Author Topic: Palflash 500, nanosecond light source  (Read 18181 times)

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Offline krivx

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Re: Palflash 500, nanosecond light source
« Reply #50 on: November 27, 2014, 05:47:12 pm »
He's claiming the 500ns pulse was measured with an optical sensor with 1ns rise-time, I don't really see why it matters if he has captured a bullet or not? Surely an optical measurement is more useful than an image of a bullet?
 

Offline Yago

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Re: Palflash 500, nanosecond light source
« Reply #51 on: November 27, 2014, 07:45:15 pm »
Ok, I am that curmudgeon, under-a-cloud, test engineer! :P
If someone says "it's bulletproof" etc.

I'll leave it alone then, with the irony of; if it's any good I might buy one (well, try making one :P)
 

Offline dexters_lab

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Re: Palflash 500, nanosecond light source
« Reply #52 on: November 27, 2014, 08:09:41 pm »
from what i understand 500ns should be good enough to capture bullets from guns and stuff.

the reason i ask is in the data sheet it says the 2J model is rated at 250ns, i can change mine by removing caps so i just wondered about measuring it to see how much of a difference it made,

i just shoved a 3mm red led in front of it directly to my scope, and get a good rising edge off it in the light beam but the falling edge faded slowly over 100s of microseconds so wasn't useful so put a 10k resistor across it and got the following which looks better, but 4uS is still no where near what it should be so i'm guessing i need to do better. Maybe there is some strange physics happening with that much energy in a short period?
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Offline T3sl4co1l

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Re: Palflash 500, nanosecond light source
« Reply #53 on: November 27, 2014, 08:39:18 pm »
To do good sub-microsecond photodiode work, you really want the diode reverse biased (so the photocurrent is cleared quickly and the junction capacitance is small), and you want it returned into as low an impedance as possible -- you can use a very low value resistor, but the signal is very weak.  (For a freaking flash, this is probably an advantage -- it's probably more than bright enough to saturate the diode, so the current will be relatively large.)

For ordinary signals, you use a TIA (transimpedance amplifier), which has (ideally) zero input impedance, and converts input current to output voltage (thus, a gain of V/I = impedance, but gain, so it's 'transimpedance'..).

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Offline miguelvp

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Re: Palflash 500, nanosecond light source
« Reply #54 on: November 27, 2014, 09:08:54 pm »
A phototransistor might be quicker, but not sure if they have any available under 1 us

What's the fastest optocoupler and what do they use inside? darlington configured phototransistors?
 

Offline T3sl4co1l

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Re: Palflash 500, nanosecond light source
« Reply #55 on: November 28, 2014, 03:22:13 am »
They don't get faster than photodiode + TIA.

No combination of PD, BJT or darlington will get anywhere near close enough, being limited by recombination time of the B-E junction(s).  Darlingtons in general are slow as molasses.

For example, this circuit uses a 4N35 as its own TIA (plus some outboard bits, of course); analog bandwidth is a few MHz, versus maybe 20kHz in saturated operation, or a few 100kHz with a small load resistor.
http://seventransistorlabs.com/Images/4N35_TIA.png

Compared to an LED, any small junction PD will probably not improve much.  I could even think of reasons why an LED would be better: the junction potential is higher (wider bandgap semiconductor), so the capacitance is smaller than that of a similar size silicon part.  PV efficiency is always crap with LEDs, but to measure a flash, that shouldn't be a big deal with the sheer excess of intensity available.

Tim
« Last Edit: November 28, 2014, 03:24:56 am by T3sl4co1l »
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Offline SeanB

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Re: Palflash 500, nanosecond light source
« Reply #56 on: November 28, 2014, 04:55:53 am »
IR LED driving the virtual ground of a beefy opamp with very low gain ( try 100R at first and increase it if it is not a big enough pulse, but it will probably work) and you should get a good pulse. IR diode because it will be silicon only. Otherwise use a metal can transistor like a 2N2222 and take the top off to use the CE junction, though you will have capacitance of the case degrading the rise time, the base will be grounded.

The laser rangefinder used a different system with a large area silicon photodiode ( because it had a 20km range it needed the light capture) with 200v bias so that it would break down and avalanche with the pulse, used because this gave a very fast response to the initial edge of the pulse, and the recovery time was not important.
 

Offline dexters_lab

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Re: Palflash 500, nanosecond light source
« Reply #57 on: November 30, 2014, 06:37:43 pm »
ahh, yea... lets leave that measurement for the moment then, i'll stick to making pictures  :o
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Offline SeanB

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Re: Palflash 500, nanosecond light source
« Reply #58 on: November 30, 2014, 06:55:37 pm »
ahh, yea... lets leave that measurement for the moment then, i'll stick to making pictures  :o

Well it would develop film directly, mostly by cooking a hole through it. That was the easy way to see if the optics were aligned by seeing spot size in the office on a test bench. Centre burnt and around it a dark developed blotch giving light levels.

Had a similar size capacitor bank and discharge methods using a large GDT.
 

Offline chickenHeadKnob

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Re: Palflash 500, nanosecond light source
« Reply #59 on: November 30, 2014, 07:20:55 pm »


Why not use a real air rifle, and not a toy BB gun.
Even UK air rifles are much nearer(than a BB gun) the velocities claimed, FAC UK, you can hop-up a PCP air rifle to around and possible above supersonic speeds.
It is easy to control too, just fire it into a block of soft clay.
There is no need to chucking military lead about to test, that is the root of my misgivings.

As a young lad I had a reasonably powerful quality but subsonic air rifle which was envied by my friends. One day by accident I discovered it could be made to "diesel". Now having fired center fire hunting rifles and also being fired at by the same* I am pretty sure the dieseling pellets were making a supersonic crack . Not healthy or safe for the pellet gun and hard to make happen consistently although I think some cold start (ethylene gas) sprayed in there would fix that. Don't try this kids!

*mushroom picking in Canada's north had some rounds make that whistling-crack just past my head.
 

Offline dexters_lab

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Re: Palflash 500, nanosecond light source
« Reply #60 on: November 30, 2014, 09:23:08 pm »
i will buy an air gun of some sorts to play around with the flash, probably a .177, i will need something better than the 25mm ball bearing i'm dropping on stuff

been a small delay on the trigger project though, i ordered some 10 turn pots from china and one of them is faulty (lol, surprise!) so its not worth doing any updates at the moment, i want to build it up off the dev board in one go
"A common mistake people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools." - Douglas Adams
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Offline jlmoon

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Re: Palflash 500, nanosecond light source
« Reply #61 on: December 03, 2014, 10:27:39 pm »
i picked up this pulse light source a while ago and want to figure out why i get no flash out of it.

its a simple device with two air-gap spark flash devices inside with a capacitor bank.

would like some ideas on getting this working, i think the problem might be more mechanical than electronic but we will see!



That thing looks like it could have a lot of magic smoke trapped in there.

Recharged Volt-Nut
 

Offline dexters_lab

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Re: Palflash 500, nanosecond light source
« Reply #62 on: January 24, 2015, 05:25:45 pm »
has been a while, but i finally made an update to the flash project.

You should expect to see some interesting pics soon now i have a controller that i can just pickup and use.


"A common mistake people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools." - Douglas Adams
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Offline dexters_lab

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Re: Palflash 500, nanosecond light source
« Reply #63 on: February 07, 2015, 08:42:35 pm »
Have been playing with my air-gap flash today, dropping a 25mm ball bearing onto a microscope slide.

Attached pictures are 1:1 crops

Next step is to make some kind of rig for the impact, and probably paint the enclosure so you can see more of the detail of the subject
"A common mistake people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools." - Douglas Adams
https://www.youtube.com/user/DextersLab2013
http://dexterslab2013.blogspot.co.uk/
 


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