Re: Photo Cell responce time

From: Anthony Fremont (spam_at_anywhere.com)
Date: 01/24/05

  • Next message: Tim Wescott: "Re: Photo Cell responce time" 
    Date: Mon, 24 Jan 2005 22:33:13 GMT

    "Chris W" <1qazse4@cox.net> wrote in message
    news:jCcJd.1096$al1.805@lakeread07...
    > Tim Wescott wrote:
    >
    > > Chris W wrote:
    > >
    > >> I was wondering if someone could point me to a source for a photo
    > >> cell that has a fast response time. I'm not sure where to find
    data
    > >> sheets on them.
    > >> Here is what I want to do. I want to hook the photo cell up to a
    > >> friends fluke scope meter and use it to measure duration and number
    > >> of pre flashes from my digital camera flash. I know a flash
    duration
    > >> can be less than 1/40,000 or a second so obviously the photo cell
    > >> needs to be faster than that.
    > >
    > >
    > > You want a PIN photodiode, not a photocell, and you want it in a
    > > fairly fast circuit. Since the flashes are intense and well
    separated
    > > you don't actually have to catch the flash -- just the fact that it
    > > happened.
    > >
    > Why not a photocell? Why do I need a fast circuit, or any circuit?
    > Won't the scope meter be able to measure changes in an applied voltage
    > drop with the light flashes? I'm also not sure what you mean when
    you
    > say "you don't actually have to catch the flash". I want to know how
    > many times it flashes and how long each flash is as well as the time
    > between each flash.

    I think the problem is that the photocell is too slow in it's reaction
    time. You are wanting to see pulses 25uS wide spaced maybe 100mS apart.
    Rise and fall times of CdS photocells is measured in 10's of mS. IOW,
    they are just not capable of giving you any meaningful data.

    You said that you wanted to measure the pulse width, so you need
    something fast (in terms of rise and fall time response). If Tim told
    me that I needed a pin photodiode, then that's what I'd be researching.
    ;-)

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