LED Tail Light 12v to 9v

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I don't think Nigel was taking about LEDs but luminous intensity vs brightness in general.

And I meant to say:
"I doubt that because that would make my 150W floodlight only twice as bright as the 15W light in my fridge - it just doesn't add up."

It is true that LEDs are generally less efficient when run close to their maximum rating but I still don't think it's as drastic as having to increase the power by a factor of 10 to double the brightness..
 
Alright so taking everything into consideration..

  1. I will use the scooter's 12v battery.
  2. I will test the LEDs and match them up.
  3. I'll keep them in sets of three using a 220ohm resistor in each set.

Sound like a solid plan? Do I need to throw a 12v regulator on the line to avoid voltage spikes? I can also just test the battery and see if it fluctuates much.

Will I be able to desolder all those LEDs from that board? Should I use one of those sucker tool things?
 
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Nigel Goodwin said:
Isn't it a log scale like sound?, so you need ten times the current to be twice as bright.
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I do not believe it is exactly log scaled but you do have two factors working to kill off efficiency:

1) emission efficiency drops off rapidly with LED's, they are most efficient at lowest output. As the carriers get more and more crowded, a smaller percentage get out.

2) Human eye response is not exactly linear, which is to say, a light source with twice the true brightness does not look twice as bright to the eye. I don't think it is specifically log scaled, but it's definitely not linear.
 
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The resistors certainly won't get too hot because they will only have about 100mW of power dissipation. The LEDs will be making most of the heat, each string of three will be about 150mW, or about 50mW per LED. I don't think you will have a problem.
 
JdotFite said:
Where did you order these? Any info or photos of that project?

Justin
Click to expand...

Here are the bulbs. They screw straight into the stock sockets.

WARNING: they are SUPER BRIGHT, you do NOT look at them with your eyes unless the lense cover is in place. Seriously.

Product Listing - CAR

BA15 3Watt LED bulb
Super Bright 3-Watt High Power LED 12VDC replacement bulb
15mm 1156 Single or 1157 Dual Intensity Bayonet Base
Super Wide 130 Degree Viewing Angle
Select Base type & LED Color
$ 12.95
 
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Nigel Goodwin said:
so you need ten times the current to be twice as bright.
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I have a little PAL LED flashlight and the thing has two brightnesses, one is low and the high appears to be about 3X as bright as the low. I measured the current and it was about 6X increase. something like 12mA low, 75mA high.
 
JdotFite said:
bountyhunter, you are suggesting using the 12v as input and using 330 ohm resistors in strings of three?ac
Click to expand...
yes, one resistor feeds 3 LEds in series to ground.


JdotFite said:
To address your comment about safety, I agree with you, but here is a photo of the original crappy LEDs that were in the tail light (this isn't the stock tail light). These aren't the best comparison shots...
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If the new light you made is brighter than the old one, you should be good to go. I was referring to motorcycles where they use the standard 18W 1157 bulbs, and most LEDs are dimmer than those. Scooters must be different standards. Only the 3W or 5W LEDs listed on superbrightleds.com are actually brighter than the 1157 bulbs.
 
JdotFite said:
Alright so taking everything into consideration..

  1. I will use the scooter's 12v battery.
  2. I will test the LEDs and match them up.
  3. I'll keep them in sets of three using a 220ohm resistor in each set.

Sound like a solid plan?
Click to expand...
I would not waste my time matching the LEds. The series resistor will keep the current in each string pretty equal. That really is a non issue.

JdotFite said:
Do I need to throw a 12v regulator on the line to avoid voltage spikes?
Click to expand...
I don't think so. Even if there is a spike, the series resistor will limit the current to a safe value.

JdotFite said:
Will I be able to desolder all those LEDs from that board? Should I use one of those sucker tool things?
Click to expand...
No, suckers are pretty useless. Solder wick is the wau to go but I don;t think you need to remove the LEDs, just change the resistors.
 
Hero999 said:
but I still don't think it's as drastic as having to increase the power by a factor of 10 to double the brightness..
Click to expand...
That might occur, but it would have to be at a current level just shy of frying the LEDs. At any normal level, I have never seen the efficiency loss be nearly that severe.
 
This is a motor vehicle.
Wide angle LEDs operating at only 10mA to 20mA will be too dim.
Narrow angle LEDs are brighter on axis but cannot be seen at the sides.
 
audioguru, I will be sure to test the LEDs at the lower mA before putting the light on my scooter. At 70mA, they are blinding bright. They are wide angle 120degrees). I am all for safety and I do appreciate your input. I will have to see how they look at the lower mA.

Thank you bountyhunter for all the tips, I appreciate you taking the time to answer my questions. Same to everyone else.

I do have some of that solder wick stuff, ill give it a try. I will only remove the resistors like you said. I was originally thinking I would need to remove everything if I was going to try and match them all up. I won't worry about that part.

Thanks again everyone! I will report back.
 
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No problem. As for brightness, you need to try it in bright daylight with a person behind and see if the light is plainly visible. It's a judgment call, but you definitely have to be able to see it especially if it is used as a brake light.
 
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It is illegal to change lighting on motor vehicles in Canada.
Kids buy "after market lights" for their little Hondas and then nobody can see them at night. The police confiscate their cars until they put back the original bright lights.
Some new cars have very bright LED lights.
 
12v brake light

Hi JdotFite, You don't need a voltage regulator,let the LEDs & resistors do
it for you.Ideally 4 leds & a 180Ω resistor will run continuously,without even
getting warm.Since you've allready built your unit,it would be a shame to take it apart (LEDs dont like to be de-soldered) the best way would be to replace the resistors,to calculate the value, check the resistance of a LED,
and add 180Ω you will now have group of 3 leds & a suitable resistor in series.
good luck Terry
 

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The LEDs are not guaranteed to be 2.5V each. They have a wide range of voltages. If all four are 3.0V then none will light with a 12V supply because the current limiting resistor will have no current in it.
 
True, I wouldn't go higher than 3 LEds per string. You will get way too much brightness variation as the battery voltage goes from 12.6V up to 14V as the engine brings the system voltage up to nominal alternator/regulator set point which is typically 14V.
 
Look at the data sheet


There's no need to get too technical to create your Brake light. Firstly,it has one function,and that is to be viewed from the rear of your vehicle,usually from a straight angle,brightness varies according to the angle viewed.It is also only seen intermittently,(as you apply the brakes) The design is of your choice.The one you have built can be modified to work on the power supply of your vechicle,and as the light will only be used when the engine is running,then it has to cope with the voltage(usually 13.8v).So look at the data sheet of your LEDs,you will see-Forward Voltage;e.g. Typical 1.7v
Max 2.6v you can now calculate the resistance to light a LED without burning it out.e.g a single led would need something like a 1K2 resistor at a stable voltage of 12v,groups obviouslly share a resistor,and if one of them blows then the group fails to light,So design your circuit within the safe range of the LED and you will have no problem.
 
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