I'm making a solder fume extractor from an old PC fan (4-pin PWM), driven by a mid-range PIC.
The fan needs 12V @ max 0.28A for power and a 5V PWM @ max 5mA to control the RPM. So I'll be running the PIC at 5V and therefore I will need both 5V and 12V available. I assume the PIC won't consume much power compared to the fan, even though I plan to have an IR proximity sensor too so I can ramp up the fan speed when my hands move towards whatever I'm soldering and then ramp down again when I'm done.
I haven't yet decided whether to use a wall wart or batteries, but would like to know the pros and cons of the available options.
So, as an example, I assume I could power the system with a 5V wall wart and use a DC boost converter to get the 12V for the fan.
Or, I could power the system with a 12V wall wart and use a DC buck converter to get 5V for the PIC etc.
Apart from the cost and availability of parts, what criteria are there for deciding to go one way or the other? This is a one-off personal project, so commercial considerations are less important (although still interesting), I'm thinking there may be practical issues that I'm unaware of (e.g. noise between power rails, efficiency?).
Could someone give me some insight into how these decisions are made?