SpudFiles

A while back I got my hands on a weeks worth of disposable cameras from the camera shop. I used about half of them by pulling them apart and shorting the resulting capacitor bank through various objects, but after I got over that the rest had just been sitting there unused.

So, I decided to try my hand at taking some high-speed photos with the simplest possible setup. My goal is to get a picture of a projectile from my mini spudgun with as little blur as possible.

So far, I have rewired the flash trigger on the camera to a couple of wires, and wired those to a clothespeg. When the clothespeg closes, the flash fires. I will set up a tripwire of some sort that will get hit by the projectile, firing the flash. This will be in a dark room with my digital camera set to long exposure, so it will capture an image of the room at the moment the flash fires.

This setup obviosly isn't ideal for a few reasons: There has to be some sort of physical switch which whatever you are photographing has to trigger, and the other downside is that I haven't used an SCR or anything else fancy to trigger the flash. This means there is about (according to my multimeter) 50V present on the exposed electrical parts. I'm not sure how much current it can deliver, as there are no sparks etc when the switch closes, and I'm not willing to test it on myself.

I haven't done any tests yet because dad is sleeping on the couch in the next room and the noise of the gun would wake him.


Has anyone else done anything similar to this? I'm sure those of you with more electronics knowledge than me could take some incredible pictures with a more advanced version of this setup (maybe use a laser trigger, like what they have on the conveyor belts at checkouts?)

Once I get a chance to test it I'll upload any good images I capture.




EDIT: Just did a few simple tests, with me trying to pull a bit of paper out of it's grip as fast as I could. All of the images just showed the paper still in the grip of the clothespeg, so it is definitely going off at the first possible moment. The only problems are that the blind in my room lets a bit of light in, and that the flash is too bright, so I need to bounce it off somthing instead of directing it at what I'm taking a photo of.

I'm sure it wouldn't be too hard to hard to have the flash triggered by the sound of the firing, though you might not catch the projectile. Wired science did a segment about that and it seemed to work quite well for popping a balloon/shattering a cd, using some cheep electronics and a disposable camera, I'll look for a link...
FOUND: [youtube][/youtube]

hope this helps[/quote]

Yeah, I've heard of using sound for it (in fact, I was hoping that there would be a script I could run on my camera via the CHDK (software you can run on canon cameras to give it extra functions), but unfortunately no-one has made one yet)

Also, I am just doing this with what I have here and now, and I don't have anything I could use to activate it via sound.

Ok, I did a couple of actual shots.

Unfortunately, the flash isn't quite as instant as I was hoping. On the plus side, it is still high enough to get a pretty good idea of what is going on.


The first pic is of me firing an alfoil ball straight at the string.
The second is the same setup with a small bit of CD taped to the string. (I've spent the last minute or so collecting the shrapnel )

I need to get either a faster flash, or figure out a way to make a delay long enough for the bits to slow down a bit, or simply try and capture things that are happening a little bit slower. It is also possible that the small amount of light coming in through the gaps in the window are enough to cause the 'fuzziness' you see in the pictures. I shall try again tonight to see what happens.

And here are the pics!






Not half bad for a disposable camera and a clothespeg

BTW, what camera are you using? Is it a little point and shoot, or a DSLR?

It's a Canon A540... Not exactly point-and-shoot, but not a fully featured camera either. It has manual mode, and you can manually adjust a fair few settings. I set it to a 10 second delay with a 1 second exposure, and have the apterture set to as small as possible, 80ISO, and manually focus it. This lets me get my hands away from it and prepare for the shot with both hands.

Also, if you install CHDK you can do a bunch of cool stuff with it (60 second exposures, motion detection, reversi (no, seriously, I have played reversi on it))

Here is another pic I took, of my ping-pong ball gun firing. The ball itself is a tad blurred (I think the gradually fading blur is from the xenon gas cooling too slowly and providing a little bit of light after the main flash)

The interesting thing about this pic is because of the toilet-roll barrel you can see all the little fibers that have been blasted out of the barrel. This is closer to the 'frozen' effect I'm hoping to acheive.




EDIT: Same as above, now with cornflour!



EDIT EDIT: The ball got a bit damaged, so I cut a little bit out of it and put a TINY bit of cornflour in. I think some of it ignited due to the fireball that isn't present in any of the other pics.




I really need a better background, that last one would have been awesome on a black backdrop.


EDIT EDIT EDIT: I did a bit of reading, and it seems the solution to my blurry images is to replace the 80uF capacitor in the camera with one closer to 30 uF or so. This will let me get a dimmer flash, so I can actually point it at what I'm photographing, and the flash will also be of much shorter duration, which should sharpen up the photos nicely.

To any of you with a bit more experience in electronics, I have a few questions regarding this:

1. Will any capacitor rated at 330V work fine, or do I need a specific type that will cope with the rapid discharge?

2. If I use a cap that can't handle it, will it just fail to work, or will it burst apart spraying it's contents in my face?

3. Does it matter if I get a cap that is rated to a much higher V than 330V? Ie will a 600V cap behave just like a 330V cap if they are both only being charged to the 300V the camera circuit can deliver?

4. Should somwhere like Dick Smith's Electronics stock suitable caps, or will I have to look elsewhere? Basically, how hard is it to find caps that would work for this purpose?

looks very cool. maybe you can use a photo sensor as a switch, like the ones people use for the home made chronys? This won't work if you are in commplete darkness. Try looking up infred lasers. Or maybe you can take one of those toy spy kits with the laser alarm and wire it so instead of ringing the bell when the beam is broken, it will fire the flash. Nice work so far!!

Wow awesome method of taking pictures!
I will try to answer some of your questions, but I am not an expert.

1-There are different types of capacitors (electrolytic etc.) Im not sure, but most of em can discharge very fast.
2-Depends on why it fails. Some may also leak or give off smoke. But when keeping it under its rated voltage and not overheating it by rapidly charging/discharging, youll probably be fine.
3-Googled it for you:

Never replace a capacitor with one rated below the original capacitors voltage! HOWEVER, a replacement rated above the original value is acceptable. That's about it. If the original value is 350 volts, then any voltage rating higher is acceptable. The voltage rating on a capacitor is a maximum value. A 400-volt, 450-volt, or even 600-volt can be used to replace a 350 volt capacitor.

Seems you can use any higher rated cap.

4-No idea. You will have to try some electronic stores. Search for them, possibly on google maps.
If they are local OR have a webshop, search the catalogue for usefull stuff.

since you seem to have a crap load of disposable cameras, you could just put the some extra capacitors in series with the one in the circuit already. this will double the max charge voltage, as well as half the total capacitor. if you wanted a huge, bright, short flash, you could put a bunch of chargers in series, and a bunch of capacitors in series and charge them all up. I could make a paint schematic if you wanted.

you could also use a thrysistor (sp?) to open the circuit from the capacitor to the flash tube, like speed lights do.

funnily enough, I just read an article on this earlier today.

To any of you with a bit more experience in electronics, I have a few questions regarding this:

1. Will any capacitor rated at 330V work fine, or do I need a specific type that will cope with the rapid discharge?
Yea and no. Any cap rated to at least 330V will work, but it won't necisarily work "fine". Photocaps are a special breed of electrolytic cap with a couple of unique properties. (1) They are designed to be short circuited without damaging themselves. (2) They have somewhat lower internal resistance (ESR; equivalent series resistance) than typical electrolytic caps.

2. If I use a cap that can't handle it, will it just fail to work, or will it burst apart spraying it's contents in my face?
Mostly likely problem with a cap with too low a voltage rating is that it just won't charge up to 300+ volts. When you overcharge a cap it starts to leak across its internal insulator. Apply too high a voltage at low current (like a photoflash charger circuit) and it'll just leak voltage once it is to the rated voltage. If you had a high current high voltage supply you might be able to get the cap to burst. With a photoflash board that isn't going to be a problem.

3. Does it matter if I get a cap that is rated to a much higher V than 330V? Ie will a 600V cap behave just like a 330V cap if they are both only being charged to the 300V the camera circuit can deliver?
Any cap rated to at least 300V will be charged 300+V.

4. Should somwhere like Dick Smith's Electronics stock suitable caps, or will I have to look elsewhere? Basically, how hard is it to find caps that would work for this purpose?
Photocap's are pretty unusual. I suspect you will have to order them online since RadioShack and the like are unlikely to have more than one type (and it'll probably match what you all ready have from the disposable cameras).

A couple things about high speed photography with a flash.

1. Typical flash units have a flash duration of about 0.1 to 1 millisecond, 0.5mS is pretty common. That puts a pretty serious limit on your ability to freeze action. At 300 FPS an 0.1mS flash means the object moved 0.36 inch during the exposure. At a 1mS flash it moved 3.6 inches.

2. It is pretty simple to measure the flash time. Get a photodiode or phototransistor (or even a cadmium sulfide cell or photocell) and connect it to the MIC input of a PC. Start an audio recording and flash the detector with your flash unit. Use something like Audacity to display the signal and measure the flash's duration. I suspect that the typical disosable camera has a flash duration of about 0.5mS.

3. Flash duration can be estimated from the equation of a capacitor discharge through a resistor (the resistor is the xenon phototube), an RC circuit. In an RC circuit the voltage will drop to about 40% of its initial value in R*C seconds (R in ohms, C in farads). You really can't do much to change the R of the xenon tube but you can change the C of the capacitor. Cut the cap's capacitance by ten and you'll shorten the flash duration by ten (using a very simple minded model of the discharge ). In the real world it won't work out quite that simple. The tube has resistance but so does the photocap. Often the cap's resistance dominates the circuit and it is it's R that should be used in the RC equation. Generic caps have internal resistances (ESR) values of perhaps 1 to 0.01 ohms. High speed photo caps have resistances of perhaps 0.01 to 0.001 ohms. There are caps with even lower resistance. They will give much shorter flash durations but they are rare and expensive. You already have a bunch of these very low ESR caps since there is one on each of your photo boards. The low ESR cap is used to supply the power to the trigger coil. Their low ESR means they'll dump their energy very quickly through the trigger coil. Since transformers work based on the rate of current change (dI/dT) the faster you can dump the cap the higher the output voltage of the trigger transformer is. The trigger cap is charged to 300 V just like the big-ass photocap. It is dumped through the very low resistance of the trigger transformer and creates the 5~10KV trigger voltage that triggers the flashtube.

So you have some high speed caps of the correct voltage rating. Unfortunately, their capacitance is probably too small to be of use. IIRC, they are something like 0.01MFD. So that cap only holds ~0.01% as much energy as the 120 MFD photocap.

A smaller cap value should give a faster flash. If you wire caps in series the capacitance goes down; 1/Ctotal = 1/C1 + 1/C2 + 1/C3 + ...
The RC equation says the lower capacitance should give a shorter flash duration (and less energy and brightness in the flash). Unfortunately, as you wire caps in series you have to add their ESRs. So the C goes down but the R goes up. Net result is that RC doesn't change all that much, and the flash duration doesn't change much.

You can buy low ESR high capacitance caps but they are pretty pricey. You might want to look at Mouser.com or Digikey.com.

Kind of a long shot, but you might scavenge a bunch of the trigger caps from your photoboards. Wire them in parallel. That adds their capacitances. In parallel the reistance is 1/Rtotal = 1/R1 + 1/R2 + 1/R3 + .... If they all have about the same ESR then the final resistance is Rtotal = R_{one cap}/(number of caps). Ten caps will drop the resistance by 10 fold. The capacitance goes up ten fold. If the trigger caps are 0.01 MFD it'll take quite a few to get the energy for a sufficiently bright flash.

There are "simple" circuits that can turn off the cap's discharge before it turns itself off. So it might be possible to get a truncated flash with a duration of perhaps 0.01mS (0.036" movement at 300 FPS). Google might help you find something helpful. I suspect that this would be the best way to try to get a shorter flash duration.

One last thing, the 60V (or so) you measure across the shutter trigger on the board is actually the same valtage as is on the trigger and photocaps, 300+V. You only read 60V or so because the switch is in series with a 1 megaohm resistor. http://www.repairfaq.org/sam/kflashm.gif is a schematic of one type of Kodak disposable camera circuitry. The low ESR trigger cap is C3. The photocap is C2. C3 is charged to the same voltage as C2 but since it holds so little energy when you measure the voltage you actually are measure the photocaps (C2) voltage through a voltage divider. The divider is the 1 megaohm resistor (R2) and the internal resistance of you volt meter (probably 100K to 10M ohm).

Ok, I've taken the photoflash cap off the board and am gonna run a couple of wires out of the camera in it's place, that way I can mess around with different capacitors and not have to worry how I'm gonna fit it all in the camera. The first thing I'll try is just two of the smallest ones I have (80uF) in series, and see if this gives a better result.

I don't intend on trying it, but what would happen if I touched the trigger leads when the cap is fully charged? Would there be enough resistance that I would only feel a tingle, or would it be, ah, considerably worse?

I'm off to finish wiring this up and trying the in-series setup. (yay, this means there are more exposed high voltage parts!) I just realized how stupid what I'm doing actually seems.... I'm firing projectiles at live electrical contacts while operating a camera, in the dark.

Has anyone else done anything similar to this?

Thought about trying it, but never had the motivation.

I don't intend on trying it, but what would happen if I touched the trigger leads when the cap is fully charged? Would there be enough resistance that I would only feel a tingle, or would it be, ah, considerably worse?

Wait, you never touched a photocap before?
It kinda hurts, but only for a second (Catches you by surprise more then anything).

Also, wouldn't pushing more voltage through the flashtube decrease the pulse time? Maybe you can get a high(er) voltage charger from one of those dollar store bug zappers.

***

While this may not be useful for capturing high speed projectiles, it would be very interesting to see it used on an Exploding Watermelon.

Exploding watermelon eh? Pity I can't get fireworks here.

Here are a few more pics. I the first one is of a rubber band gun I made out of k'nex that would close the circuit when the pressure from the rubber band was released. (This was using the old camera)

The second two are using two 80uF caps in series, and while it seems to have helped a little bit I'm still a long way from properly 'freezing' the motion. The last one is the ball going through a sheet of paper.

Impressive work so far.

Thoughts...

Your clothespin is functionally similar to what is known in the ballistics industry as a "make paper."

There are many different types of make papers, but one that I've played around with (read: experienced in my professional life) that is dirt simple that MAY work for you.

Make a sandwich...

Aluminum foil / Saran wrap / Aluminum foil.

The two pieces of foil are separated by the saran wrap. As your projectile penetrates the above sandwich, one piece of foil is shorted to the other. This works REALLY WELL with conductive (read: metalic) projectiles. With non-conductive projectiles... It sometimes works and sometimes doesn't as the two pieces of foil may or may not actually contact each other.

Hmm... I think your sandwich might work better with alfoil-paper-alfoil... My reasoning is that paper tears rather than streaches, so I think it's more likely the foil would touch the other side. I will give it a shot later and see what happens.

I still need a way to shorten the flash duration though. Might add another cap in series and see if it helps at all.

Also, what happens when you connect a bunch of caps in series if they don't all have the same uF rating?