Hi all - am planning to make a PCB christmas ornament that is 3 letters (my wife's initials) with LEDs all along the letters. Wanted to double check my design planning and calculations.
I'm thinking ideally 8-10 LEDs / letter. So that would be 25-30 LEDs total. I'd like to power it from 2 AAs.
So I'm assuming I need to power all LEDs in parallel from the battery source (which will have a basic switch on it).
If I use this 3MM LED from Digikey (Datasheet: https://www.lumex.com/spec/SSL-LX3054SRD.pdf), then the steady current would be 30mA * 30 LEDs = 900 mA of steady current.
In theory, with V=IR, that would mean I have 4=0.9*R, so I'd need a resistor of 4.5 ohms. Rather than have 30 resistors, I'd plan to have one single resistor feeding the parallel branches of LEDs. That LED would have about a 2V drop on it (because each LED has a forward voltage of around 2). That means that via W=AV, I'd need to handle 0.9A * 2V = about 2 Watts. So I'd probably bump that up for some buffer and need a 4.5 ohm, ~5 Watt resistor.
First question: Is my math and plan accurate above?
Second question -- clearly 900mAs can't be sourced realistically from 2 AA batteries. So I think I can jack up the resistance on the resistor to reduce the current. The brightness would be less, but in theory maybe it's an ok balance for a christmas ornament. I can probably test a single LED at lower currents to see what would be acceptable.
So if I plan for 10mA per LED, then I'd only use 300mA of steady current. That means 4=0.3R, so R=13.3 Ohms, and wattage could drop to about 2 with padding (e.g. 0.3A2V = 0.6W).
But since AAs have mAH of around 2.5, then I could run for about 8 hours before running dry. That's not enough. I figure the batteries need to last for 3-4 weeks realistically, of about 5 hours per day. So if I could get to ~100 hours I'd be good.
But that would mean I'd have to roughly take the current down to 1mA per LED, which I can't imagine would be enough.
So SECOND QUESTION - am I hosed? Is what I'm doing impossible without lowering the # of LEDs? Are there different "low current" LEDs?
Open to thoughts - I'm a beginning but would love if my thought process above is right.