switching circuit with multiple power sources

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.

--

________ Jim Alexander __________________ jalex@xxxxxxxxxxxxx ________________
I have yet to see a problem, however complicated, which, when you looked at it
in the right way, did not become still more complicated. -- Poul Anderson
.

Relevant Pages

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