External xenon flash for mobile phone

Discussion in 'The Projects Forum' started by pyrs9, Dec 22, 2015.

  1. pyrs9

    Thread Starter New Member

    Dec 21, 2015
    Since some weeks ago I'm developing an external xenon flash for mobile phone. I want to elevate from 5V to 300V and synchronize flash trigger with photo button. I'm using LT3485-1 controlled by an attiny85 connected to mobile phone through OTG.

    I'm not clear about components to use so this is an approximation.
    Here is the datasheet for LT3485 http://cds.linear.com/docs/en/datasheet/34850123fb.pdf
    These are the elevator circuit schematics:
    LT3485-1 circuit.png

    Attiny85 is connected between mobile phone and LT3485 to communicate them.
    I'm not sure if I will need this firmware https://www.obdev.at/products/vusb/index.html to connect attiny as USB device.

    Camera app
    I'm developing it in java. Photo process is:
    1. CHARGE capacitor.
    2. Display ready when DONE is received.
    3. Wait for take photo.
    4. Send TRIGGER before picture is taken.
    I'm learning java so this is the most tricky part for me.

    Interesting points
    Some people gave me points that makes it more interesting:
    • Rotatable flash
    • Draggable photo button
    Next week I'm receiving an asus ZenFlash to take a look into it and see how they solved this problem.

    I would like to know if I should use another microcontroller, elevator circuit or if you have any interesting point.
  2. wayneh


    Sep 9, 2010
    My biggest concern would be a reliable method of synchronizing the flash to the photo. I think doing this precisely will require an intimate understanding of the timing of the photo process, and of the time delays and so on of the microprocessor and flash circuits. The user hits the button. Then what? Several things start happening in parallel. Frankly I think it will be next to impossible unless you can access the same trigger signal that is sent to the internal flash, but even that might be too late to trigger your external flash in time.
  3. ian field

    AAC Fanatic!

    Oct 27, 2012
    Not intending to knock LT, but the traditional photoflash inverter is an enhanced blocking oscillator.

    The tail of the high voltage secondary is returned to the transistor base so the charging pulses enhance drive. This gives rapid charging when the capacitor is discharged, but as the capacitor comes up to voltage, the charging pulses diminish. The blocking oscillator has the usual base winding, but the limiting resistor in series is dimensioned so the oscillator just starts reliably. When the charging pulses settle to just maintaining the capacitor voltage, the inverter is just idling. This means the photoflash is always at the ready with minimum possible battery drain.

    With an NPN blocking oscillator; the HV rectifier has to be negative to put positive pulses at the tail of the winding, some care might be needed in the design of the trigger circuit - but its not insurmountable.
    pyrs9 likes this.
  4. pyrs9

    Thread Starter New Member

    Dec 21, 2015
    First I will try adjusting delay by trial and error between trigger and photo and if it is not enough, take that signal to the internal flash sounds very interesting also.
  5. Sensacell


    Jun 19, 2012
    I think your chances of triggering it correctly by software are very low.
    Timing will be problematic.

    Use a photodiode on top of the actual LED flash? You may need to count-off the red-eye flashes, holding off the main for a number of pulses, but this is easy compared with mucking with software in the phone.
  6. BReeves


    Nov 24, 2012
    The time delay between pushing the button and when the photo is taken depends on how automated the camera is. Not sure about phone cameras but have allot of experience with point and shoot digitals. The big problem with trying to sync a flash is the time between button and photo will vary depending on light, subject and how much info the camera processes to determine how to take the photo. Setting a fixed delay may work for a specific situation but odds are it mostly won't work.

    Might be better off seeing if a photo sensor slave will react to the internal flash as Sensacell has posted.
  7. Mike M

    New Member

    Dec 15, 2012
    See if you can borrow a remote slave flash to test it with your phone. If it ignores the red eye flashes, you can go from there.
    GopherT likes this.
  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
    @Mike M

    I saw you made your first post today and I was going to say, welcome to the site - but you joined 3-years ago. Well, welcome anyhow.
  9. pyrs9

    Thread Starter New Member

    Dec 21, 2015
    Here is Zenflash circuit. It has only 2 layers. Components annotated are described as they are on top. I don't know what are they doing in the circuit.
    As you said it might be difficult to adjust light necessities because it is using two sensors.