Re: switching circuit with multiple power sources

Jim Alexander wrote: 
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.


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
.

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