How to make a simple 18650 CREE light?

[bencan]

I thought that I could just buy a CREE bulb from eBay and wire it up to a 18650 battery, but I guess that's not going to work.

Can someone explain to me in very simple terms what I'll need? I'm trying to build a bike light that will fit into an old style light can. The battery will be external to the can.

From what I understand, I need a CREE bulb and an LED driver, but I have no idea which/what to get regarding the driver - nor how to wire it. I've searched for DIY instructions and haven't found what I need.

Thanks -

 

[RoGuE_StreaK]

Extreme simplest "driver" is just a resistor. Far from efficient, and far from ideal, but it'll work. You need to know the Vf (forward voltage) of the LED, how much current you want, and your source voltage; note, a fully-charged 18650 is 4.2V, NOT 3.7V which is the "nominal" voltage. Then just plug in the figures into any number of online "LED resistor calculator"s.
Otherwise get a proper driver, it'll only cost a couple of bucks.
Basically you need to regulate/restrict the current (Amps) going to the LED. A good driver will provide constant current regardless of what the source voltage is (not fade as the battery gets lower), others will, like a resistor, essentially just put a top limit on the current and will fade with the battery level.

You will however ALSO need heat-sinking. Even if the LED is on a metal "star", this is only really for spreading heat to attach to a proper heatsink, it will do sweet-#@!*-all to dissipate any serious heat. Needs to be attached to a chunk of aluminium, copper, etc, which your light body may or might suffice as.

 

[RetroTechie]

Extreme simplest "driver" is just a resistor. Far from efficient, and far from ideal, but it'll work.

Compared with a linear LED driver (like the AMC7135 used in many flashlights), a series resistor is exactly as efficient: battery current = LED current, excess battery voltage is 'burned off'. The alternative is a buck (aka "step-down") switching regulator, which isn't much more efficient at the low voltages we're talking about here.

A proper LED driver will provide constant output for as long as the battery allows, after which the output will drop off rapidly.

A series resistor will make for a bright LED with a fresh battery, and then slowly, gradually drops off as the battery empties - with a very long 'tail'. Note that the light will become increasingly efficient there: during the very low output near the end, battery voltage will creep towards LED voltage, the voltage drop across the resistor will be very low so the LED gets nearly 100% of the battery power.

So choice between these is a matter of which behaviour you prefer for the light. And whether you want multiple output levels (easier to do using a LED driver).

 

[Steve K]

you might want to search through the Bicycle subforum that is under the Transportation Lighting forum.

I did find a LED conversion of a dynamo light, which might be of interest. It doesn't use/need a driver, though, so it doesn't solve all of your problems:
http://www.candlepowerforums.com/vb...3-Generic-vintage-dynamo-light-LED-conversion

 

[bencan]

I'm now considering either hacking a flashlight or using a G4 12v bulb and a micro transformer. The G4 looks like they have controllers built in.

 

[CreeAddict]

Hey Bencan
It sounds like your best chance is something like this:

https://m.fasttech.com/products/160...afire-1-cree-xp-l-v5-3-mode-1200lm-6000-6500k

You can literally just wire it up to a battery. All the heatsinking and reflectors and optics are taken care of for you