Re: Flashlight dimmer for cave photography?

From: John Muchow (jmuchow_at_SPAMMENOTcamlight.com)
Date: 07/02/04 

Date: Fri, 02 Jul 2004 20:38:49 GMT

>>>PWM (pulse-width modulation) - your steady-state is a stream
>>>of pulses with a duty cycle proportional to your desired light
>>>level
>This is exactly what I was thinking. It's just like pinching a garden
>hose periodically to govern the water flow. How about a capacitor
>(expandable rubber section in-line to absorb the inrush volume) in-line
>to avoid jarring and fatiguing the filament?

Something like a sine-wave would treat the filament better than a
simple full-on/full-off square wave. Another way is to just bring the
bulb down a few volts. Not turning it off (so it starts cooling), but
just down enough to reduce its brightness enough (but still stay hot).
I guess a square wave could work here since it would get averaged out
to pulsating DC by the slow response time of the filament.

>>> halogens hate being dimmed.OK, they don't hate it, but the bulb life
>>>is significantly reduced.
>Get out of town?! One would think it would last noticeably longer at 75%
>power than at full power. Is it possible that the Wolfram gas would not
>be redeposited on the hot spot at lower temperatures? But, it's got a
>boiling point of 10,000 degrees F. There can't be much of it
>evaporating. How hot is a filament?

I don't understand it completely (but I'm sure there is a lot of data
on the WWW), but my understanding is that at less than full voltage,
the evaporated metal gets deposited on the inside of the glass bulb
and not back onto the filament, causing premature failure of the
filament.

>>>But, if I read the post correctly, the halogen light is to be turned
>>>off before the picture is taken
>Actually, it is the flash signal which would turn it off. This secondary
>light would help focusing and also reduce the power of the central
>flash reducing the dreaded white spot on people's foreheads, noses and
>cheeks, especially at night time, summer parties!

I'm not sure the timing works here. The flash fires when the
"shutter" is already open and then the lamp circuit receives a signal
to turn off. Problem is that the lamp takes a while to turn off, up
to several hundred milliseconds. This might affect the image.

But, reading your response above, it sounds like you want some of the
lamp light to hit the subject to balance out the flash. Those
reflections you mention are often highlights due to reflections from
skin oils. No matter what your light source is, it will reflect more
from those body "high points" than from others. Not sure if having
some of the lamp light mixed in with the flash will work.

>>>Ah! Then just a Watson Name flashlight with a pot or 555 PWM, and just
>>>a "bright-<~.5s>-Shoot-Off" sequence sounds like it would work. :-)
>
>Huh?
>
>Where can I find the parts to build a pair of these? Radio shack didn't
>seem to have anything of the sort online.

Rich's idea will definitely work

A standard 555 timer chip would work, or a 556 (dual-555's) might be
needed. Probably the easiest thing to do would be to have the 555 run
in "astable mode", that is creating a series of square waves with an
"on" time and "off" time (thus the frequency too) adjustable by you.

The 555 would trigger a MOSFET that is rated to handle the voltage and
current of your lamp (with a good 50% safety margin) and be set to
give you the "idling" brightness you need. I'd then have a pushbutton
switch that triggered another 555 timer (or the second timer in the
556 timer chip) in "monostable mode" to create a pulse that turns on
the MOSFET full (no square wave going on/off) to give you the high
brightness you want. The pulse from this second 555 would be set to
what ever length of time you felt you'd need for this high-brightness
mode. If you needed it on for longer, the monostable can be set to
stay high whenever the button is held down. Otherwise, the lamp would
go back to the idling brightness after this second 555 timer's pulse
had ended.

This doesn't automatically bring the light back down to idle when the
flash fires but it is a simpler circuit to build and can be modified
later to add that "auto-idling" function if you want.

John
John Muchow
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