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:
|3 NiCd or NiMH
||1 LS may be tricky here
|4 NiMH or NiCd
- 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
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:
Many thanks to Ned K. for sharing this project
with FlashlightReviews.com and its visitors!