. On this setup I should be able to change that length by very small amounts to get it as accurate as I can manage. I'm thinking this type of setup will be fairly inexpensive to build likely around $20 in materials and should out perform the cheaper chrono options and maybe even better ones in our application since as I said it could be tuned to get pretty close to exactly what it should read.

Allot of this is theory till I go ahead and build it, but I'm planning on documenting the process with some pictures and maybe video. Make it sort of an open source project for guys who are fed up with cheap inaccurate chrono's(like myself).

So any input for guys that have done this before would be appreciated. Otherwise I'll go after it with what I've learned and build it out. Current plans is something fairly small probably using a foot or so between the sensors to keep it accurate(I think longer distances will be more accurate to a point until velocity drops off). Some of the off the rack solutions are only 6in. or less between the sensors which seems like it would be way to close to get a good accurate reading. I'm planning on building the tube from PVC, lighting it with IR Led's since I've got a bag of them around here somewhere. And Building the wiring and electronics into either a larger tube around the inner tube, or something. basically a robust housing that will be mostly water tight and durable so it can get beat around without failing. Something that can handle being used on a field or hauled around to games. Planning to run on voltage provided by the MIC jack on the laptop though might swap over to battery power depending on how much power is necessary for the whole system. I'm also considering putting in a little micro controller or something to read and output FPS with an LCD but likely the first version will require a laptop, or maybe a smartphone app if I can find one that handles MIC input for chrono not sure if that exists or how hard it would be to build as I'm not a programmer at all.

So that's the plan I'll keep you guys posted how it turns out, and I might even see if I can't mail it out to some people with existing chrono's to get comparison readings since I'd really like to get this thing as close to spot on as possible.

Luke

[Snarf]

This sounds great. I've always wanted a chrono but I never wanted to pay for it . I hope this works out well.


I am not experienced at all with this kind of thing, but in theory couldn't you record the sound of firing the gun at a metal plate a foot or two away (measured precisely) and then look at the sound graph (or whatever it's called) and measure the time between the two impacts to calculate velocity? Of course it would be slightly less accurate than motion sensors or whatever you're using, but at such a short distance I don't think it would affect it too much. The problems would probably arise in the sound recording (does the BB exit the barrel right at the same time as the "pop" or not?).

Anyways let us know how it goes.

[T6e9a]

I have been having issues with my own chrono.
Will read FPS decently, but it's ROF is all over the place.

I wouldn't be opposed to building something, or modifying the current chrono to somehow maybe measure audacity for rate of fire. Or even something that might help make it more accurate or have more consistent readings.

Great idea, and can't wait to hear the outcome though.

[luke213]

You can do it with audio actually if you've got an android phone search the play store for chrono graph can't remember the name of the app but there is a lite version that does just that and calculates FPS. However it's just not accurate for me, I've tried it several times and I can't get consistent results not enough to really use it as a chrono at least. Quite a few guys though have taken that idea and used it to measure pellet guns etc and even spud guns.

I'm using photoresistors which sense light more less like a normal chrono, the resistors themselves pending they work were dirt cheap. I paid something like $2 for 20 of them and I only need 2 for a single chrono. Pending those work properly, I've read reports they will, I've also read guys saying they are too slow etc etc. So I decided I'd just give it a try.

I haven't been planning on checking ROF with this build, honestly it's just not something I care about. I never was a fan of super high ROF anyways I actually prefer something a bit slower myself. That said record a sample with audacity then measure the high points over a given time and you can get ROF really easily compared to FPS. So that's how I would check ROF myself if your chrono is acting up, should work with just about any microphone at all as long as the sample rate is high enough to pick up the individual shots.

I'm mostly planning on doing this as a cheap option for guys who are handy enough to build it, and would like to have a chrono. If it goes well then maybe we'll either see more accurate consumer grade chrono's or even fields going to something a bit more accurate. That might be too much to expect but I'm just hoping for something consistently accurate for my own use.

I'm planning on mounting one sensor likely on an assembly that rides on a fine thread bolt, so basically you can turn the bolt to adjust the distance between the two sensors effectively tuning the unit. It may be as simple as a slot in the outer tubing to adjust it, or a sliding assembly to keep the inside smooth. I'll have to see probably try the slot first then the other option. I also may run more sensors than the 2, I've read guys having trouble getting pickups with a single sensor so two on each end might make it more consistent. The end result though is the photoresistor outputs a small amount of voltage when lighting changes, and that registers via the audio cable hooked to the sound card in the computer. So just like it measures sound with a MIC, this measures sound more less but using the photoresistors rather than a microphone. Should be pretty dang accurate. If it works well I could even double the length do three sensors and take an average reading between the two to get better accuracy etc.

I'm just tired of these crappy fairly expensive chrono's and I'm ready to put a little time into building something that works;)

Luke

[Kjones]

I am actually currently making a chronograph for my digital circuit analysis class. It uses infrared sensors and should be pretty accurate since it counts the amount of clock pulses (500 khz) and converts it to time then divids the distance by the time elapsed. I can produce then pretty cheap but my time is the more expensive part. I do plan on selling them once I get them completed. I am expecting a price point around 50 but that is not set in stone. I have two different methods of making them but for my class I need to make it the harder way since it must use all digital logic. The unit I sell will be the easier and faster way.

[luke213]

Well as long as it's accurate and I'd prefer tunable at $50 that's a plenty fair price. I've got the parts on the way for this but it's up in the air how well it's going to work;)

Your method sounds like it's going to work well, and probably be more accurate but we'll see once I get this thing all together;) Keep us posted though on yours any input on the subject is of interest;)

Luke

[T6e9a]

My issue is I don't have an android phone, nor the equipment to measure audacity.

[luke213]

All you need for audacity is just a microphone really, install it on the pc record the audio and figure it out. You could record a sound file on the phone then import it into audacity and that would likely work as well. But either way if you've got a computer or some sort of phone capable of recording audio files you should be able to cobble it to work for checking ROF.

Luke

[Kjones]

I think the sound method is pretty accurate too. Do you plan on shooting sheets of paper for the second microphone? Thats the best method I seen. The first mic picks up the sound of the gun firing and the second mic picks up the bb hitting the paper. Do your math and you have a decend measurement. The only problem would be finding the exact spike from the first mic and knowing exactly where the bb is.

[luke213]

I just wanted to bring this back up since I got my parts in the other night;) I still likely won't have time to build anything for another little while but at least I can say this project is still up and running and isn't dead. The parts just took the slow boat from China so it took a couple weeks to get them.

So once I get time to build the actual device and wire it to test it expect some pictures and maybe video along with a short write up on how I did it and what luck I've had with it.

Luke

[Kjones]

I have a working prototype of my IR sensor chrono. I did about 20 minutes of testing so far but it seems to read pretty accurate. I still have a couple things to do to it but currently it does read and it does display to the computer screen, a lcd screen will be added shortly. I am not 100% sure if the FPS is actually correct compared to a store purchased chrono but the math checks out for my code and my cyma that chrono'ed at 380 fps with .2 @ no limits was hitting 349 with .25 ( I dont have any .20 for testing). I need to do a bit of repair to sensor 2 as I accidentally shot it and killed the potentiometer and a IR sensor, it should take about 20 minutes to fix so its not the end of the world.

[luke213]

Well I had some time today to fiddle with this and failed;) I think it's the photoresistors that I ordered aren't sensitive enough nor fast enough to catch the BB reliably. Once in a while I'd get close but there were far to many false readings. I tried a bunch of cobbled light sources from led lights, IR led's and even at the end lasers hitting the resistors with the same basic results.

So off to ebay I went and ordered up some phototransitors which is what was recommended in the first place. And this time I ensured they were coming from the US rather than China so I don't have to wait, and I bought some with an included matched IR led for each. I can't for the life of me find my huge bag of IR led's that are around here somewhere from another project. So rather than keep digging I'll just use the ones included. If this works it will be $2 in electronics per chrono.

So we'll see once the new parts get here.

Luke

[luke213]

This thread is continued over on the new forums: www.miairsoft.org/index.php?topic=102.msg549#msg549

Short version it works!

Luke

And I'm going to lock this one so I'm not posting and other people aren't posting in both places.