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Flashlight dimmer for cave photography?

Discussion in 'General Electronics' started by BrianP, Jun 21, 2004.

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  1. BrianP

    BrianP Guest


    I have a harebrained scheme to create a 12 Volt, wide angle flashlight
    with a dimmer. I want to be able to go from the glow of a firefly to
    that of an aircraft landing by adjusting a dial or slider.

    I am a Scientist (mechanical engineer) and Photographer and am often
    called upon to shoot in cave dark conditions. In bright light
    conditions, my camera (Nikon D100) uses the old subtract and add
    algorithm to find the point of maximum contrast to focus. In dark
    conditions, it falls back on the much less accurate infrared beam trick
    and the results are noticeably less consistent and accurate.

    I want to put a 12 Volt, wide angle, halogen bulb on a dimmer and be
    able to illuminate my subject(s) with or without blinding them in the
    process. It will also give me a second, offset light source to reduce
    shadows and shine from the flash. I need power in roughly the 10 - 100
    Watt range.

    Another, extremely useful feature would be to have the lights
    automatically turn off as soon as the main flash strobes. I have a
    photoelectric flash attachment which flashes a slave flash when it sees
    the main flash go off (or when you pass your hand in front of the
    detector and it senses a spike in illumination). This apparatus sends a
    voltage telling the slave flash to flash. Surely, this same signal could
    be used to turn a circuit off.

    Ideally, I would get ready to take a picture, press a button to turn the
    secondary light(s) on, dial in the desired illumination, take the flash
    photo and have the light of the main flash turn off the secondary lighting.

    I checked Radio Shack and all they have are either 120 V AC dimmers or a
    3 W, DC rheostat. I want a time division switch which keeps the
    connection closed for between 1% and 99% of the time. Ideally, it would
    run off of the 12 V supply it is controlling and not use the same amount
    of power all of the time by burning some of the juice off as heat and
    the rest as light. I am thinking about a time division circuit
    connected to a capacitor to cushion the sudden blasts of on-time voltage.

    Any brilliant ideas?

  2. Rich Grise

    Rich Grise Guest

    With a micro and power FET, this is almost trivial. It's a natural
    for PWM (pulse-width modulation) - your steady-state is a stream
    of pulses with a duty cycle proportional to your desired light
    level (roughly - your eye/hand would be in the feedback loop)
    and when you press the "secondary lights on" you just go to
    full brightness until you sense the "the flash just flashed"
    signal, then go back to idle.

    Nothing to it - you might not even need to do any programming -
    somebody will probably do this with a 556 and a couple of

  3. JeffM

    JeffM Guest

    create a 12 Volt, wide angle flashlight with a dimmer
    Grise covered the PWM part.
    Have you considered chromatic response over the range?
  4. John Muchow

    John Muchow Guest

    Have you considered chromatic response over the range?

    That's what I was thinking (the bulb would definitely go "red" as the
    brightness was reduced). In addition, halogens hate being dimmed.
    OK, they don't hate it, but the bulb life is significantly reduced.

    But, if I read the post correctly, the halogen light is to be turned
    off before the picture is taken (and back on again after the "shutter"
    has closed). It's primarily being used to allow the auto-focus to
    operate more accurately.

    John Muchow
    -- remove SPAMMENOT for e-mail responses --
  5. Rich Grise

    Rich Grise Guest

    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. :)

  6. barryn56

    barryn56 Guest


    if your main requirements are to a) not alert your subject and b) use
    mainly in dark environments, why not just build an infra-red light
    source (IR LED array) and turn that on to help your camera focus in
    the dark?

    You would not need to switch off during exposure.
  7. JeffM

    JeffM Guest

    if your main requirements are to a) not alert your subject and b) use
    I presume that this is in response to
    Flashlight dimmer for cave photography.

    Some newsreaders (and certainly the Google archive)
    lose track of the thread when you change the Subject line.

    If you still feel the need to change it,
    at least blockquote a bit of the original post.
  8. BrianP

    BrianP Guest

    Rich, Jeff, John,
    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?
    Simple photoelectric effect generates a voltage which energizes an
    electromagnetic which opens a rotating switch stopping the current.
    That's how my old Mustang's alternator knew when it was done charging my
    battery (substituting battery voltage for photoelectric effect). Isn't
    that how an old fashioned volt meter works?
    It would get redder as the filament got cooler ( blackbody response to
    lower 4th power K temperature). Actually, I was thinking of having 2,
    one on each side of the camera to give me offset lighting to reduce
    reflection and glare. I could turn one off rather than running 2 at half

    But, at sunset, you get a much redder light than at noon with a lower
    color temperature and that is often the best time to shoot. It's much
    better than shooting in a shadow with a white balance set for direct
    sunlight and getting blue skin.

    Or, have one of those blinding blue LEDs which would kick in in some
    sort of inverse proportion to the halogen power!
    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?
    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 want to have 2 of these mounted on flexible "eye stalks" which I could
    point where needed for the shot. Attach them to a plate planted on the
    bottom of the camera. Perhaps wear them like antennae so you wouldn't
    need to have a wire from the battery (belt) to the camera. Hmmm.

    Where can I find the parts to build a pair of these? Radio shack didn't
    seem to have anything of the sort online.

    Thank you,

  9. John Muchow

    John Muchow Guest

    PWM (pulse-width modulation) - your steady-state is a stream
    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.
    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
    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.
    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 Muchow
    -- remove SPAMMENOT for e-mail responses --
  10. JeffM

    JeffM Guest

    PWM (pulse-width modulation) - your steady-state is a stream of
    Only if you can pinch and release VERY quickly. :cool:
    PWM works by integration at the load.
    If you mean ramping up the duty cycle to its final value, OK.
    Doubt it would have much effect.
    A reference to Watt Sun.

    Take a FET that will handle the current of your bulb,
    with its gate driven by the 555 astable.
    A photosensitive latching circuit could reset the PWM.

    555 basics
  11. John Muchow

    John Muchow Guest

    halogens hate being dimmed.
    That halogens hate being dimmed, or that I should get out of town?

    John Muchow
    -- remove SPAMMENOT for e-mail responses --
  12. JeffM

    JeffM Guest

    halogens hate being dimmed.
    and the halogen you rode in on. :cool:
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