When replacing the toggle-switch by a pushbutton you must switch to the sunset-part by activating some interrupt in the program.
To change the supply-voltage you must recalculate the individual LED strips and the resistors, because with 12V you could only drive 3 white LEDs and you need a different resistor too. A workaround would be to use constant current sources, but these might cost you some bucks and use another few tens of a volt for regulation.
With 24V you could drive a lot of LEDs in one strip, to control the same amount of LEDs with 12V supply, the LEDs must be separated in two strips which are used parallel. Each of these two strips needs its own resistor and the accumulated current through this channel has more than doubled. So you see, that it makes no sense to drive all LEDs by 5V, which would be convenient, but the current would rise to an unhealthy level and the amount of resistors needed would also skyrocket.
To use high power LEDs with the ULN2803 driver you could combine two channels for a better thermal management. Just connect two inputs together on one microcontroller-pin and two outputs on one high power LED-strip. And keep in mind, that some high power LED spots come with their own constant-current circuit and might not be dimmed by PWM in the power-line!
In this setup all parts are far away from any limits. If you push the things to the edge you might get thermal problems with the FET or the darlington array.
And of course never use 230V AC or 110V AC to drive this circuit!!!
My next step beyond this instructable is to wire up an microcontroller with three hardware PWMs to control a high power RGB-Spot.
So have fun and enjoy the privilege of your individual sunset and sunrise.
