USB flashlights wiki section

[El Chupacabra]

Hi all! Please update the wiki section about USB flashlights anyone who had tested any working or not working. Thanks!

[yvanleterrible]

I don't know if anyone feels like me about USB flashlights but is'nt it a little taxing on a "un-user" replaceable battery

[ElaHuguet]

I agre, Yvan, specially considering it is already using more battery than it should...

[yokos]

Originally Posted by El Chupacabra

Hi all! Please update the wiki section about USB flashlights anyone who had tested any working or not working. Thanks!

Great idea.
mho:

• @ home I will prefer light from my reading lamp consuming 12 W.
• Reading outsides in dark night? Perhaps with USB light. A: »What the hack are you doing?« B: »Reading on my iLiad.« A: »What a geek!«

[astfgl]

Clip-on lights for various devices are already available, most handheld gaming devices need them. From memory they work on anything from button cells to one or two AAA cells, and would be easily modified for Iliad use.

BTW, I'm lighting a glass-fronted bookcase, and settled on "medium" brightness LEDs in the 500mCd range. They have a spread of almost 180 degrees, rather than the 25 degree 10000mCd LEDs I'll be using for my camera ringlight. ;-)

Each shelf has four LEDs wired in parallel and set into a wood strip attached to the bottom of the shelf above. Each strip only draws 15mA, and it lights up the room at night.

The most difficult problem with these devices is not making them low power, it's getting the light spread and positioning right so it's useable. Most solutions tend to have a bright spot which falls off rapidly toward the edges. I've even seen a Nintendo DS light which uses a CCFL, to make the lens design easier.

[NatCh]

Originally Posted by astfgl

...I'm lighting a glass-fronted bookcase....

That bookcase looks sweet, astfgl!

Thanks for the details, by the way, I'd been wondering just what brightness I ought to look at since mCd numbers don't inherently mean much to me, and I haven't had time to play around with them yet. Amps and volts and ohms (oh, my!), I grok -- candellas ... not so much.

How did you get your current draw so low? I've been staring at 10~30 mA ratings on every LED (individual) I look at! 15 mA for four is surprising to me. Have you current limited them down to a level that you found to be "enough" light? Or do you have something less obvious going on?

[astfgl]

A local pine furniture place makes things like this to order, and does a really good job. It's actually two double-door units bolted together, I havn't finished wiring the left hand side yet.

EDIT: Forgot to say, when LEDs are in series, the current draw is just as if there were one LED in the circuit, but the voltage supply must be the sum of each LED's voltage drop, plus a little bit. When in parallel, the current is the sum of each LED's current and the voltage supply must be larger than the LED with the highest voltage drop. That should make the rest of the post easier to understand. Remember, LEDs are semiconductors and do not behave in a strictly linear fashion like resistors, capacitors, and inductors.

On topic, I've scavenged power from my answering maching plugpack, which delivers 15V. Each LED drops about 3V, so with 4 LEDs in series I have 15V-4x3V=3V overhead. The appropriate resistor is 3V/15mA=200ohms. I used a 500 variable resistor(potentiometer) and tweaked the final value whilst looking at a current meter, as there are manufacturing variations in both LED current draw and resistor resistance.

If I had a 12V or 9V supply, I would have used two sets of two leds in parallel, which would have doubled the current draw. When selecting LEDs, look for the "forward voltage drop", of Vf. This is the minimum voltage for the LED to illuminate. Below this, the LED is dark or glowing so dimly it's useless as a light source. Once you have this minimum, you must add a resistor in series with the LED(or LEDs) to limit the current to(or below) it's rated current.

The rated current on a LED is usually a MAXIMUM and much above that the color will (briefly) change as the semiconductor junction overheats, increasing resistance in a runaway condition, to the point where the epoxy/acrylic/plastic housing in contact with the chip vaporises and there is a *crack*, or if you're really lucky, a *pop* and shrapnel.

As to brightness, 500mCd is bright to look at, and will leave a "spot" in your vision after a few seconds which lasts less than a minute. 10,000mCd is very unpleasant to look at and will leave several spots and streaks in your vision which persist after several minutes. I plan to avoid repeating that experience in the future. ;-) Also, the lens shape is critical. 10,000mCd would be great for lighting, but mine have a beam spread of 25 degrees. Great for spot illumination, bad for area. The 180 degree LEDS look longer to find and cost 50c each, but are well worth it and more than bright enough. I'll be adding a dimmer potentiometer to the common supply when it's done, as it lights up the room at night.

[NatCh]

Series, yes, that does make it clearer.

I've never slagged an LED, but I have a very clear memory of all the insulation simultaneously melting off the wires (and onto my breadboard) the time we connected the power to an op-amp ... wrong.

I got a lovely square burn on the end of my finger from that little adventure -- we realized just how wrong we'd connected things when I pressed on the top of the op-amp to make sure it was seated completely (about .5 seconds before the insulation gave it up, but well after it was too late for the poor chip).

It's all about the magic smoke -- you can't let the magic smoke out of the chips!

[Adam B.]

I've added another one to the wiki. It's a generic "10 LED USB light" I got off ebay for ~$5. It works suprisingly well. It's bright enough that it shines a uniform light over the whole screen. I tried the 3 LED light listed in the wiki, and the 10 is much nicer.

[JSWolf]

Originally Posted by Adam B.

I've added another one to the wiki. It's a generic "10 LED USB light" I got off ebay for ~$5. It works suprisingly well. It's bright enough that it shines a uniform light over the whole screen. I tried the 3 LED light listed in the wiki, and the 10 is much nicer.

Would you mind taking some photos so we can see this light in action?

[Adam B.]

Here you go... The only light is from the reading light.

[Robert Marquard]

10 LEDs? It will suck the battery dry in no time. Each LED draws at least 20 mA.
BTW USB powered LED lights are no valid USB devices so you can run into severe problems. In this case the USB port may not deliver more than 100 mA and you may harm the electronics that way.

[Adam B.]

Harm the electronics on the iLiad? In what way?

I'm pretty much in the dark when it comes to electricity and currents...

[Robert Marquard]

USB ports can deliver either 100 mA or 500 mA. The device connects drawing only 100 mA and then negotiates for 500 mA if it needs more than 100 mA. That can be denied. Since the USB light is no proper USB device it cannot negotiate at all and simply draws more than 100 mA. As a battery device it is unlikely that the iLiad is built to deliver more than 100 mA. It obviously has no over current protection so it is entirely possible that these lights can harm the iLiad. It is entirely dependent of the electrical design of the iLiad so only the iRex techs know. They have probably designed it to withstand some abuse, but you never know.

In the end promoting such shitty abuses of the USB spec like USB lights or fans or coffee warmers can only lead to dead computers. They sell you a $5 item which can kill a $2000 computer. And what for? I can get a real big fan for $30 which is not even crap from China. A proper coffee machine costs $20. A clip-on light for my bed is not really costly either.

[JSWolf]

So basically, a $20 -$30 light might end up damaging a $600 Device. Correct?