Powering white LED with 2 x AA batteries

Dear expers,

I'm working on a small project that consists of Attiny85 and one 3mm white LED, the power source is two AA batteries. The controller switch on/off the LED based on some conditions (data from analog input). After two month of nonstop running the batteries voltage felt to 2.5v. The controller still works, but LED light is barely visible.

I want to make it always bright till the battery completely die. I thought about 1.2v LED and voltage regulator LD1117S12TR.

But so far I found only 5mm 1.2 white led (OSM5DK5111A). All others are infrared. White are always 2.6-3.3 V.

Maybe you can suggest any solution to achieve such result?

It would be ideal if there will also be a possibility to control LED brightness from the controller.

Thank you

The simplest is 3 AA batteries, or change to a single LiFePO4 cell (3.2V nominal, pretty flat discharge curve),
but that means getting a different charger.
LiPo’s change voltage more, but are nominally 3.7V which is even more headroom.

Or change to a green LED, which are usually 2.5V and bright.

LED brightness is done with PWM.

MarkT: The simplest is 3 AA batteries, or change to a single LiFePO4 cell (3.2V nominal, pretty flat discharge curve), but that means getting a different charger. LiPo's change voltage more, but are nominally 3.7V which is even more headroom.

Or change to a green LED, which are usually 2.5V and bright.

LED brightness is done with PWM.

Thanks for reply - the batteries should be easily available for the end users, that's why I prefer popular AA. Aso 3 batteries will make the device too heavy. And the LED should be white.

I want to put voltage regulator in between of the controller (PWM output) and LED. But I suspect it will not allow me to control the brightness. Still waiting for regulator to try such setup.

A boost converter you mean? PWM will control the brightness, but with a boosted voltage rail you'll probably need to drive the LED down to ground and use inverted logic.

Boost converters can be quite heavy on quiescent current unless you choose one designed for micro-power operation.

How much of the time is the led on? You can consider having the chip boost it’s own running voltage for everything, or just boosting the led output with something like the jewel thief circuit.

MarkT: A boost converter you mean? PWM will control the brightness, but with a boosted voltage rail you'll probably need to drive the LED down to ground and use inverted logic.

Boost converters can be quite heavy on quiescent current unless you choose one designed for micro-power operation.

I'm thinking about something like this: |375x500 The only problem is - I cannot find 1.2 v 3mm white LED, only 5mm. brightness control is nice to have

westfw: How much of the time is the led on? You can consider having the chip boost it’s own running voltage for everything, or just boosting the led output with something like the jewel thief circuit.

It is switched off 99.9% of the time. Also I want to keep the circuit small 10mm x 40mm x 5mm

here is the circuit that I hope will work with 5mm 1.2v LED.|375x500

But for me 3mm would be better, and looks like 3mm White LED for 1.2 volts does not exists.

looks like 3mm White LED for 1.2 volts does not exists.

Correct, it is down to physics why such a thing can not exist. As the wavelength of an LED decreases the voltage required to drive it increases. That is why IR LEDs have the lower voltage followed by red, green, blue and finally white have increasing voltage requirements,

In fact white is actually an UV led shining into a phosphor.

A voltage regulator like the one you are proposing to use will only reduce the voltage it will not increase it. For that you need a boost converter hitch is a circuit with inductors, capacitors, resistors and a chip. You buy these as a prebuilt board you can’t make them yourself without a great deal of skill.

You also need a series resistor with an LED of any colour.

But so far I found only 5mm 1.2 white led (OSM5DK5111A).

Look again, that LED needs way more than 1.2V, more like 2.7 to 3.3V.

See the data sheet https://www.tme.eu/Document/75a7e57a46726b7bee66db176cae89cc/OSXXXX5111A-CRLED18.pdf

Grumpy_Mike:
Look again, that LED needs way more than 1.2V, more like 2.7 to 3.3V.

Thanks,

Check this one - http://www.optosupply.com/uppic/2016813486372.pdf

I got it a couple of days ago from ebay, it is white and really bright on 2.3v, didn’t try on 1.2 yet.
I think the clue there in “drived module”
I was wonder, if the same but smaller exists

Regards,

You can use the tiny YX8018 module that is found in solar garden lights to drive a white LED, from as little as 1.0V.

I think the clue there in "drived module"

Yes in fact calling it an LED that works off 1.2V is wrong. It is a boost converter connected to an LED, because as I explained physics in this universe will not permit a white LED at 1.2V.

I was wonder, if the same but smaller exists

I doubt it, but you never know. The old joke in the 60s, when transistor radios came in was:-

Did you here about the transistor radio manufacturer who was so good at miniaturisation they had to move into smaller premises.