FreeTrack Forum

Hello, I'm planning my first setup but can't decide how to power the leds. I'm going to use the recommended osram SFH485P leds.

My options, as i see it, are either 2xAAA (2.4V?) or USB (5V).

Is it correct that 2 parallel AAA will provide 2.4V and therefore be too little for the leds wich are specified to 3V. Will this make the leds illuminate too little?

I'm thinking of attaching the leds and batterypack to my not very lightweight FM earphones so more batteries will prove to be heavy to my neck, i think. =)

Thanks for any input!

Gustav

Hello Gustav

My options, as i see it, are either 2xAAA (2.4V?) or USB (5V).

If you're not familiar with electronics, i don't recommend powering your led setup using USB power. One falsestep and you can damage your PC motherboard, so better not try this.

Instead , use batteries or an old power adapter.

Is it correct that 2 parallel AAA will provide 2.4V

To get 2.4v, you need serial mounted batteries, not parallel.

the leds wich are specified to 3V

The SFH485 data are :
Forward voltage : 1.5 V
Forward current (max continuous) : 100 mA
And my 2cents opinion : recommended max forward current for FT use : 70 mA, or even 50mA or less for a batteries powered setup



You can power your led setup using NiCD or NiMH batteries.

I would recommend using AA NiMH batteries

So your setting will be :
- Power supply : 2.4v (2x1.2v AA or AAA batteries)
- setup type : 3x parallel led
- led forward voltage : 1.5v
- led forward current : 50mA (each)

- Resistor : 3 x 18 ohms 5% 1/4w resistors (1 resistor per led)

schematics are avail on the led wizard page, on the website main page.


total current consumption will be : 150mA, so you can expect more or less  10 hours battery lifetime without reloading, if you use 2000 mAH NiMH AA Batteries.

Hope this helps.


greetings.

Wow, thank you for a very thurough answer, that answered a lot of my follow up questions aswell.

The risk of USB is short circuit?

Cheers!

edit:


edit:
Ok, just saw the thread about USB-power, that answe

Hello, i've found an "power pack" (dont know if thats the name) i think i'm going to use. Anyways i can set it to provide 3, 4.5, 6, 7.5, 9 or 12V. i guess i should set 3V

Also, the shop im buying from doesnt name their resistors in ohms instead they have names like "100r2" ,"15r" etcetera. Is for example 30r (the resistor i should use with 3V right?"the same as 30ohms? Wich one should i use 27R or 33R, they dont seem to have a 30r?

Thanks!

Hello gustav

If you can set your power supply voltage , i'll suggest you to go for a serial assembly , and use the appropriate power supply voltage.
This way :
- you'll limit the current since there's only 1 net.
- you'll need only 1 resistor (less components).


Since you want to make a 3point setup, using SFH485P you'll get :

- Led forward voltage : 1.5v => total led forward voltage for a 3 led serial setup : 4.5v.

So, you'll need at least :
4.5v (led forward voltage) + about 1v (to have enough drop voltage to get a consistent resistor value) => 5.5v power supply

We'll go for a 6v setup
Using :
- 6v power supply voltage
- 3 sfh485p , serial mounted
- 70 mA current

You'll need a 22 ohms 0.25w resistor

cya