Best way to trigger Oscope

Thread Starter

DC_Kid

Joined Feb 25, 2008
796
R changes the sensitivity since a given amount of light impinging on a photo-diode generates a given amount of current. Thus the larger the R, the more voltage drop is generated across that R.
Not quite.
You didn't include any stray capacitance from the layout and the 'scope input.
Even with a 10:1 probe for the 'scope the strays will likely be at least 20-30pF.
ok, i wont be able to accurately test the total capacitance (and real delay), but i can effectively combat that issue by adjusting the trigger threshold on the scope. if it takes 1ms to get a rise to 1v trigger threshold and 1ms is too long, i can turn the trigger threshold down.

at 15x the pF i mentioned earlier (so about 22.5pF) the RC is just 225ns (2t is 450ns) = 0.450μs. i would be concerned if 1RC was close to 10μs as things are happening at the speed of ~12μs.

the DSO gear i have (cheap stuff) will likely have cap in the area of 120pF, which pushes the 1RC up around 1.2μs.

If you don't want to put anything near the path of the bullet then maybe a sound or vibration sensor would work. I have seen this done in shooting simulators.
interestingly enough, the light beam approach seems easy from the electronics side, but very hard from the physical alignment side, of which seems to be a difficult challenge.
 
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GopherT

Joined Nov 23, 2012
8,012
Why do you want to trigger a scope? I thought you would want to trigger a flash-lamp (or high power LED) to capture a bullet. Are you using a camera or what exactly are you doing?
 

Thread Starter

DC_Kid

Joined Feb 25, 2008
796
Why do you want to trigger a scope? I thought you would want to trigger a flash-lamp (or high power LED) to capture a bullet. Are you using a camera or what exactly are you doing?
no, i am not capturing images. i am capturing muzzle blast sounds using a PCB 378C10 w/ 426B03 preamp. the PCB components are built for this purpose but fast inexpensive data acquisition is not easy to find. there are lab style (chassis/card) systems out there for $$k, but i found that my ~13yr old owon pds5022s DSO (~$260 back in that day) can outperform a lot of the acquisition gear being sold today costing 10-15x as much. and to boot, all my stuff can easily be 100% portable, so it can be way out in a field if need be w/o the need for util power, etc. literally just two small bags worth of gear.

so, since my old DSO can do the job i will use it to capture waveforms produced by the mic setup.

so, all that said, i need to trigger the scope just before the sound blast event happens (which happens darn fast). technically, a sound wave starts to form just as the bullet gets to the end of the barrel, but i am interested in peak dbSPL and not really the full event. my DSO can aquire long enough at 100MHz to capture the peak dbSPL. this is all i need.

but, as i noted in a post or two back, the electronics side is easy, but what's not so easy using a light beam is physical alignment. its a challenge, thus i am still open for ideas on how to trigger the event. perhaps some form of a trigger switch (adjustable pressure) so that the finger pull is the trigger, but there is a delay for pin to hit round. i could build a time delay RC to account for this. i would need a piece of silicon that could turn on fast, and then i could build in needed delay with a RC circuit (pot to adjust delay) giving me two controls for delat, one from trigger pull and the other being threshold adjustment on the DSO. all that said, and the challenges i see with light beam alignment, a trigger pull switch seem like an alternative i need to look at.
 
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GopherT

Joined Nov 23, 2012
8,012
Then that is easy. Use two microphones. One right by the receiver of the rifle/shotgun and a second microphone near the muzzle. Sound travels about 340 meters per second so your mic by the receiver will trigger the scope and the sound will reach the second microphone about 2 to 3 mSec later. You can calculate exactly when the sound arrives - if you account for air temperature. There are smaller corrections for humidity and pressure but even temp will be a relatively small (less than 5%) error.
 

Thread Starter

DC_Kid

Joined Feb 25, 2008
796
but the 2nd mic also needs to be very very fast, which = $$

the IR led way is ~$2
trigger switch with RC circuit is ~$2

i am still looking at proximity vs beam using same IR led/diode pair. proximity might not have physical alignment issue like beam does.


or, use a vibration sensor that gets attached somewhere on the rifle? i think sensitivity would be ok, but what about rise times? this LT opamp has slew rate of 4V/µs



if this becomes too cumbersome i might just skip the detection piece altogether and just trigger the DSO when the main mic preamp output reaches a threshold dbSPL (volts).
 
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