# Best way to trigger Oscope

#### DC_Kid

Joined Feb 25, 2008
796
i need to do a ext trigger for a scope capture. the only catch is, i need to trigger on a bullet just as the nose of the bullet meets the start of muzzle exit. i was thinking of using optic path to trigger (like the eyes on a garage door). as reference, the speed of the bullet can be in the range of 600-4k ft/s

any ideas for me to look at?

#### crutschow

Joined Mar 14, 2008
24,716
You could use a small laser as a narrow light source directly across the barrel exit and a photo-diode (not a photo-transistor, which are slow) to sense the bullet as it leaves the barrel and blocks the light from the laser to the photo-diode.
The photo diode anode is grounded and the cathode is reverse-biased with a low voltage (say 2V) through a low value resistor (say 1kΩ or less).
This will give a 2V pulse when the light is blocked.

#### DC_Kid

Joined Feb 25, 2008
796
thanks crutschow, that was my 1st idea. would the beam be perhaps partially hitting the end of the barrel so that only a small x-section of the beam is used to hit the diode, this way i dont run into hassle of trying to get the beam edge right up against the end of the muzzle?

for a laser trip, i guess i could use a small bat powered laser pointer, one of those tiny keychain types. then the other side is the "IC" side.

wondering if there might be a simpler way.... perhaps there isnt, but this site is full of great ideas.

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#### ISB123

Joined May 21, 2014
1,236
That is the simplest way.

#### crutschow

Joined Mar 14, 2008
24,716
thanks crutschow, that was my 1st idea. wondering if there might be a simpler way.... perhaps there isnt, but this site is full of great ideas.
It requires 3 devices.
Don't see how you can get much simper than that.

#### OBW0549

Joined Mar 2, 2015
3,304
any ideas for me to look at?
How about just stringing a short length of fine magnet wire across the muzzle? Ground one end of the wire, connect the other end of the wire to a pullup resistor and to your microcontroller input (or whatever you're using to process the signal), and the other end of the pullup resistor to some positive voltage.

Bullet breaks wire, voltage goes positive. Easy as pie.

#### spinnaker

Joined Oct 29, 2009
7,835
I just saw something like this being done. Pretty sure but is on this forum. Search around a bit.

#### RichardO

Joined May 4, 2013
2,271
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.

#### GopherT

Joined Nov 23, 2012
8,012
i need to do a ext trigger for a scope capture. the only catch is, i need to trigger on a bullet just as the nose of the bullet meets the start of muzzle exit. i was thinking of using optic path to trigger (like the eyes on a garage door). as reference, the speed of the bullet can be in the range of 600-4k ft/s

any ideas for me to look at?
I just saw something like this being done. Pretty sure but is on this forum. Search around a bit.
Here it is. Super simple.. Although it seems the sensor needs to be moved a little closer to the rifle to get the bullet before it hits the target (I think that is what the OP said he wants).

#### DC_Kid

Joined Feb 25, 2008
796
a wire will likely stretch into a delta-t area i dont want to be, triggering the scope too late. but yeah, thin wire is very very simple, i like the ideas.

photo beam - well, i have a concern there too. barrel rifling does allow some gases to get past the front of the bullet, thus it may be possible that the exiting gases ahead of the bullet block the beam and the scope triggers too soon.

that piezo item wont work, there are no vibrations yet to capture. connecting it to the barrel i dont think will work, where would it go? and, that project is great and very low cost for a camera pic, but i would have ventured into 1kfps video for capturing that stuff.

my scope need to record some data just as the bullet is exiting the barrel. trigger scope right when nose of bullet reaches muzzle exit zone, scope (DSO) will record a few ms worth of data.

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#### RichardO

Joined May 4, 2013
2,271
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.
Never mind. After re-reading your requirements and replies I realize that this won't work since the sound/vibration happens well before the bullet exits the barrel.

#### crutschow

Joined Mar 14, 2008
24,716
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photo beam - well, i have a concern there too. barrel rifling does allow some gases to get past the front of the bullet, thus it may be possible that the exiting gases ahead of the bullet block the beam and the scope triggers too soon.
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That certainly could be a problem.
It depends upon how opaque the initial gases are to the beam.
Since the particles in the gas are less reflected by longer wavelengths you could use an 0.9μm IR wavelength laser beam (near the maximum sensitivity of a Silicon diode detector) and by adjusting the sensitivity of the detector, you likely could differentiate between the gases and the bullet.
But that's just an educated guess.

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#### DC_Kid

Joined Feb 25, 2008
796
what about using a simple narrow beam 940nm IR led (not a laser)? the led and diode receiver (vishay TSOP382/384) are like $2 total. this Vishay receiver is not the right part, the led needs to be driven at 38kHz, too slow for my needs, my timing are in the 800kHz(min) area, more like 1MHz, thats how fast things are happening. but anyways, IR led in the ~940-950nm area? something like this pair Vishay VEMD2000X01, VEMD2020X01 photo-diode TSAL6400 led Last edited: #### crutschow Joined Mar 14, 2008 24,716 what about using a simple narrow beam 940nm IR led (not a laser)? the led and diode receiver (vishay TSOP382/384) are like$2 total. this Vishay receiver is not the right part, the led needs to be driven at 38kHz, too slow for my needs, my timing are in the 800kHz(min) area, more like 1MHz, thats how fast things are happening.

but anyways, IR led in the ~940-950nm area?

something like this pair
Vishay
VEMD2000X01, VEMD2020X01 photo-diode
TSAL6400 led
An IR LED is okay as long as it's powerful enough and you can make the beam narrow enough or mount the LED close to the barrel so the bullet blocks most of the beam.

#### DC_Kid

Joined Feb 25, 2008
796
An IR LED is okay as long as it's powerful enough and you can make the beam narrow enough or mount the LED close to the barrel so the bullet blocks most of the beam.
that Vishay part i listed is 25mW 25degree. if i went this route both sides would have to be located at the muzzle (or very close to it). my concern here is, movement of barrel during a firing. i didnt want to have to use a rifle mount to hold it firm. it really should be free stance position, which makes me think an light beam would have to be held in place by some for of barrel attachment. most barrels are magnetic so i could make a mag mount, but some are stainless, etc.

so something as basic as this? i can literally adjust sensitivity by using the trigger threshold on my oscope.

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#### crutschow

Joined Mar 14, 2008
24,716
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so something as basic as this? i can literally adjust sensitivity by using the trigger threshold on my oscope.
Yes, that's the basic circuit (if that top device is a voltage supply).
The sensitivity I'm concerned about is if the LED light is sufficient to give a good signal with the low resistor value you need to get a fast signal response.
You may have to do some experimenting to determine that.

#### DC_Kid

Joined Feb 25, 2008
796
ah, yes, the top in that schematic is just a bat. i made that pic small so it wasnt to big here, i made it too small.

so why not i use a 10k instead of a 1k on the receiver diode? why does that R change the sensitivity on the output?

i am wondering what the reverse impedance is of the opto-diode if its hit with 25mW from just a few inches away.? i'll order up the Vishay items and get my breadboard out.

and while looking at Vishay app guide (its more of a "this is what we can do with our IR products"), they have a proximity IR module. the module itself wont work for me, but what about using the led and diode on same side and measure the reflected IR instead of using the pair across from each other? the circuit becomes different in proximity setup, but is that a better way?

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#### ISB123

Joined May 21, 2014
1,236
Because of the resistive-capacitive delay.

#### DC_Kid

Joined Feb 25, 2008
796
Because of the resistive-capacitive delay.
the diode will be in the 1.5-2pF zone for the reverse voltage being used. the RC with 10k (R) is just 15ns. 2t is 30ns. that is fast for my needs. in 30ns a bullet @ 1kft/s will have moved 30uFT. if it takes 2t in RC to get to my trigger level.

do i have all this right?

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#### crutschow

Joined Mar 14, 2008
24,716
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so why not i use a 10k instead of a 1k on the receiver diode? why does that R change the sensitivity on the output?
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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.
the diode will be in the 1.5-2pF zone for the reverse voltage being used. the RC with 10k (R) is just 15ns. 2t is 30ns. that is fast for my needs. in 30ns a bullet @ 1kft/s will have moved 30uFT. if it takes 2t in RC to get to my trigger level.

do i have all this right?
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.