Technically competent but electronically a bit "fick": LED project help PLEASE !

[SteveWrona]

Hi all!

A bit about me, a bit about my project idea and then my questions.

First thing is; thanks for starting this site/forum, whoever that was! I've been lurking for a while now trying to get a grasp on what seem like quite basic electronics principles to most but comes across a bit like Sanskrit to me! I can solder, I know what a resistor is and I know how to wire up things like LiIon batts to electronic speed controllers to motors and receivers for RC planes but this LED project I have in mind is making me head go all funny.

The Project: "The Bike Lights of Mass Distraction"
3 x 3w Cree LEDs with a control board of some sort that I yoinked out of some Tesco torches.
3 x Maplin toggle switches (I want each light to be ON/OFF'd individually)
A PVC pipe setup that will house the LEDs and optics
Project box to house the batts which will be bolted to the water bottle bolts on the bike frame
Silicone black/red wire that I have from my RC plane g00dies
I think that's about it.

The Question (so far!):
1: I intended to use 2 x AA alkaline cells for each of the lights and run them as individual circuits with their own toggle switch. Is using each circuits own batteries the best way to go for 3-4 hours of use before batt replacement?

In my mind  I can "see" how it will all be laid out on my bike and where the batt box will be and the switches on the handlebar and one light on each leg of the front forks and one light up on the handlebar but I'm just a bit stuck on the values of optimum brightness in relation to power into each light and power available in the batts. Is there some sort of Excell spreadsheet that can calculate this because I dont have the money to hire an electronics grad to act as a consultant on my piddly little project! I guess I could go get a book from the library and read up on the basics but I've just started a new job and I'm needing to take my bike on the train and then the return leg home is in the dark and I need lights so what better time to get this project underway than now which is why I come before you, oh learned ones, to guide me through my darkest  hours, regarding my project of course! 

Thanks again and ANY help will be accepted and greatly appreciated!

Steve
www.clematisimages.com

[cadstarsucks]

not going to happen at 3W per.  each lamp takes 2.5A at more than 3V so you really can not get that out of 2AAs (3V) that can only supply 2.5A for an hour

what was the power source for the original tesco?  a 6V lantern battery?  If so you would be better off powering it with one of your 6V RC batteries and the control circuit.

[SteveWrona]

Thanks for the reply! This is the torch that I am gutting for my bike lights. http://direct.tesco.com/q/R.203-8191.aspx

The power source for the torch is 2 AA's. It is impresively bright for a £10 Tesco torch. )it's £10 in the Tesco close to my house; I don't know why it's £12 on the Tesco Direct site.)

The Cree LED sits atop a circular circuit board in the torch head. Taking the LED/circuit board out of the torch head allows me to stick that and the optics into my own design housing with power from, what are essentially, the same two AA's in a remote battery pack elsewhere on my bike. This x3, with a toggle switch for each LED/battery pack circuit would do the trick. I'm getting acceptable run times from the torch in its current state; not sure what the run times actually are but I use the torch for about 15mins a night 4-5 nights a week and I havent changed the batteries in just over 2 weeks! But I assume that my removing the LED/board and adding a metre of appropriate diameter wire would have a negligible effect on the LEDs performance if there's any effect at all. I'll take a pic of the LED/board in the morning and post it here.

Steve

[minster9]

  If you intend to ride your bike alot at night,seriously consider a set of NiMH (nickel metal hydride) rechargeable batteries w/home charger. They don't develope a "memory" as far as charge/discharge goes.  Also consider one of those DC generators that ride against your bike tire.  Between the 2 charging systems(bike & home),the rechargeable batts will save you a lotta bucks over throw-aways.  Don't know how many watts the bike gen. cranks out,though. .  How would one wire that up,Cadstar? I'm guessing a dc/dc converter between batts & led,with a diode in the middle coming from generator? Interesting project,stevewrona. Was just looking around ebay,a typical 12volt,6 watt system was selling for 20 bucks(incandescent front & rear bulbs). You could eliminate bike generator if you incorporate a "blink" chip/circuit  of some sort,blinking lights @ night get more attention & your batts would last much longer. Cheers! 

[cadstarsucks]

Thanks for the reply! This is the torch that I am gutting for my bike lights. http://direct.tesco.com/q/R.203-8191.aspx

The power source for the torch is 2 AA's. It is impresively bright for a £10 Tesco torch. )it's £10 in the Tesco close to my house; I don't know why it's £12 on the Tesco Direct site.)
That says it runs on 2Ds and a total of 3W.  Well I can see it lasting for maybe an hour or so on 2AAs or 5-10hrs on 2Ds.

The Cree LED sits atop a circular circuit board in the torch head. Taking the LED/circuit board out of the torch head allows me to stick that and the optics into my own design housing with power from, what are essentially, the same two AA's in a remote battery pack elsewhere on my bike. This x3, with a toggle switch for each LED/battery pack circuit would do the trick. I'm getting acceptable run times from the torch in its current state; not sure what the run times actually are but I use the torch for about 15mins a night 4-5 nights a week and I havent changed the batteries in just over 2 weeks! But I assume that my removing the LED/board and adding a metre of appropriate diameter wire would have a negligible effect on the LEDs performance if there's any effect at all. I'll take a pic of the LED/board in the morning and post it here.

Steve

[cadstarsucks]

If you intend to ride your bike alot at night,seriously consider a set of NiMH (nickel metal hydride) rechargeable batteries w/home charger. They don't develope a "memory" as far as charge/discharge goes.  Also consider one of those DC generators that ride against your bike tire.  Between the 2 charging systems(bike & home),the rechargeable batts will save you a lotta bucks over throw-aways.  Don't know how many watts the bike gen. cranks out,though. .  How would one wire that up,Cadstar? I'm guessing a dc/dc converter between batts & led,with a diode in the middle coming from generator? Interesting project,stevewrona. Was just looking around ebay,a typical 12volt,6 watt system was selling for 20 bucks(incandescent front & rear bulbs). You could eliminate bike generator if you incorporate a "blink" chip/circuit  of some sort,blinking lights @ night get more attention & your batts would last much longer. Cheers! 

Thats a tough one... you would want a diode OR, but  a simple one would drop some battery voltage forcing you to add an extra cell.

Complicating things further is the variable output of the generator that likely exceeds 12V since they usually drive 12V incandescents, forcing you to add an extra regulator to keep from blowing the LED drivers that likely max out at 5V in.

Dan

[justDIY]

is it really necessary that each light be independent?  why would you want less light at some point?

use a 4 to 6 cell lithium ion battery back and a single buck converter, wire the leds in series.  If you need less light for some reason, tell the buck converter to dim the leds. 

[SteveWrona]

is it really necessary that each light be independent?  why would you want less light at some point?

use a 4 to 6 cell lithium ion battery back and a single buck converter, wire the leds in series.  If you need less light for some reason, tell the buck converter to dim the leds. 

Now there's a thought...
I'd been looking at the Pucks for a while now and was going to go the route of the smallest of the Pucks; trying to find it now on the UltraLeds site is fruitless, it's not on there now but it was a booster puck or some such thing and I thought that would work.

Which of the Buck Pucks would the crowd reccomend for what I want to do if I am going to run a 6C NiMH sub-C pack of, oh, let's say 3000mAh?

Thanks again!

Steve

[justDIY]

six cells is not enough ... you want at least two volts over the forward voltage of the three leds, in series, which is going to be roughly ten volts.  Figure a final voltage of 0.8v per cell, you'd want fourteen cells.  You could probably get by with eleven cells figuring on a final voltage of 1.0v per cell.

quoting an LED Lighting Design guide from National Semiconductor: "Whenever possible, buck!"

The problem with a boost converter is suppling enough power

I think this is the correct math:

Vf = 10v multiplied by If = 0.8a equals 8 watts

Lets round that up to 10 watts to cover the overhead and losses of the boost converter.

With fresh batteries:
10 watts divided by 7.2v equals roughly 1.4 amps.   

With low batteries:
10 watts divided by 4.8v (assuming 0.8v per cell) equals roughly 2.1 amps

In summary, your six cell 3000mAh battery pack is going to get drained pretty quickly, and it gets worse as the voltage drops.  A boost converter with 8-10 watts of output isn't impossible, but it is not off the shelf either.

[minster9]

Read the data sheet on 2008B power pucks from ledsupply. A 700ma unit should be ideal for all 3 leds. Run your switches off of a bus from output side. The unit will feed your 3 watt leds singly or all. The leds must be grounded on a bus on the - side of the driver output(not common frame ground).  On the input side you will need a junction for power in from either batts or generator. Between this junction & power sources you need a diode in each feed wire. Each lets current flow away from power source but prevent power from the other source from entering it.  Wish I knew how to display a drawing on a post.   Driver input, batts,& generator share common frame ground. It is IMPERATIVE that you turn on one or more leds before applying power to the input side of the driver or it'll fry itself. The descibed driver will handle 5-32 vdc input. A fail-safe of some sort should be incorporated aside from memory.