YES, that worked! I tried playback button and it works. It boots up. Then I have to half press shutter button, and it goes to shooting mode, but this can be done with chdk-ptp?
Yes, the 'rec' command in chdkptp switches to shooting mode.
Also, if playback works, I'd guess the reason power doesn't work is either related to the startup key handling in CHDK code, or is something that happens late enough that we could work around it. So if play has other issues, there's a decent chance we can get power working.
Also, can something go wrong if playback button is pressed all the time during 12hour shooting?
Well, that's for you to find out and let us know
I would generally expect if it works enough to get into rec mode and shoot, it's probably fine.
One side effect known from holding the power button (on at least some cams) is that it interferes with Canon power saving setting that turns the screen off after a certain time. It wouldn't be surprising if holding play had similar or different side effects.
(Screen-off power save is useful for an intervalometer because the sensor is also turned of since it no longer needs to drive the live view. If the interval is much longer than the camera continuous shooting rate, this significantly reduces temperature and power consumption)
Can it be software "depressed" just in case?
Yes. The issue above can be worked around by using post_levent_to_ui"UnpressPowerButton" in CHDK Lua. The levent for the playback button should be "UnpressPBButton", so if you run into issues, calling that at startup would be the first thing to check.
CHDK also masks most physical button state in alt mode (except the power button) so running a script at startup should make Canon firmware stop seeing the button as pressed. If your script isn't running continuously (for example, if your controlling from chdkptp using individual lua/luar/rs calls) you may want to use call enter_alt at startup. Also, if the alt button is set to play (default for most cams), you may want to change that since CHDK seeing the alt button continuously held might cause other problems.
If none of that helps, we can make CHDK always mask the play button state, but that would require a custom build.
Finally, note that physically holding buttons for a long time has been associated with hardware damage in some cases
https://chdk.setepontos.com/index.php?topic=11434.0The D10 mentioned is in that thread is an underwater cam, so the buttons may be different from most, but generally Canon's designers probably don't expect users to hold a button down for months on end.
Some other long term, tethered timelapse projects you might find useful
http://escursionisticivatesi.it/webcam/ - code is at
https://github.com/alesanmanoweb/multilapse-CHDK/tree/master (it looks like it stopped updating this year, but it ran for something over 7 years). The project was mostly discussed in IRC but you can search user manoweb to find some information (edit: including this thread, which I didn't notice at first
).
https://www.youtube.com/channel/UCEJHg--ujxLkjMrevJXh-Gw - long discussion
https://chdk.setepontos.com/index.php?topic=14302.0User mlapse has also done very long term (years), not tethered timelapses. For example
https://chdk.setepontos.com/index.php?topic=13856.msg150298#msg150298
