My skull is shining too brightly. What can I do?

mickledore

mickledore

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I have a skull. In fact I have two. One I use more or less regularly and one that only comes out in winter. This is a real skull. Real plastic!
In the eye sockets I have put two blue LED lights. These are from a £1 box of Xmas tree lights from Poundland. They are powered by three AA batteries. So far so good.

Only problem is when I'm trying to do nighttime shoots and using long exposure the lights are blowing, and instead of a nice blue effect all I get are blown white highlights. Even 15 seconds is too long, but nowhere near long enough for the shots I am trying to take.

So techy guys - I need help! I presume that there will be some form of voltage reducer that I can wire into the circuit. But which one is suitable? Other than guess work how do I know which wire to cut into? Obviously I don't want to spend a fortune for what is probably a very small control type unit. I don't want to buy a pack of 10 with extortionate postage for somehting that will get used two or three times each winter.

Can any of you techy guys point me in the right direction? I've no tools other than a screwdriver and wire cutters, and no real knowledge of these things. I wired everything together very much on a trial and error basis!
 
How about dimming the blue lights with an ND filter type "thing" placed in front of the leds?

Or do an exposure blend, one shot exposed for the leds, turn them off then take your long exposure shot, merge in PP.


ps: this is possibly in the wrong forum? the lighting forum might be better?
 
Trouble with putting something infront of the leds is that I'd have to fix it somehow in an irregular shaped hole. Plus if it I made it big enough to be handled easily it would be at the front of the socket, and visible.

Have tried the double exposure method but it ain't right. The glare coming from the LEDs doesn't show up properly, so the whole jobby looks fake - which it is, but I like my fake to look real!
 
Couple of resistors should do it, see here: http://www.raltron.com/cust/tools/voltage_divider.asp

e.g. with an input voltage of 12 volts DC, using 2 x resistors of 2k (R1 & R2) will give you an output voltage of 7 volts DC - adjust the value of R1 & R2 to suit your needs.
 
OOH Thanks Gramps - I think! Not sure what that all means, but understand the principle. Only problem is will three AA batteries give 12 volts?
 
Don't put the resistors in a divider network like that link. Just put a resistor in series which each LED... or just one if the LEDs are already in series.

Normally, an LED wants about 20mA through it. Blue LEDs drop about 3.5v leaving 8.5v across the resistor. Using Ohm's Law, v = IR, the resistance necessary for this normal operation is 8.5/0.02 = 275. Closest available value is 270 ohms.

However, if you want them less bright, you need less current. Lets say 3ma. The resistor you need now is 8.5/.003 = 2833. Closest available value is either 2.7K or 3.3K. Either will do.

You don't need much current as even the slightest glow shows up brightly in photographs when the ambient light isn't too bright.

mickledore said:
Only problem is will three AA batteries give 12 volts?
Click to expand...

Three AA cells gives 4.5 volts so the resistors needed in my examples will be smaller.

i.e. for normal 20mA operation, about 47 ohms and for about 3mA operation, about 330 ohms.

EDIT: Here is a better explanation: http://www.evilmadscientist.com/2012/resistors-for-leds/


Steve.
 
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mickledore said:
OOH Thanks Gramps - I think! Not sure what that all means, but understand the principle. Only problem is will three AA batteries give 12 volts?
Click to expand...

Doesn't matter what the voltage is, the formula works accordingly ... establish the voltage your batteries are producing (possibly 4.5 v but check) and check the resistors needed to reduce the voltage to something that obtains an acceptable light output - variable pot resistors would allow you to adjust with a small screwdriver.
 
Tha LEDs are wired in parrallel at present. Would that greatly change things? It would be easier to work on the cable from the battery pack, as there isn't much room for my chubby fingers to reach inside the skull to relatively short leads from the lights.
 
mickledore said:
Tha LEDs are wired in parrallel at present. Would that greatly change things? It would be easier to work on the cable from the battery pack, as there isn't much room for my chubby fingers to reach inside the skull to relatively short leads from the lights.
Click to expand...

See #4
 
What about using one battery instead of the 3 you are currently using. (no pun intended)
 
Or you could try running on 2 batteries + a bridge across the gap of the 3rd battery.
 
That might be worth a try. Easier than trying to grapple with resistors etc!
 
And/or post a pic of the battery compartment up so we can see if its possible to bridge it somehow.

Or what about holding a large card in front of your skull for the first portion of your long exposure and then whipping the card away towards the end of the exposure. Experiment to determine the right time at which to pull it away
 
ancient_mariner said:
Or you could try running on 2 batteries + a bridge across the gap of the 3rd battery.
Click to expand...

Two cells will only give 3 volts. Blue LEDs typically have a Vf of 3.2 to 3.6 volts so not enough.

You might be lucky and get a little bit of light with 3 volts but it's bordering on the operating voltage of the LED. A higher voltage with a series resistor is the best way.

DorsetDude said:
What about using one battery instead of the 3 you are currently using. (no pun intended)
Click to expand...
Less than half the required voltage.

gramps said:
Doesn't matter what the voltage is, the formula works accordingly.
Click to expand...

It does when the circuit is free standing. As soon as you connect the LED in parallel with the bottom resistor of the divider, unless the resistor values are very low, the formula will no longer work as most of the current will be going through the top resistor and the LED with only a small amount going through the bottom resistor.


Steve.
 
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Let me get back to you this afternoon. I'm in the middle of something important - called work!
 
Steve Smith said:
Two cells will only give 3 volts. Blue LEDs typically have a Vf of 3.2 to 3.6 volts so not enough.

You might be lucky and get a little bit of light with 3 volts but it's bordering on the operating voltage of the LED. A higher voltage with a series resistor is the best way.



It does when the circuit is free standing. As soon as you connect the LED in parallel with the bottom resistor of the divider, unless the resistor values are very low, the formula will no longer work as most of the current will be going through the top resistor and the LED with only a small amount going through the bottom resistor.


Steve.
Click to expand...

I agree, but sometimes there's enough tolerance in a system to work on the edge. A resistor is going to be better because it's controllable, but they require ordering & soldering etc.
 
ancient_mariner said:
I agree, but sometimes there's enough tolerance in a system to work on the edge.
Click to expand...

Yes. It might just give out enough light on 3 volts.

ancient_mariner said:
but they require ordering & soldering etc.
Click to expand...

Well, there is alwyas the old 'universal connector' method of stripping the wires, twisting them together and taping!

Still needs ordering... although if the OP wants to send me a postal address by PM, I will send a selection of resistors.


Steve.
 
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mickledore said:
Only problem is when I'm trying to do nighttime shoots and using long exposure the lights are blowing, and instead of a nice blue effect all I get are blown white highlights. Even 15 seconds is too long, but nowhere near long enough for the shots I am trying to take.
!
Click to expand...

aside from all the techy stuff isnt the obvious answer to take a short exposure of the skull then turn the eyes off, do your long exposure and then blend the files in photoshop
 
big soft moose said:
aside from all the techy stuff isnt the obvious answer to take a short exposure of the skull then turn the eyes off, do your long exposure and then blend the files in photoshop
Click to expand...
You've not read it all have you. #2
 
if you want to keep the BLUE effect i would suggest some of the blue sweet wrappers from a tin of roses sweets you can double or treble wrap them to get a drop in light output ,just hold in place with elastic bands .if you keep different coloured wrappers you can even change the colour .probably the simplest solution
 
the black fox said:
if you want to keep the BLUE effect i would suggest some of the blue sweet wrappers from a tin of roses sweets you can double or treble wrap them to get a drop in light output ,just hold in place with elastic bands .if you keep different coloured wrappers you can even change the colour .probably the simplest solution
Click to expand...
That does have the drawback of having to munch through half a tin of Roses though.

Hang on, did I say drawback?
 
OK. here are a couple of shots of the skull.

Top one shows the face with the LEDs inplace. Putting a cover over the lights might be difficult with the lack of space to provide a secure fixing, whilst still remaining unseen.


IMG_4746 by mickledore on Talk Photography


The second one shows the battery tray. The LEDs are connected, but only on short cables so that is all inside the skull. Ideally I would like some simple form of variable resistor to simply insert into one of the cables from the battery tray. I have a soldering iron, but would probably use use simple screw connectors as the easy option.

IMG_4747 by mickledore on Talk Photography
 
Again I've thought of that, but I want everything inside the skull when it's in use, and I can't find a dimmer small enough to go through the hole. Don't want to enlarge the hole as I'm getting to the thin bits and I'm scared I might crack it - as I did in the left eye socket!
 
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I'd certainly (as already suggested) try a dab of water based marker pen over the eyes,
so you can wipe it off if it didn't work.
or maybe something like cling film doubled / trebled over until you get
the required dim-ness?
 
Have you tried flat batteries? Raid the TV remote and anything else you have with AAs in!


Steve.
 
Surely it would be easier to run a thin cable with a remote switch out of camera view, and turn on the led's for the last seconds of exposure.
Then establish exposure time without the lights, and work from there.
That would then be a simple single exposure with a flick of a switch part way through.
 
Steve Smith said:
Have you tried flat batteries? Raid the TV remote and anything else you have with AAs in!


Steve.
Click to expand...
I tried one of the flat 9v batteries, but it nothing lit up at all. I will try that again just in case I connected it wrongly, but wouldn't that be even brighter?
 
david357 said:
Surely it would be easier to run a thin cable with a remote switch out of camera view, and turn on the led's for the last seconds of exposure.
Then establish exposure time without the lights, and work from there.
That would then be a simple single exposure with a flick of a switch part way through.
Click to expand...

That's an idea I hadn't thought of. It would need a bit of trial and error, but that's the beauty of digital.
 
mickledore said:
That's an idea I hadn't thought of. It would need a bit of trial and error, but that's the beauty of digital.
Click to expand...
What about blocking off the leds/skull with a large "flag" to achieve a similar effect? No wiring required.
 
Not sure I get that! I'd like if possible to be able to take some front end shots so nothing too obstructive is wanted. Plus if I do side on shots I don't want to have anything stcking out of the sockets.
We spent most of last night trying to bodge something, but gave up and went to the pub - without the skull!
 
One more thought. If it's a long exposure, can you turn it on for just a fraction of the exposure time? Run a couple of wires out with a switch on the end or just hold the two ends together for a short time.


Steve.
 
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