Using CHDK on a canon S5is to trigger shutter on a Sony Alpha 77

Well the subject pretty well describes what I want to do.. I want to use a CHDK script to somehow trigger the shutter on my Sony Alpha 77.   I really don't know where to start with this, ideally I would like to somehow get chdk to output some signal that could be used as a trigger for my a77 without actually taking a photo with the canon s5.     The use of this is to be able to run something like a motion detection script but to actually take the photo with my far superior a77.    Since I will be using the external trigger input on the A77 in the way it was intended, I will have no issues with the warranty on it.    I would like this interface to be fast as I would like to use it to capture lightning strikes or perhaps falling drops of water.  it would also be great if I could have the chdk script do an adjustable time delay in milliseconds after detecting motion before triggering my shutter on my a77.  Had anyone else used a canon running chdk to trigger the shuter on a DSLR?   I'm sure the concept would work with just about any dslr out there, they all have remote shutter cables available for them.    I'm pretty new to chdk.. I have it installed and working on my S5 and I have used things like intervalometers and outputting raw files, but I have not attempted my own script or anything so advanced

What you want to do is rig up a phototransistor to the "focus assist" lamp on the S5is and use that as an input to your DSLR's external trigger input.  Does the A77 support an IR remote trigger?  That would make things easier but the exact circuit needed will depend on what the A77 expects for an input.

I don't think you are going to find anything commercially available though.   

You then modify one of the CHDK motion detection scripts to turn on the focus assist lamp rather than fire the shutter when it sees motion.

That sounds like a good solution.

The A77 does have an IR remote control,  I have 2 of them.. one has just 2 buttons, one for instant shutter, one for 2 second delay.  the other remote has a ton of buttons for basically hooking it up to a TV and looking at all the photos and videos on it's memory card.    I suspect these remotes must use some sort of blink codes so it knows what button is pressed, I'm not sure if I could make the focus assist lamp on my S5is duplicate these, although that would be completely awesome!

wiring up a photo transistor to detect the focus assist lamp sounds like it would be quick and easy, and sounds like it would be very reliable and probably better for what I'm trying to do because I'll probably almost always have both cameras pointing at the same event, so even if I could figure out how to make the S5is output the 'ir code' needed it may not be practical anyway.   the A77 does have a socket for a remote trigger, it's a simple 3 pin interface.. it has only 2 functions, auto focus and shutter release.. I'm pretty sure you just short the common pin to which ever function you want.  I bought a cheap shutter release cable from ebay to take apart which will not only get me the weird plug I need, but I can also take it apart and verify how it works

Google for "light operated relay" or "photosensitive switch" - tons of circuits and products out there.

for example :  http://dx.com/p/fc-17-l-12v-light-operated-switch-photosensitive-resistance-and-relay-module-blue-173023

thanks,  I think that should pretty much take care of the hardware aspect.   Could someone point me in the right direction as to the CHDK command to turn on the focus assist lamp?

thanks,  I think that should pretty much take care of the hardware aspect.   Could someone point me in the right direction as to the CHDK command to turn on the focus assist lamp?

http://chdk.wikia.com/wiki/CHDK_scripting#set_led

you might also find this useful : http://chdk.wikia.com/wiki/CHDK_Scripting_Cross_Reference_Page

Thank you very much for the help! 

Here's my progress on turning on LEDs on the Canon S5is.   It's a little different than the above article which had LEDs listed for the S3.   here is what I found:

0 Power LED Green
1 Power LED Orange
2 Power LED Both Green and Orange
     *note you can use 0,1, & 2 interchangeably, set_led 2,0 will always turn them both off, no matter how they got to be on.  If you turn on both leds with 2, you could turn off just orange with 1 and it will end up green... etc.

3 Red memory card LED
4 Blue print button LED
5 Green AutoFocus Assist LED
6 Orange Self Timer LED  This one is the only one that seems to use the brightness parameter.
from 0 to 27 it is off as near as I can see
28 barely on
100-full on 
101 off then it repeats 128 barely 
200 and higher full on

It looks like the autofocus LED and the self timer LED's would be the best choices for triggering a photo transistor, they are both very bright and not in the way of anything.  The memory card LED could also work but its so close to the battery/memory card door, my rig could possibly be in the way, although once I get this working I will probably use external power and not really need to mess with the memory card.. so if I want a third thing that could work.  The power led is just too close to the constantly needed power switch and the blue led is in the center of the constantly needed print button to get into the chdk <alt> menu.

now I just need some photo-transistors with suction cup mounts LOL