Photo studio LED Strobe

Discussion in 'The Projects Forum' started by nedix, Jan 22, 2014.

  1. nedix

    Thread Starter New Member

    Jan 21, 2014
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    Hi all,

    A couple of weeks ago I bumped into some LED panels (48 piranha LED) which were cheap enough to consider them for the following project. I was thinking to building a studio strip-light strobe; for whoever is not familiar with the term this is simply a photo-studio strobe but in a shape of a long strip not a square, triggered either by another flash (slave) or directly by a remote (master). I will not get into the details of why building it.
    I used the attached schematic, which is fairly simple and so far the test results are ok. The logic behind it is to actually overload the panels with an impulse of max. 0,1s with 10% duty cycle. The panels are rated at 12V/300mA and yes, I know I might burn them, or one of them, by increasing the current, but I'd really like to build them. The characteristics I found on the Internet so far are saying about a ratio between 1/3 - max 1/10, between the rated current and the peak current. While I'm not sure I will peak the panels with 3A individually from the very beginning, I'd like to experiment with 1-2A peak.

    0,1s is more than enough for studio shooting, most of the cameras go out of sync after 5ms so I can easily lower the duration of the impulse. The FT is not very sensitive, which is good, right now it can be triggered either by a very close LED flash-light or by the built-in camera flash. In the schematic, only one panel is shown, I will actually use 5 either in parallel or put more MOSFETs. I still have to tweak the MOSFET saturation and the panel currents, this is an ongoing work.

    My questions so far are:
    1. How can I reset the 555 timer in such a way that after 0,1s (T) is stays off? In other words, if the input impulse is longer than T, let's say 1 sec, then how can I make sure that the output is reset after 0,1s? By this, I would like to prevent burning the panels in case of an accident when there is a continuous and strong light in the FT. Also in other words, I would like the 555 to be triggered on the raising front of the IN-impulse and not to actually care about the duration of the IN-impulse.

    2. Just as my regular strobes in the studio, I would like to have some sort of a modeling light. In the classical strobes, this is achieved by having 2 lamps in the head, one regular bulb and the strobe itself. The bulb is continuously lit and it is used not only to actually see what you shoot but also to have an idea about the shadows and help the camera to focus; when the camera is triggered then the strobe flashes.
    I was thinking that maybe I can achieve this by having the LED panels normally powered and then trigger only a spike for actual shooting (mimicking the strobe). This idea is in the 2nd picture, in which just a "black-box" is shown; I need some sort of a circuit that can control the current going through the LED panels and in the same time to respond to the impulse given by 555.

    I'm opened to suggestions.

    Nedix
     
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  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    1.a. As drawn, your 555 one-shot will produce a high on pin 3 for only 11 milliseconds (.011 seconds.) To get the .1 seconds you want, increase R1 to 1MΩ (which will actually produce a .11 second pulse) give or take for component tolerances.
    1.b. The 555 one-shot is triggered on the falling edge of a pulse on pin 2. It doesn't matter how long the pulse is; the 555 won't trigger until the pulse goes low. However, there may be a potential problem with sync if the input pulse is too long.
    2. You should be able to have the LEDs on for use as a modeling light by simply powering them with a low current supply in parallel with the flash power. I think you would need to isolate the flash output from the low current supply to avoid damaging the low current supply with the high current pulse, but I haven't really thought that through.
     
  3. nedix

    Thread Starter New Member

    Jan 21, 2014
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    Hi,
    1a. Yes, 11ms is enough for my needs. My Nikon's sync freq is 1/200 which is 5ms.
    1b. Are you sure? If I test the circuit with a flashlight in the FT, then the Output stays UP as long as the FT is lit. From what you are saying, the circuit should not go UP when lit (rising edge) but when the light is actually going -> gone; this is not currently happening.
    2. Still under assessment
     
  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    1.b. Yes, I am sure. See page 9 of the datasheet.
     
  5. nedix

    Thread Starter New Member

    Jan 21, 2014
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    You're right, I got the signals mixed up, I missed the fact that the FT is keeping the Trig High when it's dark and Low when lit.

    So the question becomes: how can I trigger the 555 on the falling front and then to reset it after T?
     
  6. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Here's the way I would try it for an 11mS pulse. But I would prefer a longer flash...at least 100mS.
     
    Last edited: Jan 22, 2014
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  7. nedix

    Thread Starter New Member

    Jan 21, 2014
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    Thank you, I will definitely put it on a breadboard and test it. I will get rid of LED2 and R6 and instead of LED1 I will adapt the MOSFET+LED panel. I actually thought about using a capacitor between the FT and Trig but I haven't realized (last night) that I have to actually amplify the signal. I take it that VR1 controls the sensitivity, right? Anyway, if this works then I'm done with #1 and I only have to find the answer for #2 - "controlled current supply"
     
  8. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Yes, VR1 controls the sensitivity, but depending on your phototransistor, you may need to adjust the value of R5 as well.

    And although you didn't ask, I think you need a pulldown resistor on the MOSFET gate, and I would use a logic level MOSFET depending on Vcc.
     
  9. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    That should be easy. Just post the details of your LED panels and your supply voltage.
     
  10. nedix

    Thread Starter New Member

    Jan 21, 2014
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    The details are in the 1st post: each panel is made of 48 Piranha LEDs and rated at 12V / 300mA. I plan on using 5 of these so the minimum current will be 1,5A. Maximum ... I will take it step by step; from 300mA per panel, which will be the modeling light, I will gradually take it to at least 1A per panel, resulting in 5A total current. Depending on the result and my willingness to buy new panels I will raise it to 2A per panel or even go crazy to 3A.
     
  11. nedix

    Thread Starter New Member

    Jan 21, 2014
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    PS: in the schematic you are saying that R4-C4 act as a conditioner for the Trig signal. Do you have a formula behind those values?
     
  12. nedix

    Thread Starter New Member

    Jan 21, 2014
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    Ah, and I'm using the IRFZ44N MOSFETs because I have them around; I got carried away with another schematic and bought 10 from China :)
     
  13. RichardO

    Well-Known Member

    May 4, 2013
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    Since he is not trying to stop motion with a very short flash, I think you are right. I have never measured it but I think the time constant of the phosphor in the LED is long enough to limit the minimum duration of the flash.
     
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