Help translating and/or sourceing electrical components for the LED headlight project

[cywelchjr]

Do you know how to set the 555 up to vary the resistance?

Not off the top of my head, but being that I have a little bit of a background in that area I may well be able to figure it out for you. Give me a few days and then rattle my chain if you haven't heard back from me.

[randy]

Why not set up the 555 as a free running oscillator and driving a darlington?

[b.walker5]

Try Here

http://home.cogeco.ca/~rpaisley4/LM555.html

Everything you never wanted to know about triple 5's

[81xsproject]

Why not set up the 555 as a free running oscillator and driving a darlington?

Whoa there, Guru... You're speaking that other language. If you could be more specific, I would be very thankful. If you don't have the time that's okay.

[randy]

Oh, sorry. I got the impression that some of the respondents were familiar with 555's.

The 555 is designed as a timer. You adjust (set) the timing intervals by selecting appropriate r/c (resistor/capacitor) values.

My comment was that if you set the timing to for arguments sake, say 10Hz, use the output of the 555 to drive a high power Darilington transistor, you could flash a light with it.

Keep in mind that the light filament may suffer shortened life from modulating it

[randy]

Ok, scratch that part about the filament, we're discussing LED's Du-uh!

Excuse me for not scouring the above posts for this info, but how much current will the LED array be drawing? You have the value in watts or in amps?, either will suffice.

[psycoreefer]

looks like 12 LED's x 900ma = 10800ma or about 11amps max?????????

[randy]

11 amps? Whoa. If that's correct, it will draw double what the standard high beam draws. 65 watts / 12 = 5.41 amps! I think that would be a step in the wrong direction, especially with the anemic charging systems we have.

[psycoreefer]

ok, I think I screwed up because I'm sure the LED's don't take 12v. I don't think I've ever seen an led that takes more then 5v...

This is the second time I screwed up today, maybe I need to take the day off from posting and just lurk....back into the shadows I go.....

[randy]

Don't feel bad, it depends on how he's connecting them. Could be a series / parallel combination. For instance, four 3.3v LEDs in series would need 13.2V, damn close to 13.5 nominal battery voltage and all four in series would only draw the equivalent current of one LED. On one aspect you're right, we need more info.

[81xsproject]

What I had planed was this; two lights, one for low beam, both for high beam. Each light housing will consist of 2 constant current circuits driving 3 LEDs (3.7Vf @ 900mA) in series. If I incorporate the 555, it would modulate the low beam light, but in high beam, it would be bypassed .... GO!

[randy]

OK, describe this 'constant current circuit'. Try to explain why you think you need it and how it functions. GO!

[psycoreefer]

I'm going out on a limb here, seems how I already posted trash in this thread once, but I'm almost positive the Constant Current circuit he wants to use is shown in the very first post...

[81xsproject]

So, as many of you may know, I am attempting to build a 2400 lumen LED headlight. Last time I posted I was looking for answers on building a constant current power supply. Well, I have finally found one that is perfect...

Here is the schematic;



And here is a link to the bill of material:
BOM

You can disregard the circuit to the right, as it is just there to provide the 5v power supply for running other things like a 555. These high power leds need a very constant current that cannot be obtained from the bikes power soure. I know that my bike with the XJ stator varies from low around 12.2 V at idle to 13.8V at 2500 RPM. I am sure the current also varies greatly with RMP. Even with a resistor, you have very poor efficiency, and the best you can do is try too calculate for the absolute highest current/voltage supplied by the bike and set it up conservatively from there which means when you aren't getting full power, your LEDs aren't either.
There are simpler circuits for this, but they lack the efficiency. The constant current allows you not only set the LEDs near their upper limits, but increases the lifespan of the fairly expensive ($5 each) LEDs. HERE is the link to the data sheet on the driver IC.

THIS GUY here goes into a lot more detail on why high power leds need a constant current supply.

[tw1980]

Hey XS81 looks to me like this circuit would need about 16 - 18 volt source to supply constant current at 12 volts,depending on the load.If you need a constant 12 volt supply my guess is you may need to use a switching supply with a transformer then regulate the 12 volt supply off the secondary.IMHO Good luck. Terry