From: Greg Harp (gregharp_at_antispamdomain.not)
Date: Thu Nov 27 2003 - 01:33:20 EST
In my opinion and experience it should be safe to use a fairly high value
resistor, and it would be no more or less safe than using opto-isolators.
The design I went with used 47k ohm resistors between the parallel port and
the base of the transistors, and I saw no reason to vary from that. By my
math, even if the parallel port voltage gets all the way up to 5V (which I
suspect many do not, these days) the current draw is going to be about 0.1
milliamp. It would be safe even if the voltage was ten times that. If 0.1
milliamp or even 1 milliamp blows out your parallel port, your computer
already has serious problems. :)
Looking at the opto-isolator circuit documented in the DSLRfocus Yahoo
group's Photos section, it's actually going to draw more current from the
parallel port than that. I don't know the voltage drop across that
particular opto-isolator, but at 5V it's going to draw 5 milliamps with that
1K resistor. Not that the amount should be an issue either, but the point
is that it's definitely not safer.
One nice thing about the "two transistors, two resistors" approach is that a
monkey with a soldering iron could build it correctly. The more complex
circuit documented requires an external power supply, is (slightly) harder
to build, and depends upon moving parts with contacts that tend to bounce
and may corrode over time and exposure to dew. These are all major
down-sides.
To sum up what both of us are (I think) trying to say: One must be careful
whenever one wishes to interface an expensive computer to an expensive
camera. But, constructed properly, neither of these approaches should pose
any real risk to the hardware.
Your mileage may vary...
--Greg
> From: cengell [mailto:cengell_.at._yahoo.com]
>
> Hello All, what I want all to know if you are going to make a device
> for controlling the camera from the computer, is when you make a
> device that attaches to the printer port, you must be very carefull
> not to draw to much current from it..
>
> Using 2 transisors and 2 resistors, unless at the correct value can
> damage your printer port, and if it's part of your motherboard bad
> forever.. The circuit here in Yahoo DSLRfocus fourm does not use
> transistors, they are opti-couplers 100% safe..
>
> Just want to help...
>
> Christopher
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