No diode is needed with the transistor because there's no coil, no inductance. So it's a very efficient switch. Not sure what you mean by isolation. Anything discussed so far will be grounding the camera's shutter contact to take a picture, so the device and the camera will share a common ground. But nothing will be sourcing current into the camera.
I have not done the range test yet. I'll try to do that tonight.
For future reference in case others come across this thread, I am attaching the schematic for the interface to trigger a Canon DSLR from a walkie talkie's headset jack. It uses an LM358 dual op amp and various resistors, capacitors and diodes, but doesn't need an Arduino. It is powered by a 9V battery, but really anything from 5V up would work.
I've changed the previously posted circuit to reflect the different power supply, the change in the output polarity of the second opamp stage to make it active LOW, the use of R8 to provide some hysteresis in the triggering, and of course the direct connection to the DSLR with no Arduino in between.
As the audio signal is received, amplified and rectified by the first stage, capacitor C1 is gradually charged more on each audio wave cycle. But in between those pulses, the capacitor is discharged just a bit through R5. That means it is possible that the voltage on C1 could rise just barely above the trigger point, then fall just below it, then rise convincingly above it. But that might trigger two pictures. So R8 adds positive feedback. Normally the opamp output is high. But when the input voltage goes above the reference divider voltage, the output goes low to trigger the DSLR, but it also lowers the reference voltage because R8 is now connected to ground, so the input voltage has to drop more to be interpreted as being below the reference. Moving the goal posts, so to speak. So the net effect is that the instant the C1 voltage goes above the reference, the feedback resistor reinforces that transition.
Diode D2 is added to prevent the 9V supply from interfering with the DSLR. With that diode in place, the circuit can only ground the shutter contact, which triggers the shutter, but cannot inject current into the camera. By the way, the focus and shutter contacts in the camera are at 3.3V, so we don't want the 9V supply messing with that. I've confirmed that grounding the contact through a diode does work to trigger the shutter.
I should say that while I thoroughly tested the previous door opener circuit, I haven't tested these changes.
I was able to do the range test on the TP2262/2272 remote trigger set this evening. I had the receiver running on 5V, and found that a straight-wire antenna a little over 10 inches seemed to do best, although there really wasn't much difference at various lengths of 9 inches or more. With the transmitter antenna telescoped fully out, I was able to get a clean, solid trigger at 234 feet (71.3m) direct line of sight with no obstructions. Then adding about 3 inches to the transmitter antenna, I got it out to 268 feet (81.6m). In each case, I was able to find one orientation of the transmitter that worked, but it was very touchy. So the practical range would be maybe 25 feet less to avoid getting multiple frames while not having to hold the transmitter exactly the right way. ("You're holding it wrong.") And of course individual copies of these triggers may vary a good bit in performance.
I think these results are actually pretty good. 200 feet is a pretty good distance. Other things that could be investigated are helical antennas for the receiver, or even some kind of directional antenna. which could make a big difference. And then there's that slug adjustment on the receiver that might give better tuning. Oh, and running the receiver at 9V might also make a difference in range - I was not able to test that.
Edit: We've previously assumed that the momentary version of the receiver chip should be used. But in the range test I used a relay driving a nightlight, and connections that weren't so great produced a flickering light. That's no good for a momentary receiver, but I think the latching version might work well. The first transmission, even if not so solid, would latch the output, but repeated flickering latches in theory would have no effect. Then to release the shutter trigger, you would just push a different button.
Thanks ShermanP.
I am not up to customised electronics as per yr drawings, perhaps others can manage it. I like the plug and play approach, so the audio coupler is appealing to me.
The range you achieved is brilliant. More than enough for most birding scenarios where there will be no obstructions. I'm thinking the walkie talkie approach is overkill.
Just waiting for China to make the parts for me.
J.
I think the PT remote triggers would be the simplest and lowest cost solution. I think further work needs to be done on the antennas, the power supply for the receiver, the use of the Latching PT chip (the L4), and possibly the slug adjustment. The walkie talkies would give you essentially unlimited range, but if the range on the remotes is enough, that seems to be the best choice.
I will keep you informed as I develop the project.
