Hi. I'm not an electrical engineer and besides designing simple ALU for university course and some basic wire repairs have no relevant experience. So any help and tips will be greatly appreciated.

Goal: Make ceiling lamp that can:

• change color from "cold" to "warm"(~6800K-2800K)
• change light intensity
• automatically transition from cold to warm light when bed time is near
• automatically transition from off to warm to cold before the alarm rings
• takes bed/alarm times via Bluetooth from phone/computer app

Lamp design (can be easily changed):

• about 1 x 0,5 x 0,15 (m)
• wooden box
• ~5 cm from ceiling a sheet metal to which the LEDs will be glued to
• above the metal plate are fans for cooling and all electronics
• ~10 cm below the metal would be sand blasted glass to diffuse the light
• (3-6)x 75W LED strips while generaly 1/3 working at any given time, but still being possible to turn all on.

Circuit design:

• Wire colors: blue - control wires, green/yellow/red/black - 3.3V/5V/12V/24V
• Controlled by Arduino Uno/Mega, Bluetooth via HC-05
  
• Temperature sensors, fans and LED strips are connected in parallel
• Each LED strip controlled by PWM
• Two options for power supply : 24V with down step to 12V/5V or 12V/5V/3.3V and step up to 24V (if 24V LED)

Problems:

• Should I feed power to temperature sensors and Bluetooth from Arduino board or straight from power supply?
• If I turn one strip off then the system resistance changes, is this problem?
• How is it possible, that I can cut LED strips without doing any additional changes as it would increase the resistance?
• Which power supply schema to use? Or use two power supplies PC PSU for 12V, 5V and 3.3V components and 24V for LEDs only?
• Where to put fuses to make sure that in case something fails the power will be cut off?
• Do the NPN-transistor leak current? Do I have to put relay before each to conserve energy?
• Which LED-s to use? The variety is so big (5$ - 100$) and very little reviews/benchmarks about specific products.

Hope someone can makes sense of all this mess. Sorry for so many questions.
Also I have a LED strip that says 75W on box but has included 60W PSU, whats up with that?

 

What power are you after? 75W sounds pretty high to me.
If only about 12W total you could run it all on a single 12V plug pack. The Arduino can run on 12V and the LEDs too, running a number of strings of 4LEDs in series each string with a current limiting resistor, and having a number of strings in parallel, and all driven with PWM via a logic level power FET.
Repeat for each of red, green and blue.
But a single 12V switch mode supply of twice your expected power is a good idea. Bigger will run cooler and be more reliable.

Or have a 5V supply running addressable colour LEDs.
So decide what you want to drive, a group of low power red, green and blueLEDs,
Addressable RGB LEDs or high power LEDs.

You will not need relays, the FETs will turn off ok.
A fuse in the power supply +Ve line will be a good idea.
Power the sensors from the Arduino as long as they don't draw too much current.
Cutting the LED strips is ok as long as you do so in the correct place. They are actually wired as I mentioned above. If you are using LED strips, they are usually 12V so feed the LEDs from 12V and also the Arduino.

 

75W is a mistake I made but can't edit any more.

I want white LED strips with three different tones.
I bought a LED strip for testing and it is 3300 lumen @ 34W 24V (3m).
It worked very well, can run it at half power and is still enough on regular day.

I'll try to get an Arduino board tomorrow and test it out.

 

I am getting 12V PSU, should I get 5V DC/DC or second 5V PSU?

 

A 12V to 5V buck converter will be the way to go.
https://www.ebay.com/itm/DC-DC-Buck...493337?hash=item1a2fe20d19:g:dCsAAOSwdd9aVjD6
or
https://www.ebay.com/itm/5A-DC-DC-B...090598?hash=item4b23517ea6:g:d0kAAOSwYlJW2k01