( easy ) Powering LEDs off of triple A batteries

[Wolvenmoon]

Okay, this is obscenely easy and that's why I'm confused on it.

http://teachwithtech.com/Full-Product-Listing/Vishay-TSAL6400-10-Pack-p21.html

What I'm trying to do is power two of those LEDs off of a single triple A battery, if at all possible. I don't know what the specifications mean. I have a neck injury and am trying to use these to make a wiimote based headmouse. The LEDs will mount onto a wireless bluetooth headset.

I know most LEDs require resistors, but when I run these through a resistance calculator ( if I understand the specs right ) I only need a 0.75 ohm resistor. Other LEDs, to my knowledge, require resistors of much higher values. It looks like I could just get by with wiring the LEDs in parallel directly to the triple A battery ( which will be a rechargeable ).

Proof I've done some of the legwork:

My full project thread is here: http://reclaimyourgame.com/showthread.php?4178-Wolvenmoon-s-hardware-mischief-thread-Wiimote-headmouse&p=50250#post50250 and I have another thread posted here: http://forums.pcper.com/showthread.php?t=473619

Thanks for your help!

 

[NobodySpecial]

Just connect them directly across the battery (in parralel) the LEDs need 1.35V the battery puts out 1.5V you don't really need a resistor as you calculated.

With a rechargeable the LED won't run at full power - NiMH or NiCad onlyput out around 1.2V

 

[Wolvenmoon]

Any thoughts on a rechargeable 1.5V power source?

Should I bother trying to step the voltage up on rechargeables or should I consider putting two batteries in series and using resistors from there?

I seem to remember something about rechargeable batteries having a voltage drop as they discharged, if they START at 1.2V then am I going to have issues with the voltage getting too low?

 

[NobodySpecial]

NiMH are fairly constant output until they are almost empty, but 1.2V is a bit low for these - especially if you want the full 100mA output

I would use 2xbatteries and a resistor, or 3xbatteries and 2LEDS in series (giving 3*1.2V/2=1.8V across each)

ps a good cheap place for flashligth parts http://www.dealextreme.com/details.dx/sku.2399

 

[MATLABdude]

As NobodySpecial mentions, NiCd are around 1.2ish volts, fully charged. All batteries (that I'm aware of) have a battery curve and the (unloaded--i.e. with nothing except a multimeter attached) voltage will drop as the charge goes down:
http://www.mpoweruk.com/performance.htm

A quick glance at the actual datasheet indicates that the typical value for the forward voltage of your LEDs is 1.35 V, but the more surprising thing is that the expected operating current is around 100 mA:
http://www.vishay.com/docs/81011/tsal6400.pdf

100 mA isn't a really heavy load, but you might be better off with NiMHs rather than NiCd as they have higher power density, and don't have that annoying memory effect. Or running it in at a lower voltage / current with a ballast resistor. Most LEDs don't have sharp turn-on characteristics (i.e. no light below their operating voltage, full light at / above)--when you're in range (~90% of the value) they'll be on, but faintly. As you increase the voltage / current, it'll get brighter. Until at some point, you burn out the LED because you put too much voltage / current through it. So your 1.2 V might work just fine, especially if the light is strong enough to be picked up by your photocells.

How might you be able to tell if your IR LED is working? Years ago, I picked up a little IR detector card from Radio Shack--it was treated with a chemical that's excited by IR and reemits in the red. Unless you have some night vision glasses, or some such.

 

[Wolvenmoon]

So, are there any easily rechargeable, interchangeable 1.5V power sources I could get? These LEDs will be detected by a wiimote mounted over my monitor and will function as a head mouse for me. It's sort of important that they be as close to their maximum brightness as possible.

I'll be able to tell if they're working by if they show up to the wiimote. Coincidentally, that's why I chose this specific LED because it's known to work well with wiimotes.

 

[MATLABdude]

Unfortunately, you probably won't be able to any that won't require specialized chargers. You might be better off with 2 batteries in series (giving you around 2.4 V) and a ballast resistor to drop the remaining 1.05 V (2.4 V - 1.35 V)

A 10 ohm (1/4 W) resistor should do the trick--assuming that component values are on the better side of tolerances and their nominal values, the resistor will be dropping 105 mW -- probably quite warm, but okay, although a 1/2 W resistor would probably dissipate the heat better with the larger surface area.

The best easily-purchased AAA NiMH that I've seen (Duracell) are 1000 mAh, giving you around 10 hours or so (definitely less, but that's at least a ballpark value).

 

[Wolvenmoon]

Well, whatever I do I have to wear as part of a headset. I've heard of rechargeable alkaline batteries, but I'm skeptical as they seem to be niche market items.

Do you have any specific resistors in mind from any specific stores? I'm going to attempt to run the LEDs off of 1.2V first, but if that isn't giving me good recognition I'll go for something more complicated.

 

[MATLABdude]

For your purposes (one-off prototype, and well-within device tolerances at DC frequency) any 1/4 or 1/2 watt 10 ohm resistor should do the trick. Even your local Radioshack will probably stock a 10 ohm of at least a 1/4 watt (but if you want to do more electronics, you're probably better off finding your local hobbyist shop: look in the Yellow Pages under electronic components, or electrical supply or the likes). Go to DigiKey or Mouser or the likes as a last resort, since you're paying an arm and a leg for shipping (relative to your single resistor), and the guys and gals at the hobby shop are much better for shooting the breeze / consulting with (and IMO, that's worth paying the dollar or three for the single resistor you end up buying).

As for rechargeable alkalines: I had some Rayovac alkalines back in the day, and they worked reasonably well, but these have apparently been discontinued in favour of NiMHs (I don't know if it's a matter of Betamax vs VHS or whether it's actually a better product, however). I do recall that they didn't always last the longest (even compared to the NiCd that were available).

EDIT: There are still rechargeable alkalines available, and available fairly cheaply--check out PureEnergy (plus, the chargers are, IIRC, cheap enough that you're willing to plug in used-up 'single-use' alkalines to experiment on):
http://www.pureenergybattery.com/

 

[Wolvenmoon]

Since I'm already $20 in if I do this, anyone know of any 1.5-4.0V rechargeable power sources that would take a USB plug as a charger/tether so I could switch between battery power and 'plugged in' whenever I needed to? Otherwise, it's rechargeable alkalines ( I'm afraid to try out non-rechargeable alkalines in the charger as the battery pack is going to be very near my head )