I have just done a very quick test booting a RPi with the filesystems
mounted read-only. Basically it just worked. There were some 'failure'
messages at startup where it was trying to save things to disk, but they
aren't things which would matter. If the filesystems are all mounted
read-only then there's no problem just turning power off. There's
nothing waiting in RAM to be written to the disk if it's read-only!
I assume that "dawn" and "dusk" will be time intervals like "every few
minutes", rather than actually following clock-time? I'm sure you are
aware that the RPi doesn't have a real-time clock, so every time you
turn it on its clock will start from the same time. However for around
£10 you can get a GPS receiver module which can be connected to the
RPi's 3.3v serial pins, which would provide an accurate time if it were
needed.
If it doesn't know the real time and gets restarted every day then the
RPi might actually keep running past year 2038!
If the filesystems are mounted read-write then you shouldn't power off
by just removing the power. If you do then any data in RAM which hasn't
been written to the disk will be lost, and the filesystem will still be
marked as dirty, so even if there's no un-written data it might take
longer to start the next time.
If you do end up using a UPS, perhaps a large capacitor would be better
than a battery as it shouldn't require replacement.
The "interface kit" required to connect a computer to an Arduino is a
USB cable.
--
Andrew.
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