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tade 8th November 2006 10:37 PM

DIY camera flash
i got a bunch of used flash units from one time use cameras and I want to make them useful.
I want to know if i can daisy chain the flash lamps. considerations there will be increasing the capacitor size, whether whatever controls the voltage will be able to handle the extra current, and what to do with the trigger wire. Do i simply extend it along the length of my linked flashbulbs?
I guess that when you trip the shutter, it sends a high voltage pulse to the trigger wire. this ionizes enough of the gas in the tube to allow the current to flow, at which point most will flow out of the capacitor in a very short pulse.
My other option which i want to pursue would be making some sort of controlled trigger so that i could have three or so units trigger at an interval which the user can set.
I would also like to know how the hot shoe on a camera triggers the flash. Does it supply some voltage, or does it simply close a circuit?


Pano 9th November 2006 09:27 PM

A shocking subject!

You have to be careful, that stuff can really hurt, especially if you have a lot of them together. Might be the last thing you ever feel. :dead: :dead: :dead:

I once discharged some huge flash caps from one hand to the other - 450V. Lucky to be alive. (it was in Greensboro, BTW).

The trigger is usually a very high voltage supplied by a flyback coil. It ionizes the xenon inside the lamp so that current can flow. Once the current flows - it happens very fast - in a flash! Then the caps are discharged.

If you look up strobe circuits, you will see how they work. Radio Shack used to sell a kit. There are certainly other kits around; I see them all the time.

The flashes you have will not have big caps; the flash tube is too small to use much power. The high voltage is supplied by a DC to DC converter. It steps the battery voltage up to the higher voltage needed by the flash. The DC-DC converter working is the whine you gear when the flash is charging.

The hot shoe on a camera is a contact closure. But now there are lots of other contacts for smart flashes. Those contacts carry information.

Be careful!

pixpop 9th November 2006 10:18 PM

Many modern cameras have electronic trigger circuits connected to the hot shoe. This means that it's easy to fry the camera if you connect unnatural things to it. The modern flashes that go with the modern cameras only put low voltage on the hot shoe.

Old cameras, maybe built before the 80s, used real contacts to fire the flash, and will work fine with high voltages. When the contacts close, the flash electronics generates a trigger pulse that ionises the gas in the tube.

Most likely, the disposable camera works like the old cameras, with a mechanical contact. If you measure the voltage across these contacts when the camera is ready to fire, you can figure out what kind of transistor you would need to trigger it. Be careful.. it's likely to be a few tens of volts, but could be as much as a few hundreds. I have an old Sunpak flash that puts 190 Volts across the flash contacts. If the disposable flash uses contacts, you can fire it with an NPN transistor or an SCR, as long as they can withstand the voltage.

Beware of using a larger capacitor. Too much energy will destroy the flash tube. Also flashing too often. If you want it to flash faster, you could make the capacitor smaller.

You don't want to be connecting your flash tubes together.

Sam's laser faq has info about camera flash circuits.

DigitalJunkie 9th November 2006 10:34 PM

Here's my 0.02.

Be careful with raising the value of the capacitor,I once replaced the cap with a 450V 2000uf monster,and promptly vaporised the flashtube!
I was picking hot glass out of my arms and face. :hot:

Just FYI. ;)

dnsey 10th November 2006 12:37 AM

There's some useful information here

tade 10th November 2006 01:56 PM

good call digital junkie! hmmm, i may see about designing a circuit to trigger them manually and maybe sequentially. I want to mostly use it for high speed photography. The easiest way to do it is to design some sort of application specific to close the flash trigger. Like burying a pair of small pieces of aluminum foil in a container of sand. When you shoot the sand when the projectile is deep enough, it triggers the flash. I think the flash may be as fat as 1/4000 of a second?
let me think on it a bit,

djQUAN 10th November 2006 02:06 PM

I have experimented with those in the past.

I would recommend a small SCR in TO-92 package will be enough for the trigger. applying a voltage triggers the flash. as far a sI can recall, the high voltage spike can be positive or negative or whatever as long as a high voltage trigger is applied is enough to trigger the tube.

small tubes like those discharge the cap down to 48V only. not sure how long they will last though. continuous flashing with a 33uF cap makes the inside of the tube silvery.

Pano 10th November 2006 08:08 PM


Originally posted by tade
When you shoot the sand when the projectile is deep enough, it triggers the flash.
"Aiming" to follow in Doc Edgerton's footsteps, are you? :cool:

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