ghetto chrono help

[singularity]

i am in the process of making a ghetto IR chronograph to get a reasonable estimate of the fps im getting. its a basic idea have two IR LEDs hooked up to a power source and two IR photo transistors hooked upi to a headphone jack. one pair (led +photo transistor) would be paced a foot from the other and they would both be wired into the same head phone jack. then the head phone jack would be hooked into the mic port of a computer running audacity. when a projectile broke one of the beams there would be a negative spike.

now for the question do i need to apply voltage to the collector of the photo transistor or are mic ports powered (as in they put out some voltage)? i would love to spend hours going through data sheets trying to find this out but i just thought it might be faster to ask you guys.

ps - on another subject thats completely off topic... does anyone have an AK clip?

[Mishkan]

Here's a circuit that works relatively well. It uses a potentiometer so you can adjust the sensitivity of the interrupt circuit. Then it goes through a schmitt trigger to clean up the signal




Was a circuit I setup a little while back (using part of the one above), I messed up the 555 circuit so you'd have to fix that. Anyhow that worked by itself but it only works at certain FPS.


Edit: What type of AK mag..

[Spuddin]

there is some guy that made a chrony just like your describing but i cant remember where it was, maybe u could google "homemade chronograph" and i bet u would find his site. he was using it on a bb gun also. if u get it figured out i wanna know cause i want to build one also when i finally get a chance in between projects.
______________________
www.freewebs.com/spuddin

[singularity]

jut found this image while searching for the homade chrono you speak of spuddin. it appears as though there is 5v going into the mic so i think im fine

<img src="http://home.earthlink.net/~jimsluka/_im ... rInput.gif">

and about the clip im just looking for a classic ak-47 clip preferably mad out of metal i don't care if its from an airsoft gun or whatever... my friend told me he could get me one but he gone and i have no clue where he went

[shud_b_rite]

I don't think mic inputs on a sound card are powered, there is no reason for them to be.

If you were going to make one like this I would probably use the line-input instead of the mic input, this is because whatever signal you put into it, it will get amplified quite a lot which could damage the sound card whereas the line input doesn't amplify anything so it would be safer. I have hooked photo transistors up to a soundcard before(not for a chrono but for another project) and it works fine on the line input.

[singularity]

the mic shouldn't be amplified until it gets to my speakers atleast thats the basic idea. as far as i know the sound card doesn't amplify the signal all to much it just coverts it from analog to digital and back againrthe speakers are what do the amplifing

[jimmy101]

shud_b_rite, wrong, MIC inputs are almost always powered, they are designed for Electret microphones which require a power source.

Very easy to tell if, and how, the MIC input is powered. Stick a plug into the jack and use a voltmeter to measure the voltage from the tip to the body (and the ring to the body, and the tip to the ring since it is probably a stereo plug even though it is used for mono.)

Usually you'll see ~2.5V (open circuit) and ~1mA short circuit on either the tip or the ring, relative to the body.

If the tip is more than ~1V and less than 50V then everything will work great, just wire the phototransistors (PT) in series from the tip to the base of the plug. The PTs collector goes to the plugs tip and its emitter to the plugs base, the polarity matters. If you get it backwards it won't work, if the plug supplies less than 6V then it won't hurt the PTs.

Hook up the PTs and then connect a voltmeter accross one of the PTs leads. You should read a voltage. Cover or shine a bright light on the PT and the voltage will change.

The IR LEDs are probably not needed, ambient light should be more than enough. Just orient the PTs so they are looking at the sky.

If you use LINE INPUTs then you'll need to supply power to the PTs. A 1.5V battery and a 100 to 1,000 ohm resistors should do the trick. (you might be able to leave the resistor out)

The "homemade chrono" pages is mine, as is the wiring diagram that singularity posted.

[singularity]

well the photo transistors are 880nm (IR wavelength) so that would be why i have the LEDs. should give me slightly more dependable results and i could use it in any weather. i originally got the LEDs and PTs for a coil gun project (ordered 5 of each) but i only used 3 of each so i figure why not use them i have them lying around.

[jimmy101]

Do you have any idea how much IR there is in daylight? Much more than the IR LEDs will put out. WAG, 10 to 100 times more IR in hazy daytime sky than the LEDs will produce.

Nothing wrong with using the IR LEDs, it just isn't needed. And, it limits the size of the projectile. To use the LED + PT on a BBMG they will need to be pretty close together. To use the same setup on a 2" spud you'll need to open up the gap by a lot. Much simpler to just use ambient light.

Easy enough to test after the rig is built. Just do a recording of projectiles with the LEDs on and with'm off.