Discussion in 'LEDs and Optoelectronics' started by kiwi5000, Apr 14, 2017.

1. ### kiwi5000

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Dec 27, 2012
Hello,

I'm not very electronic savvy but love a good challenge with a fun project. I'm wanting to build a LED headlight for a motorcycle that, with the push of switch can go from 35w to 55w (running lights to bright lights). 12 volts. I've searched for a solution and yes, i can buy a whole unit that will fit into my headlight housing but I'd much rather build something if it's possible. It's not a piratical solution I'm looking for but a fun one. Where do i start? I have a soldering iron, bread board, volt meter, soldering helping hands, ammeter and wires. Any help or advice to get me started would be much appreciated. Thanks for your time.

Michael

2. ### Joosh

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Feb 25, 2017
You will require a PWM driver circuit. Setup to give you the correct pulse for 35W, 55W and I guess an off position.

You can buy ready made LED dimmers, build a circuit with a 555 timer or even use a small microcontroller and write some code to do this. The later two being the most enjoyable depending on how involved you want to get.

Josh

3. ### kiwi5000

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Dec 27, 2012
Hi Josh, thank you for your reply. Okay, all that sounds complicated but I can definitely follow directions well. Let's start with one thing at a time. I was thinking of getting one of those high power LED chips but the only ones i can find that's close are a 50w or 100w. 50's not enough so if I get the 100w, can I regulate the output some how to only 55w or 60w maximum?

4. ### Joosh

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Feb 25, 2017
All your driver is doing is turning the LED on and off very rapidly to adjust the light we can see. How much the LED is on and off is called duty cycle. 100% = full duty; always on supply, 50% half duty; 50:50 onff etc. Anywhere between 0-100%.

So roughly you could say for your LED:

100W at full duty, 50W at half duty. You may need to play around with the duty cycle to get exactly what you need.

You will also need to use a transistor (the part which switches on and off) capable of taking the load and I would imagine, attach it to a heatsink to stop it from overheating. This all depends on the LED you have chosen.

If you search for PWM circuit on Google you will find a few examples of circuits you could modify to do what you need. Have a look at those and you will get an idea of which way you would like to do it.

5. ### Audioguru

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Sep 24, 2016
Here in Canada there are laws about tampering with lighting on motor vehicles so that the light is not too bright, too dim, too wide angle, too narrow angle or the wrong color. Your idea might have more than one of these hazards.
You talk about Watts but you do not mention brightness.

6. ### kiwi5000

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Dec 27, 2012
Josh,

So I build a PWM with a 555 chip and some diodes, resisters, capacitors, transistor etc. Instead of a potentiometer, could i use a switch somehow? The switch would toggle the duty from 35w to 55w. Am i getting close?

7. ### Joosh

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Feb 25, 2017
Yes. Have a look at http://www.circuitsgallery.com/2013/02/pwm-led-dimmer.html

You will need your switch to do the job of the potentiometer. So you can switch between set resistances which relate to the required duty.

You need to take on board what Audioguru is saying too. Here in the UK we have rules regarding beam shape, glare and road worthiness of parts which are independently checked on a yearly test. Even if they pass that, it may still invalidate your insurance if you have a crash. However I do not know what the rules are for you.

Josh

8. ### kiwi5000

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Dec 27, 2012
Once this project is fully realized, we can then discuss the legality regarding my state. Thank you for your concern.

Audioguru - you bring up very valid point about brightness. Lumens are what I should be measuring correct? Not the wattage of the LEDs. What are your thoughts on this?

9. ### Audioguru

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Sep 24, 2016
I do not know where you found 35W and 55W. I think their brightness look almost the same. Our vision's sensitivity to brightness is logarithmic (then we can see in moonlight and in sunlight) so 10 times the lumens look twice as bright.

The datasheets for many LEDs show that the lumens increase less than double when the current is doubled.

Here in Canada we have daytime running lights on motor vehicles. I am blinded every day by Chrysler and Jeep and lately Kia and VW cars using high beams in the daytime with no dimming. School buses and motorcycles too. They are blinding on a dim cloudy day. All other vehicles use dimmed low beams or marker lights.

10. ### kiwi5000

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Dec 27, 2012
Josh,

Okay i think i've settled on a 12v 50w 4200mA led light I'll use for the PWM. Looking at the circuitgallery post, do i need to adjust the R2? Their voltage is 5v, where as mine will be 12v.

Thanks,
Michael

11. ### Audioguru

3,151
698
Sep 24, 2016
The circuit was designed to drive an ordinary little 20mA LED not a monster LED headlight. R2 limits the current to almost nothing (the schematic wrongly shows 4700 ohms but 47 ohms is used in the video) and the maximum output current from a 555 IC is only 200mA (your headlight draws up to 4200mA). We do not see the spec's for the monster LED light so we do not know if it needs a resistor. The 555 IC can drive a Mosfet through a low value resistor then the Mosfet can drive 4200mA into the monster LED light.

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Dec 27, 2012
13. ### Audioguru

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Sep 24, 2016
The 50W monster headlight has 48 extremely bright LEDs as a floodlight, not for a headlight on a motor vehicle.
A vehicle low beam is supposed to be a spotlight aimed at the ground with the bright part blocked from shining ahead. The monster LED s has a very wide viewing angle and is much brighter than an ordinary headlight and it will blind all oncoming drivers.

50W is a lot of heat in that tiny package. Its hot parts are not in an insulating vacuum like in an ordinary light bulb. How will you cool it? With liquid nitrogen?

Its maximum allowed voltage is 13V so it will burn out unless the voltage is limited somehow.

14. ### kiwi5000

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Dec 27, 2012
Okay so after digging and looking into a good amount of high power LEDs, I think i'm gonna go with something a little more simple. New headlight will feature four 5w LEDs. Two low beam and two more for high beam. That's simple right? I still want to add a trim pot the both sets though. Two might be too bright for low beams so I was thinking of using the trim pot to increase the resistance as needed.

So for each 5w LED it has 7v drop and 700ma current. That means I'll need a 3500 ohm resistor for each one? Is that right? Battery power is 12v.

15. ### Audioguru

3,151
698
Sep 24, 2016
An LED is not an accurate length of tungsten wire as in a light bulb, instead it is a special diode that has a range of forward voltage. The datasheet should say its minimum which might be 6V and say its maximum which might be 8V, then you can calculate the resistor value when the LED is 6V and the battery is charging at 14.4V. (14.4V - 6V)/700mA= 12 ohms. If the LED forward voltage is 8V and the battery is fairly low at 12V then the current will be only (12V - 8V)/12 ohms= 333mA. Do you want to operate the LEDs at their maximum current when they might melt in summer if you do not cool them properly?? I think you should use a constant current source at 500mA instead.

Your calculation was (12V - 7V)/3500 ohms= 1.4mA which makes a very dim LED.

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Dec 27, 2012
17. ### Audioguru

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Sep 24, 2016
The circuit does nothing more than one large resistor will do. It does not control the voltage, instead it simply reduces the current in the LED. If the forward voltage of the LED (it does not say) is 7V and the battery is 13.8V then The current is 80mA and the power in the LED is 0.56W. It will not drive two 7V LEDs in series. Its Coil T1 does nothing.