# Wanted: Small LED with low current



## krullebolle (Aug 28, 2007)

Hi everybody !

I am busy making a keyhangerlight, you know how hard it is to find the keyhole in the dark, for your car, house, etc.. 
Now, because I want it to be a challenge, I have certain parameters I wanted to meet:

- It has a led driver (PR4401) so even with low battery voltage, the LED always gets the same current. (until voltage gets below 0,9V)
- It uses one AAA battery (even old ones should be sufficient)
- I want it to use as less current as possible so I don't have to change the battery all the time...

So my question is: Does any of you know of a white LED, who runs on really LOW currents? I've found SMD LED's, but I prefer a standard 3 mm. Light output doesn't need to be much, just enough for viewing the keyhole.
I find it hard to find white LED's, the traditional colours aren't that hard to find for low current's for example 2mA instead of the standard 20mA. But white LED's seem harder to find..

I see lot's of light power here, with all the new technologies on cree LED's, and here I am asking the opposite


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## 2xTrinity (Aug 28, 2007)

Just about any 20mA LED, or even 1A LED Like Cree/SSC/Rebel for that matter _can_ run at a reduced current, or alternatively, using Pulse-width modulation where the light is run at a high current, but switched off and on fast enough as to create a dimming effect. At super low outputs this method is often MORE efficient than outright current limiting. The light that I actually use, which admittedly is probably not what you're looking for, is the Liteflux LF2 -- which on a 10440 LiIon will run 600mA regulated, and on AAA will run 300mA regulated at 100% output, but can be programmed to run as low as 0.2% output. personally have programmed mine to have a super low level (the minimum amount of light possible) and a super high level.

I actually like this much better than standard 3mm or 5mm LEDs because the tint/color rendition is much nicer -- more of a neutral white/not as blue as most 5mm/3mm LEDs, and the beam pattern is more practical -- more of a wide flood akin to natural moonlight than a bright "circle" of light, which can create a tunnel-vision effect. Another option would be something like the Photon Freedom -- which is a coin cell that uses PWM dimming to offer lower output, and conserve battery life from the coin cell. The only problem there is that the frequency of the on-off switching is low enough that flickering is noticeable, with the LF2, the PWM frequency is so fast as to be undetectable.


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## krullebolle (Aug 28, 2007)

Well, the pr4401 is a PWM dimming module, and it switches at 500kHz, so that's undetectable with the eye. And even so I have set a diode and a capacitor behind it, because a LED gives more light output with pure DC. 
The pr4401 gives a constant output of 70mW right now, because I have set it with a coil of 18 µH.
One loading fase from the pr4401 with a batteryvoltage of 1,5V is 1µs.
Right now it gives a 3,1v output for a white led and 20mA for the LED.
So according to you it doesn't matter if I use the same LED at lower current...
Didn't know that..
Will try to switch the coil, so the current is lower, and battery life will be longer.
Also, unfortunately I don't have any experience with cree, or for that matter even with luxeons, and for a simple keyhanger I'm not gonna use a 3$ cree if a normal LED is sufficient


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## datiLED (Aug 28, 2007)

Get a batch of 3mm high brightness LED's from e-Bay. 

I am partial to this simple LED driver circuit. I have built it several times, and it performs very well. It is easy to cobble together the parts from any electronics supplier, too. 

The instability of the PR4401 circuit may blow your LED at any time unless you add extra components per the datasheet. It kind if defeats the purpose of having such a simple IC in the first place.


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## krullebolle (Aug 28, 2007)

Instability?
There are some components surrounding the circuit, a coil mainly, as mentioned in the datasheet. I already have this circuit, and already tested it...


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## datiLED (Aug 28, 2007)

krullebolle said:


> Instability?
> There are some components surrounding the circuit, a coil mainly, as mentioned in the datasheet. I already have this circuit, and already tested it...


 
The initial press release touted the PR4401/2 as a three component circuit solution. But after reading the datasheet, it is suggested that an additional diode and capacitor be added to reduce current spikes. With the extra parts, and price of the PR4401 factored in, I would rather use a Zetex IC. The efficiency of the Zetex is better, also. But, there is something to be said for simplicity. If the bare bones circuit works well enough for a keychain light, then by all means go for it.


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## cave dave (Aug 28, 2007)

I'm not sure why you need to make something. :thinking:

You could just get a Fenix E0. It will run something like 10 hrs on a single AAA. Put in a Lithium AA and it will last a year or two for keyhole duty.

For really small and cheap get a white photon2 take out the 2CR2016s and replace with one CR2032. Its dim but will also last a couple yrs.

ARC AAA is also a good solution.


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## MikePL (Aug 30, 2007)

Why don't you use a CR2032 (or 2025) lithium battery. It's small, gives the proper voltage and current, and keeps it's voltage stable throughout most of its life. Next just pick any 3mm or 5mm LED and you have it done. 

I don't want to spoil your project, but such simple things as keyhole lights don't need drivers. With such a battery your keyhanger could have the dimensions of 2,5cmx2,5cm, included battery, LED and switch.

You will probably have 10 hours out of this setup. Imagine that you use the light for 30 seconds a day. Two days is a minute, 120 days is an hour. It gives you more or less 3 years on a single battery.


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