Efficient, cheap way to drive a flashlight LED from a single AA Cell? - Page 1
Forum section: electronics / projects

author: rfmerrill
author_rank: Regular Contributor
timestamp: May 05, 2013, 01:49:38 pm
content: So I was trying to figure out how to do this and I came up with this design a while ago:https://www.circuitlab.com/circuit/ndvm4j/screenshot/1024x768/I haven't had the opportunity to test it. The idea is that Vsupply can be less than the forward voltage of the LED.The PWM out would come from an ATTiny, one of the ones that can run off 0.7 volt. Obviously you'd use the biggest inductor you can get away with. I want to be able to do blinking and variable brightness.My assumption is that if you pick the right PWM frequency and duty cycle, you can get max brightness out of the LED without blowing it--because I assume that the max current ratings on LEDs are for continuous/RMS current, yeah?Tell me if there's anything majorly wrong with this circuit.

author: SeanB
author_rank: Super Contributor
timestamp: May 05, 2013, 01:54:24 pm
content: For another idea google Joule thief and you can do it with fewer components and a little cheaper.

author: c4757p
author_rank: Super Contributor
timestamp: May 05, 2013, 02:05:39 pm
content: Quote from: rfmerrill on May 05, 2013, 01:49:38 pmYes, but do remember that diodes will also have a maximum peak current that isn't astronomically higher than their continuous rating. You can overload it a bit, but it's not going to let you get away with calling a 10A pulse at a 0.05% duty cycle "5mA continuous"I would guess that at least a few LED datasheets would specify max peak, but I don't really look at LED datasheets much.

author: SeanB
author_rank: Super Contributor
timestamp: May 05, 2013, 02:26:29 pm
content: A lot of the higher power ones have a pulse rating. The general run of the mill 3mm and 5mm ones, along with the SM ones, generally are limited to 50mA peak. Some might do 100mA, but generally only IR emitters in a 5mm package are given a pulse rating over that, they generally come with a max rating of 1A for a 1us pulse at a repetition rate of 40kHz, subject to temperature derating and suitable heatsinking on the leads.

author: Hydrawerk
author_rank: Super Contributor
timestamp: May 05, 2013, 03:09:05 pm
content: I have good results with this circuit. Well, the MAX756 is not much cheap.http://memristor.rajce.idnes.cz/LED_svitilna_domaci_vyroby/EDIT:There is a big advantage that this MAX756 is sold in SMD and non-SMD DIP version. Good for beginners who are not good at soldering SMD.

author: Hydrawerk
author_rank: Super Contributor
timestamp: May 05, 2013, 03:16:23 pm
content: If you don't want to use the MAX756, try this circuit. It works fine, but not as good as with MAX756.

author: rfmerrill
author_rank: Regular Contributor
timestamp: May 05, 2013, 04:19:21 pm
content: Quote from: SeanB on May 05, 2013, 01:54:24 pmHow is that fewer components? And how would I do variable brightness and blinking?

author: BravoV
author_rank: Super Contributor
timestamp: May 05, 2013, 04:31:44 pm
content: Another easy solution is using Linear LT1932, and can operate down to 1 Volt, although max out only at 40mA for the led current ->http://www.linear.com/product/LT1932Another alternative using simple discrete but with current regulation I found at the net while ago, in this example the led current is set approx 25 mA.As for blinking or PWM for dimming, I assume it will be sort of pull down at the base of Q1 for controlling it.Here the circuit & simulation using LTSpice :

author: rfmerrill
author_rank: Regular Contributor
timestamp: May 05, 2013, 06:52:53 pm
content: Lots of people are replying with their own circuits but not telling me why mine is bad...

author: TerminalJack505
author_rank: Super Contributor
timestamp: May 06, 2013, 06:04:26 am
content: You'll need a MOSFET with a lower VGS(th)then that. The low voltages might make a BJT more suitable.Other than that, I think your circuit is pretty sound. Like you said, it's probably just a matter of finding the right duty cycle, frequency, and inductor size.

author: BravoV
author_rank: Super Contributor
timestamp: May 06, 2013, 06:17:03 am
content: Dimming for variable brightness probably yes, but blinking as on and off within the speed for human eyes to see like half second on/off, I have doubt its doable.

author: rfmerrill
author_rank: Regular Contributor
timestamp: May 06, 2013, 09:19:33 am
content: Quote from: TerminalJack505 on May 06, 2013, 06:04:26 amOh ignore the mosfet part number. It's just a generic circuit.