Re: switching circuit with multiple power sources


"Jim Alexander" <jalex@xxxxxxxxxxxxx> schreef in bericht
news:dcjv3p$1dv2$1@xxxxxxxxxxxxxxxxxxxx
>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


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 data*** 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



.

Quantcast