AVR LED Booster

Thanks to Ned K. !

This mod/circuit is supplied in its entirety by Ned K. Thanks Ned!

AVR LED Booster

After making a Luxeon Star Flashlight design using a LT1308B, I started thinking how nice a similar booster would be if it were intelligent. My big concern is over-discharging NiMH or NiCd cells; if I could have a circuit that switches itself off before the cells get destroyed, it would be nice.

I found a nice microcontroller by Atmel called the ATtiny15L that works on 2.7 to 5.5V and has A/D conversion, a built-in reference voltage, 100-200KHz PWM, and a 20x differential amplifier.

The minimum output voltage is about 0.5V below the supply voltage. That means you can't have more than 3.5V input for a single Luxeon, though individual (higher voltage) ones may work with 3 NiCd or NiMH cells.

Note that since this is a step-up circuit, the combinations of LED and batteries you could use are limited:

Batteries Luxeons Notes
3 alkaline 2-4
3 NiCd or NiMH 1-4 1 LS may be tricky here
4 NiMH or NiCd 2-4


  • Lower Rsense because of the 20x amplifier. Only need 100mV or so for sensing.
  • Higher efficiency because of lower Rds FET and lower frequency
  • Voltage sensing prevents trashing batteries
  • Voltage sensing allows a switch to low-brightness mode before the batteries are trashed
  • Multiple brightness levels easily obtained using an external pushbutton switch

Of course, since the operation is determined by firmware, I can make multiple versions that behave differently for different battery and LED combinations.

Here's the schematic. Actually, L1 needs to be bigger (10-39 uH) than is marked here.

And here's a 0.820 inch diameter board:

Uploaded Image:boosterBoard.png

Many thanks to Ned K. for sharing this project with FlashlightReviews.com and its visitors!


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