Another question please. I have several of these. Again, I am doing some zero-budget film making. In one short crime thriller movie I helped on, they had a scene with a crime scene photographer taking pictures. The director wanted the flash from the camera to be seen in the background while the characters talked about the crime in the foreground. However, there's a problem with that. The electronic flash is so brief (1/1000 of a second or faster) that half the times it doesn't even register in a video frame. (many higher end video cameras can imitate film camera's "shutter angle" which traditionally is 180-degrees. This means that for 24 frames per second, the video sensor records light for 1/48 of a second and does not record light for the other 1/48 second of the 1/24 second frame. This can be important for achieving that "film look" when shooting video.) So there's a 50/50 chance that the 1/1000 electronic photo flash will be recorded.

So, my idea is this. I have a couple old electronic flash units. I am considering removing the electronic flash guts from them, along with the xenon flash tube, and replacing them with the guts of a cheap (like $1) LED flashlight.

Problem 1, is that the flash unit has a battery compartment (which I intend to keep) for 4-AAs whereas the flashlight is powered by 3-AAAs. So my question is would over-driving the LEDs with the extra voltage from 4 batteries be fatal to the LEDs made for 3 batteries? A couple solutions come to mind. One is to put a conductive battery placeholder, like a metal rod cut to fit, in place of one of the 4 batteries. Or, use rechargeable 1.2v lithium AAs who's voltage would add up to 4.8v instead of 6v. Or don't I need to worry at all? Would 6v for a fraction of a second be harmless to the LEDs?

Issue 2. Most camera flash triggers close the trigger circuit for the duration of the exposure. So by setting the shutter speed of the prop camera to something like 1/8 of a second would allow 2-3 frames of video to record the "flash." This might be important for addressing another video problem called "rolling shutter" which I won't go into. But again, there is a voltage concern. I read that modern electronic cameras (DSLRs) should not have flash trigger voltages exceeding 5v. So, should I put somekind of isolating transistor circuit in the flash prop rather than just wiring the batteries/LEDs directly through the prop flash hotshoe? I want to keep this as simple as I can. I'm guessing that the 5v limit might be conservative and the circuit will easily handle 6v rather than 5v. Again the 1.2v 4-lithium battery idea above would provide only 4.8v. But I can't be sure that someone on set wouldn't replace the batteries with 1.5v ones.

This sounds trivial, I guess, but I don't want to be accused of damaging someone's $1000+ DSLR.

Thanks again.

 

One is to put a conductive battery placeholder, like a metal rod cut to fit, in place of one of the 4 batteries.

That's the best option, IMO.

Would 6v for a fraction of a second be harmless to the LEDs?

Probably.

I'm guessing that the 5v limit might be conservative and the circuit will easily handle 6v rather than 5v.

With an expensive camera at stake you shouldn't be guessing . Check with the camera manufacturer. The limit for some cameras may be the camera's battery voltage (3.7V?).

 

Thanks.

I do have an old DSLR (Canon 30D) that I don't use anymore. I might make that my movie prop camera.

But, what would be the simplest circuit (like a transistor and a couple resistors) that would feed all the voltage (6v) from the battery to the LEDs yet allow only half that voltage (3v) to the camera's hot shoe? I used to tinker with some of this long ago, but I am already overwhelmed with the filming. Last week we shot for 7 hours to get 2 minutes, at most, of on screen footage.

Thanks

 

I think what you need is a ubiquitous '555-based' pulse-generator circuit driving the LEDs. The hot shoe contacts would simply trigger the circuit and need not have the 6V on them. The pulse-gen could provide a decent fixed-length pulse (say 1/4 sec) regardless of shutter speed (if speed <1/4 sec), to make the LED clearly visible.
This should do the job:

Note: The 555 supply current is ~6mA in standby, so you would need to remove the batteries when not in use, or else provide an 'off' switch.

 

Here's a variation on Alec's 555 circuit that adds a transistor switch in the power to the 555 to give zero current in standby, and thus doesn't need an off switch.

 

A useful mod, but note that it does put 6V across the hot shoe contacts, whereas it's 3V with the post #4 circuit.

 

Here's a re-work of my circuit, adopting Crutschow's suggestion of a switched +ve rail (though done somewhat differently) to reduce standby current to zero. The 6V battery is tapped at 3V, so that the maximum voltage across the camera contacts is 3V.

Edit: A PNP would do instead of the FET.

 

Why are you concerned about 6V across the contacts?
It wouldn't seem that would be a problem.

Note that the output of the MOSFET could drive a larger load, if needed.

 

Why are you concerned about 6V across the contacts?

The OP thinks it might be a problem. He said in post #1 "I read that modern electronic cameras (DSLRs) should not have flash trigger voltages exceeding 5v."
If the contacts are volts-free then no problem; but are they so in all cameras?

 

Last week we shot for 7 hours to get 2 minutes, at most, of on screen footage.

Working from memory, details might be a tad off...

2nd Raiders film, big chase scene in a runaway mine car. Between two closeups of screaming, terrified riders is a wide shot of the car crossing a wide expanse of the mine. The wide shot was all model work, 33 days in a British soundstage to build, light, and shoot. The segment in the final edit is 28 frames long.

ak

 

Thanks everyone.

I've used 555 timers in my youth, but I don't recall many details and am sure I have some in my boxes of jun... er, "resources." 1/4 second is probably too long to look like a brief enough camera flash. 1/8 second might work. 1/10 or 1/12 second is maybe better. I think It needs to be long enough to show up on at least 2 frames.

Now, 1/4 second might be good for simulating an old style flash bulb that has a noticeable fade out of the lighting for a retro period piece.

Some movies simply overlay a white or brightened frame or two, but I can tell when they do that.

So I need to find my thinking-ma-cap and go through storage boxes to see what I can find.

Thanks.

 

Back to the original question. You could use 4 rechargeable NiMH batteries, which produces 4.8V which is about the same as 3 fresh alkaline batteries.

Bob

 

The value of R6 in Alec's circuit can be changed to control the flash duration.
You can make it a 500kΩ pot if you want to vary it.

 

DYI - found that there as a standard for camera flash triggers. ISO 10330. Which seems to say that any camera flash circuit should be able to withstand 24v, but it is unclear in the discussion forum if that is only for the X-Sync connection (which many modern camera lack) or also includes the hot-shoe mount. Other people posting that thread say that the hot-shoe connection should not exceed 6v, but other posts claim that anything under 20v is safe. It's also claimed that some camera manufactures might not follow this standard or even let know what the max trigger voltage can be and state that you should only use their own brand of flash units compatible with that model of their camera.

 

Did someone post a simple transistor circuit and then take it down;/ I seem to recall a simple circuit diagram somewhere.