LED Reading Light? Noobie, please help...

Hello. I have been re-learning basic electronics over the past couple days (still don't understand everything!) in my quest to build my wife a better reading light. I came across CPF and it has been really helpful and I hope that some the experts out there will help me understand a couple more things.

And yes I have read the welcome mat!

1. I built a basic reading light while working on my soldering skills with 2 AA standard alkaline batteries and a radio shack (gasp!) 5mm white LED (3.6Vf, 20mA), no regulator just DD. i know I should have a resistor of some type to help regulate current (I think) but would love to know what type and why?

2. Building a reading light I don't want high lumens, just a nice soft light, long runtime, re-chargeable batteries, and little heat. Suggested setups? I know this is the opposite of what this forum is normally about!

Thanks for any help. I also do love flashlights (I have a surefire Exec elite with LED head) and never knew this whole world of modded lights existed!

Rather than modding a light, why not just get a headlamp which adjusts with a dimmer to your desired output? I recently bought an LED Lenser 42.5 lumen headlamp from Costco for $19.99 and it has a dimmer switch. Its all flood and not much throw, so using it as a reading light works very well.

The setup you built will work, as you've seen, but the light level will dim very considerably as time goes on, and the light will become too dim to use before the battery is completely dead.

The output from an LED is approximately proportional to its current. The current, in turn, is a very strong function of the forward voltage -- if the forward voltage changes just a little, the current changes pretty dramatically. So if you want a light with constant output, you need something that approximates a constant current source. That is, it provides about the same amount of current to the LED regardless of the LED's forward voltage -- or the battery voltage. There are several ways of going about this, the simplest of which is by using a battery voltage considerably higher than the LED voltage, and a series resistor. This is the least efficient way to do it, but the simplest and least expensive.

If you've learned anything at all about electronics, you surely know Ohm's law. So let's put it to good use.

You're now using two AA cells, which when fresh and under a very light load will have a voltage of around 1.5 volts each for a short while. So your LED has a forward voltage of about 3.0 volts when running at the current that gives you a good level of light for reading. The cell voltage will drop to 1.25 volts somewhere around the point at which the cells are halfway discharged. At that point the LED will get around 2.5 volts, resulting in an LED current and output light level somewhere in the vicinity of 1/4 or less the amount when the battery was at 3.0 volts.

I'm going to have to guess at the current you're drawing now at 3.0 volts -- let's suppose it's 50 mA. So what would happen if you used three AA cells in series with a dropping resistor?

When the batteries are fresh, you'll have about 4.5 volts. The LED voltage will be 3.0 volts at the desired current of 50 mA. So the resistor has to drop 4.5 (the battery voltage) - 3.0 (the LED voltage) = 1.5 volts at 50 mA, meaning R has to be 1.5/0.05 = 30 ohms. Now, when the cell voltages drop to 1.25 volts, you'll have about (3.75 - 3.0) / 30 = .025 A = 25 mA of current. The LED voltage will actually be a little less, so the current will be a little more. Although you've got only about half the current as you did with a fresh battery, it's an improvement over the original design. The cells have a voltage of about 0.9 volt when completely discharged, at which time you'll only have 2.7 volts available. The LED will be very dim, but not out. So you would be able to get all the energy out of the cells.

Ok, let's try 4 cells. Now we have 6.0 volts when the cells are fresh, so we need (6.0 - 3.0) / 0.05 = 60 ohms. Now when the cells drop to 1.25 volts each, we have 5.0 volts, and the LED current is about (5.0 - 3.0) / 60 = 33 mA, higher than it was with three cells and 30 ohms. When the cells are essentially discharged at 0.9 volt each, the LED current will be about (3.6 - 3.0) / 60 = 10 mA. Not great, but considerably more than before.

You can continue this exercise with more cells and higher resistance values, and you'll see that the current regulation gets better and better. But the efficiency drops as you do this. With 3 fresh cells the LED consumed 3.0 volts at 50 mA = 150 mW; the 30 ohm resistor dropped 1.5 volts so consumed 1.5 volts * 50 mA = 75 mW. Of the 225 mW supplied by the battery, the LED got 150, so the efficiency (the fraction of the power supplied by the battery that's delivered to the LED) is 66.7%. This increases, though, as the battery voltage drops. With 4 cells, the efficiency is 60%. And with more cells it's less. But chances are this won't be any big deal for your application, and many of the small regulators aren't an awfully lot more efficient than this, anyway.

The next level is to use a linear or switching regulator, which can keep the LED at a very constant output level as the battery voltage drops, but this should get you started with some experimenting.

c_c

Curious_character said:

The setup you built will work, as you've seen, but the light level will dim very considerably as time goes on, and the light will become too dim to use before the battery is completely dead...

c_c

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Thank you - that was very helpful.

Couple more questions if possible....

1. What is a better LED to use? I don't need or want high output, but I am sure there is better light than a radio shack 5mm!

2. Are the new LifePO4 batteries safe to work with? And what is a good reputable brand.

Thanks!!

aleainc said:

Thank you - that was very helpful.

Couple more questions if possible....

1. What is a better LED to use? I don't need or want high output, but I am sure there is better light than a radio shack 5mm!

Click to expand...

The most efficient LEDs available today are ones by Cree and Seoul Semiconductor (SSC). In turn, these are sorted into different "bins" according to the amount of light produced for a given current. The previous generation of power LED, made by Luxeon, is about half as efficient but still probably considerably better than the one you're using. A bit of looking around this newsgroup should reveal many sources for the newer LEDs, and a lot more about them. I'll bet you could get some Luxeons for free or nearly free if you post in the Marketplace/Lights/Buy section what you want them for, from people who have pulled them from lights they've upgraded.

2. Are the new LifePO4 batteries safe to work with? And what is a good reputable brand.

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From everything I've heard, they're safe. I've got a few and like them, although they have considerably less capacity than the more common 3.7 volt Li-ion cells. You'll get excellent quality and service from AW - see his sales thread in the Dealer's Corner section of the CPF Marketplace.

But I'd recommend sticking to NiMH or alkaline AA cells for what you're doing.

c_c

How about building a Joule thief?

1. There are plenty of high output LEDs out there, and many of them (especially the current gen ones) have no problem lighting up your whole room (with good heatsinking).

Since LEDs can be dimmed without loosing efficiency(most get more efficient at lower currents), you can have a potentially blinding LED set on low power to light your book.

2. From what I've read, the lower capacity seems to be the only down side. I'd stick with the batteries you have now.

:welcome:

The previous post was a really good introduction to simple resistor current limiter circuits.

If you got lost in the technicalities, I would encourage you to read through it again. The math isn't to complicated, and understanding it will let you design a circuit with exactly the performance you want.

As noted, with a simple resistor current limited circuit, the tradeoff is :

- more cells = better regulation but less efficiency.

- fewer cells = better efficiency but poorer regulation (light dims as batteries run down).

Three 1.5 volt alkaline cells (4.5 volts) is a good place to start (a lot of commercial lights use this arrangement.

Four 1.2 volt NiMH rechargeable batteries (4.8 volts) is also a very good choice if you want an economical to operate rechargeable design.

Either option will give good initial brightness with only a little dimming as the batteries discharge.

In both cases you can use a 25 to 50 ohm resistor in series with each 5mm white LED you want to drive, depending on how hard you want to drive the LED.

If you would like to experiment with a solid state constant current driver, here are a couple very simple circuits to get your started:


The 5mm circuit is about as simple as possible. The 2n2222 NPN transistors are not critical at all, and can be substituted with any NPN medium power (300mw to 650mw) general purpose audio or switching transistor with a gain of 100 or more (90% of the small NPN TO-92 transistors shoud work). Look for 2N2219, 2N3904, etc. PNP types like the 2N3905 can be used by simply flipping around the LED anode and cathode connections and reversing the polarity of the battery connections . The cost of the components should not exceed 2 or 3 dollars total.


The high power CREE/SSC/Rebel LED circuit is very handy for automotive lighting applications. The IRF520 MosFET is obsolete but can be replaced with an IRF540 or any equivalent common N-Channel Enhancement mode power MosFET (most MosFets scavenged from old computer power supplies should work fine). The MosFET device has a TO-220 tab which should be screwed to a small heatsink mount (A few square inches of metal is enough). If the heatsink is not electrically isolated, the device needs to be isolated from the heatsink with a mica or silicone sheet and mounted with a screw that has an insolated washer (standard TO-220 mounting kits are available).

Thank you. The last post above is a little over my head :shrug: as I do not understand what the function the transistors perform in the circuit described.

Is there a good type of 5mm standard LED? Again, this is for a small portable attached-to-book style light - it is not supposed to keep me awake! But there has too be a better quality than the radio shack unit I am running now.

Also weight is an issue so is one battery type known to be less weight than another?

Thank again!

To clarify - I would love to use a luxeon led or similar but I think it would be too bright, right? Unless these can be driven at lower powers? I think that 20 lumens or so would be plenty. I do not want to dim because the rapid pulsation may not be god for reading.

Thanks!

I just got a Freeplay Indigo lantern to read by.

aleainc said:

To clarify - I would love to use a luxeon led or similar but I think it would be too bright, right? Unless these can be driven at lower powers? I think that 20 lumens or so would be plenty. I do not want to dim because the rapid pulsation may not be god for reading.

Thanks!

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Sure, all LEDs can be driven at any level from barely visible to very bright, and they don't change color appreciably when doing it like an incandescent bulb. The advantage of using a Luxeon or later type of efficient power LED for your application is that you can get the same amount of light at lower current. That means your battery will last longer, or you can use smaller, lighter cells for the same run time compared to a less efficient LED.

If you use a simple series resistor or a linear regulator like the ones Luminescent described, dimming is accomplished simply by increasing a resistor value. No pulsing occurs with either type of regulator. For that matter, the current from most switching regulators can be decreased, resulting in dimming, without introducing visible pulsing.

c_c

I'd suggest a Cree or a current gen LED since you would get more then double the runtime for the same output then if you used the Luxeon.

Find a lower powered driven(or mod one, or use 3 AA batteries and a variable resistor) and don't use a reflector and you should get a very floody light. You should end up with something similar to the Zebralight.

that was an interesting link Gunner12,thanks

Ok. So for efficiency sake it sounds like a cree is a good choice. If I understand correctly they can be ran at a low current level which would produce a lower light level, say 20 lumens or so? This can be done with just a resistor?

I like this idea as I am sure that I can get a better quality of light color than a standard 5mm led right? All you input has been great!

Taking the info from the Cree XR-E P4(this post), the LED should be drive at around 60-80 mA if you want 20 lumen, 50-60 mA if you se a Q5(this post, a normal 5mm LED is usually around 3-4 lumen when driven at spec).

You should be able to do this with just a resistor, and looking at the low drive current, you won't need much heatsinking(a star itself might be enough). Even direct drive from 2 Alkaline AA batteries would work.

aleainc said:

Ok. So for efficiency sake it sounds like a cree is a good choice. If I understand correctly they can be ran at a low current level which would produce a lower light level, say 20 lumens or so? This can be done with just a resistor?

I like this idea as I am sure that I can get a better quality of light color than a standard 5mm led right? All you input has been great!

Click to expand...

To answer your last question about driving a CREE at lower current levels first, YES THIS IS A VERY GOOD IDEA.

Some time back the fine folks at Nichia were promising that 5mm LED's with 130+ lumens/watt were 'just around the corner', but these never seemed to materialize.

What a lot of people don't realize is that highest grades of 1W to 3W power LEDs like the CREE XRE, SSE-P4, and Rebel, are now rated over 100 lumens/watt at 1 watt or so of drive, but that the efficiency INCREASES at lower current levels (down to 30ma or so, then it decreases again), so at 40 or 50 ma they are a lot more efficient. In fact they are so efficient that they actually hit the kind of incredible numbers that Nichia promised (but never delivered on) in their 5mm products.

Another thing which is great about the CREE XRE, SSC-P4, and Rebel LED devices, for your application, is that they throw a super wide, even, very uniform pattern right out of the LED when used without a reflector (will illuminate a full book from only 6 to 8 inches away).

If you get a CREE mounted on a star board, and run it at , let's say, around 50ma or so, the star board will indeed provide enough heatsinking (and it will greatly simplify making solder connections).

For the recommended 3 battery power pack, all you need to limit the current to the correct level is a simple 33 ohm series resistor (1/4 watt rating is fine).

So all you will need is :

3 cell pack (AAA will give 20 hours plus; AA will give 50 hours plus).

CREE-XRE (a standard P4 would be fine, but a Q2-Q5 grade will give better efficiency).

33 ohm 1/4 watt resistor (this is a standard value you should be able to find at radio shack).

If you would rather run rechargeable NiMH cells, you can either use a 22 ohm resistor with a 3 cell pack or use the same 33 ohm resistor with a 4 cell pack. I would use the 33 ohm resistor but go with a 4 cell battery pack, most chargers only work with even numbers of batteries, and this will give a little more brightness. (the reason that an adjustment in the number of cells is needed is that NiMH cells have lower voltage 3 x 1.5 volt alkaline = 4.5 volts ; 4 x 1.2 volt NiMH = 4.8 volts)

If you want more light than given by the 33 ohm resistor above, try dropping the resistor to 22 ohms.


The 4 NiMH cells will work GREAT and a set of 4 will only cost about $10 at any local Target store (I have had good luck with Duracell 1000mah AAA's).

NiMH cells have such a flat discharge curve that the resistor will be all that's needed to keep the brightness almost perfectly uniform.


If you do decide to use NiMH batteries, for best battery charge cycle life, you will want to charge the NiMH batteries as soon as they start to dim even a tiny bit. Running them farther into discharge will risk reversing a cell (this is good advise for any multi-cell rechargeable battery pack).

Thank you all for the input, it has been very helpful! I really appreciate the time you put into your replies.

I think I am going to go with the Cree XR-E setup with NIMH batteries.

Where can I get a good cree star led? I would prefer the higher efficiency models in the 3500-5000k range.

Thanks!

aleainc said:

Thank you all for the input, it has been very helpful! I really appreciate the time you put into your replies.

I think I am going to go with the Cree XR-E setup with NIMH batteries.

Where can I get a good cree star led? I would prefer the higher efficiency models in the 3500-5000k range.

Thanks!

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That color temp range is a little hard to find, because the 'warm white' parts loose about 20% on brightness, and since everyone is keyed up on lumens, no one wants to throw them away with a 'warm white' emitter.

So most folks only stock parts that are all technically 'cool white', but even within this catagory, I have found that the warmer end of the cool-white range looks very nice (around 5000k to 6000k)

I have delt with DX a couple times and they are pretty reliable on simple items like emitter stars, and their prices include shipping.

Also, they take PayPal so you don't have to worry about your credit card info.

Here's the Deal Extreme DIY parts link:

http://www.dealextreme.com/products.dx/category.917


Here's a CREE XRE Q5 on a mini-star in color bin WC (the WC bin will be a bit on the cool side or 'daylight white' up around 6500k to 7000k)

http://www.dealextreme.com/details.dx/sku.11022


Here a CREE XRE Q5 on a full sized star mount (about 20mm). This one says it is color bin WG which should be a little on the warmer side than the WC part above (about 6000k).

http://www.dealextreme.com/details.dx/sku.2394


If 8 dollars is too much, and you want an even 'warmer' tint, here's a CREE XRE P4 grade part on a small mount for less than 5 bucks. It says it's color bin WD which is about 5500k to 6000k, which is about as close to 'warm white' as you get in the CREEs :

http://www.dealextreme.com/details.dx/sku.3417


Here's another CREE P4 WD on a slightly different mount for $6.30

http://www.dealextreme.com/details.dx/sku.3393


Most folks report that the Rebels are a warmer white (even though, again, all of these are technically 'cool white' versions) A Rebel 80 is about equivalent to a CREE P4 in output efficiency.

http://www.dealextreme.com/details.dx/sku.4622


Note that both the Rebel and SSC-P4 parts have a 'Gum-Drop' soft silicone lens which is much easier to damage than the hard lens of the CREE-XRE (so I would recommend you stay with a nice WC or WD bin CREE until you get a little more comfortable with handling these parts).