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Detect Photo Flash? — Parallax Forums

Detect Photo Flash?

baumentbaument Posts: 5
edited 2006-02-03 03:45 in BASIC Stamp
Hi guys, new here.......
Need a circuit· to detect photo flash of short duration. I've tried CDS photocells and they just don't seem to be able to react quick enough. The venerable LASCR's are tough to find.

I've thought of a photodarlington driving a couple 2n2222's with a trimpot to control sensitivity. Anybody done this before or have a circuit to share? We're in the 1/250th of a second in the flash duration so it's got to be able to react quick (and perhaps would need some sort of pulse stretcher, not for sure)...

Any info would be appreciated.......

Comments

  • steve_bsteve_b Posts: 1,563
    edited 2006-02-02 16:44
    Do you need to see the flash? or can you see the signal going to the flash element?

    not sure a phototransistor would be fast enough.

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    Steve

    "Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."
  • baumentbaument Posts: 5
    edited 2006-02-02 23:30
    Yea, need to see the actual light coming from the flash. I've thought about buying a slave flash cheap on ebay and stripping the circuit out and adapting that. I'm sure it triggers an scr (because it seems most do) but perhaps I could hack the signal off prior to it reaching the scr........
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2006-02-02 23:51
    What about a standard IR transistor? The response time should be at least 25uS (40kHz) but nyquist suggests it would actually be
    twice this value, so 12.5uS.
    You said...

    I've thought of a photodarlington driving a couple 2n2222's with a trimpot to control sensitivity. Anybody done this before or have a circuit to share? We're in the 1/250th of a second in the flash duration so it's got to be able to react quick (and perhaps would need some sort of pulse stretcher, not for sure)...

    If this approach does not work, I would try a current mirror for this application. Let me get back to you on a schematic that
    might work for you based on this approach.

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • baumentbaument Posts: 5
    edited 2006-02-03 01:47
    That would be super Beau, thanks!
  • baumentbaument Posts: 5
    edited 2006-02-03 02:17
    Here's the circuit I was going to try.......
    SlaveStrobeTrigger.jpg
    500 x 379 - 24K
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2006-02-03 03:19
    baument,

    Hmmm.... I'm confused about the 3k resistor that goes to the BASE of both 2n22232's and it looks like
    MAL12 is getting shorted to GND.


    I have attached a circuit that I used in a light pen project...The 74HC14 is not necessary, you should be
    able to omit this.

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
    trigger.jpg
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  • baumentbaument Posts: 5
    edited 2006-02-03 03:45
    Correct on the 3k, there should have been a join symbol between the 3k, the collector of q1, the base of q2 and the 220 pullup.
    Looking at it now I agree the base on the darlington should float. I also omitted a 47k pot in series with the 3k pullup on the collector of the darlington to adjust the trigger level.
    Thanks for the current mirror, I'll give it a try!
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