PaulRB:
I suggest tpic6b595 chips. They are like a 74hc595 shift register but with higher voltage and current capabilities. Each chip has 8 outputs which have built-in mosfets and can drive your groups of LEDs. Two chips, daisy-chained together will give you 16 channels and needs only 3 Arduino pins.Tpic6b595 outputs can switch up to 150mA and up to 50V. If you can find them more cheaply, there is also tpic6c595 which can switch 100mA and up to 33V.
tpic6b595 and tpic6c595 sound interesting. So if 1 LED is around 20ma, I can drive up to 7 LEDs per output of the tpic6b595, right? I will research about how to connect them.. And since you said they have build in MOSFETs, won't I be able to just series LEDs to main power and connect as much as I want just like with regular MOSFETs?
Also, if I could not find stocks of the tpic here in my area, do you think the 74hc595 will be fine for my project?
Hutkikz:
I agree with @MorganS about the phone charger and just hardwire it. no need for a long cable ???
If you want use a buck converter get one that outputs 7 volts and run it thru the arduino regulator for a little extra input protection.
I couldn't find a good car regulator that outputs 5v on a female USB here in our country, they all ship from abroad which takes weeks to deliver.. I guess I will just use a buck and set 7v and connect it to Vin like you said..
Hutkikz:
For the brake switch sensing instead of a voltage divider use an optoisolator like this: H11L1 Optoisolator
cheap, total circuit protection and automatically handles switch debouncing. Protect your Arduino - use an Opto Isolator!
Okay thanks. I will research about it.. But if I can't find stocks here, will the voltage divider work fine?