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switching circuit with multiple power sources

Discussion in 'Electronic Basics' started by Jim Alexander, Aug 1, 2005.

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  1. I have three digital cameras on which I need to remotely trigger the
    shutters simultaneously. Triggering a single shutter is normally done
    with a remote that simply shorts two pins in the remote cameras connector
    together (the shutter pin gets shorted to ground; there's actually also
    an autofocus trigger pin, but a circuit that works for the shutter should
    also work for the autofocus trigger).

    I don't want to risk just connecting all of these cameras in parallel
    directly to a single switch since I don't know anything about the
    internal electronics except what I can read off a multimeter, and so
    I'm not at all sure that shorting their internal power sources together
    is safe (and the cameras a very expensive, so I can't just try it and
    hope for the best).

    So my thought was to connect each pair of terminals to its own NPN transistor,
    and connect all three base terminals together, and connect the bases to yet
    another power source (probably consisting of a battery and a resistor) through
    a switch. Pressing the switch would allow current to flow into the base
    drive all three transistors to saturation, current would flow across the remote
    terminals, and a picture would be taken. Schematically, it looks something
    like this:


    /-- shutter 1 (V1)
    --------- T1
    | \-- ground 1
    |
    / | /-- shutter 2 (V2)
    ----Rb---------/ ----+-------- T2
    | | | \-- ground 2
    ---bat---- |
    |A /-- shutter 3 (V3)
    --------- T3
    \-- ground 3

    Now my basic semiconductor electronics knowledge is pretty rusty. I know
    how to pick the right base resistor when I am trying to drive a single
    transistor to saturation, but it seems to me I will need more current
    to get all three transistors saturated. The transistors are not really
    connected in series or parallel since their E and C are connected to
    3 independent power supplies, and their B terminal gets fed by a fourth
    power supply. Is it even kosher to not have the base and emitter connected
    to seperate supplies? Every switching circuit I've ever built used a
    single supply, but I don't know whether this is a necessity.

    Can anyone tell me if there are any serious problems in how I am trying to
    accomplish my goal? Or can anyone suggest a better way? Note that one
    reason I want to use transistors is I want the cameras triggered as close
    to possible to simultaneously, so switching needs to be fast. Also,
    the resistance across the switch needs to be negligible, so I don't think
    a 4066 IC would work for me. Any ideas would be appreciated.
     
  2. Dan Hollands

    Dan Hollands Guest

    It seems to me a much simpler solution is to just use a 3 pole switch or
    relay

    --

    Dan Hollands
    1120 S Creek Dr
    Webster NY 14580
    585-872-2606

    www.QuickScoreRace.com
     

  3. Jim,

    You don't want to connect the switches in parallel but this way you do not
    fuly separate them either. More important, the resistance of a fully
    saturated transistor cannot be neglected. Usually it's higher then the
    onresistance of a 4066. Nevertheless I think a 4066 will do fine. I'd give
    it a try.

    As for the current required to drive a transistor into saturation: Three
    transistors require three times the current that one requires. This current
    depends highly on the specifications of the transistor involved. You will
    need the datasheet to find out. You'll also have to find out the plus and
    minus of each camera. Tie the minusses to each other (including the minus of
    your battery) and tie the plusses to the collectors of the transistors.

    cam1 cam2 cam3
    | | |
    | | |
    | | |
    | | |
    ___ |/ ___ |/ ___ |/
    +-|___|--| +-|___|--| +-|___|--|
    | |> | |> | |>
    | | | | | |
    minus----------------+-----|----------+-----|----------+
    _/ | | |
    +bat-o/ o-+----------------+----------------+

    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de


    If you really need hard contacts, you can try three (reed)relay, one relay
    with three NO contacts or a simple one throw, three pole switch.

    petrus bitbyter
     

  4. Jim,

    You don't want to connect the switches in parallel but this way you do not
    fuly separate them either. More important, the resistance of a fully
    saturated transistor cannot be neglected. Usually it's higher then the
    onresistance of a 4066. Nevertheless I think a 4066 will do fine. I'd give
    it a try.

    As for the current required to drive a transistor into saturation: Three
    transistors require three times the current that one requires. This current
    depends highly on the specifications of the transistor involved. You will
    need the datasheet to find out. You'll also have to find out the plus and
    minus of each camera. Tie the minusses to each other (including the minus of
    your battery) and tie the plusses to the collectors of the transistors.

    cam1 cam2 cam3
    | | |
    | | |
    | | |
    | | |
    ___ |/ ___ |/ ___ |/
    +-|___|--| +-|___|--| +-|___|--|
    | |> | |> | |>
    | | | | | |
    minus----------------+-----|----------+-----|----------+
    _/ | | |
    +bat-o/ o-+----------------+----------------+

    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de


    If you really need hard contacts, you can try three (reed)relay, one relay
    with three NO contacts or a simple one throw, three pole switch.

    petrus bitbyter
     
  5. [ my reply was accidentally mailed rather than posted - sorry! ]

    ]It seems to me a much simpler solution is to just use a 3 pole switch or
    ]relay

    I should have said this needs to be a momentary push-button switch. I
    don't think such switches are made to guarantee simultaneous (say within
    1 ms) contact across all of the poles. If you press the switch off
    center, for instance, the contacts on the side you are pressing on will get
    connected first. How much a delay this causes depends on the construction
    of the switch and how hard you are pressing.

    I am also considering driving the switches with the output of a timer chip
    so that I can control precisely how long the terminals are shorted. The
    cameras can be set to take multiple exposures if the shutter switch is
    held down. That's a lot more difficult to accomplish using a mechanical
    switch.

    As for relays, electromechanical relays aren't fast enough, and reed or
    solid-state relays with multiple poles don't seem to be very easy to
    come by (and are certainly going to be a lot more expensive than low-power
    transistors).
     
  6. Bob Monsen

    Bob Monsen Guest

    Sorry, your picture isn't going to work. For the transistors to be
    turned on, a voltage must be applied between the base and emitter. Thus,
    the grounds all need to be connected together. In your picture, the
    transistors probably won't turn on, because the camera ground will move
    wrt the base of the transistors.

    Thus, you need to connect up ground 1, 2, and 3 to make it work
    properly. You also need to connect the - terminal of the battery to that
    same ground. Then, when you apply the voltage to the base, the
    transistors will turn on for you.

    If you are afraid of ESD, for example, and don't want to connect the
    grounds and possibly mess up the electronics in the cameras, one way
    would be to use optoisolators. For example, a 4Nxx series optoisolator
    could be used, one per camera, with a reasonably large current (maybe
    20mA) pushed through all the LEDs at once, in series, with the button.
    That would give you a (nearly) simultaneous activation of all of the
    output transistors, but no danger from ESD or ground loops.

    http://www.fairchildsemi.com/pf/4N/4N25.html

    They are pretty cheap.

    --
    Regards,
    Bob Monsen

    If a little knowledge is dangerous, where is the man who has
    so much as to be out of danger?
    Thomas Henry Huxley, 1877
     
  7. Rich Grise

    Rich Grise Guest

    ....

    Well, you've already been told your thing won't work. If you've
    determined that a transistor switch will do the job, then use three
    optoisolators. You drive the 3 leds in series, and each one sits
    at its own camera and the transistor turns on and shorts (closes)
    the switch.

    Good Luck!
    Rich
     
  8. John Fields

    John Fields Guest

    ---
    I just posted a circuit for you on abse under the same subject, but
    it has an error which I'll fix this afternoon. Basically, what
    happens now is that in CONTINUOUS mode the outputs to the cameras
    arent de-skewed. The fix will have them de-skewed in both MOMENTARY
    and CONTINUOUS modes.
     
  9. quietguy

    quietguy Guest

    I would have thought using 3x optocouplers driven by a pulse generator would be
    the way to go

    David
     
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