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switching circuit with multiple power sources

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.

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They are pretty cheap.

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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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Bob Monsen
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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.

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________ Jim Alexander __________________ jalex@cis.upenn.edu ________________
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
Reply to
Jim Alexander

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

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Dan Hollands

1120 S Creek Dr Webster NY 14580 585-872-2606 snipped-for-privacy@USSailing.net
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Reply to
Dan Hollands
[ my reply was accidentally mailed rather than posted - sorry! ]

In article , Dan Hollands wrote: ]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).

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________ Jim Alexander __________________ jalex@cis.upenn.edu ________________
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
Reply to
Jim Alexander

"Jim Alexander" schreef in bericht news:dcjv3p$1dv2$ snipped-for-privacy@netnews.upenn.edu...

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

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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

Reply to
petrus bitbyter

"Jim Alexander" schreef in bericht news:dcjv3p$1dv2$ snipped-for-privacy@netnews.upenn.edu...

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

formatting link

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

Reply to
petrus bitbyter

...

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

Reply to
Rich Grise 

--
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.
Reply to
John Fields

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

David

Jim Alexander wrote:

Reply to
quietguy

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