dual bright leds

ok im new here so please bear w/ me. i have a couple ?s i would like answered if possible.

1. how do they do the leds so they are 2 different brightnesses? i am makin taillights and i see every1 use 2 sets of bulbs. i want to use 1 set.

2. what is a good brightness to use for tails? i was goin to use 10k mcd. then if i used 2 sets 6k mcd and 10 k

Experimenting with different value resistors won't hurt anything.

There are too many variables for me to determine what constitutes "a good brightness". Simply try different resistor values until you are happy with it (and law enforcement as well)

Good Luck

Oh..and "microfarads" should be expressed has "mfd"   or  "uf"

"mcd" doesn't really mean anything.

Keep us posted on how the leds work out and feel free if you have any more ?'s.

how would i do that? run different resistors from the tail wire and brake wire?

also, whats the difference in inverted and regular leds?

The answer to the first question is "Yes".

You want more resistance on the tail so when you hit the brake the leds will be brighter.

The only difference I believe with inverted leds is that they are brighter. If you want more info on inverted leds do a google search and knock yourself out.

You will need different resistance values for each brightness you want.  You can run ~5K ohms to the LEDs when you want them to be brighter you would run them through a 500 ohm resistor.  (values are random, testing/data sheets will need to be used to determine correct resistance values).

mcd refers to millicandellas, a measurement of brightness.  He was not referring to capacitance at all.

Opps sorry had a brain fart with the capacitance, don't know what I was thinking.  doing too many replies at once and getting confused..lol

Anyhow, let me know if you think this diagram will work. I used diodes to isolate the turn/brakes.

led_tail_lamps.jpg

Wait, This diagram makes more sense to isolate the tails from the turn/brake.

led_tails_revised.jpg

That does make sense haemphyst,

However, for some reason 95poopscort wanted to use one set for tail/brake/turn because he is making them himself and everyone else uses two.

I did as best I could to help him even thought I confused millicandellas with microfarads. (doh!)

Perhaps I am not the best person to give advice on this topic. But I do believe the 2nd diagram I posted should work. I would hope that the proper resistance is used to keep it legal. Anything "painfully" bright most likely does not fall into the legal catagory.

I think it wise to make sure the brightness of your tail/brake/turn lights do not pose a safety hazard or legal violation before taking it on the public roads.

I'm going to have to agree with haemphyst here, simply on the fact that you don't know anything about LEDs.  I'm all about trying to do something yourself, and as long as it is close I'm sure you won't have to worry about getting an unsafe vehicle ticket.  However, if you don't set up the LEDs correctly (proper current) you will end up burning up LEDs pretty quickly.  Also, changing from a bulb to an LED will eliminate your 'quick turn when a light is burnt out' feature.  At least commercially availible units will have gone through extensive testing and quality assurance, so you can have faith that they will hold up. 

Furthermore, automobiles provide a VERY harsh environment for electronics.  Vibration, temperature changes, moisture, dirt and about the worst electrical environment imagineable are some of the things you need to consider when designing electronics for cars.  One example would be in the power supply that runs the LEDs, as you won't be able to drive them directly off of the battery because the battery voltage fluctuates too much while driving.  You would most likely want to go with a 5vdc regulated system to insure that the lights are the same brightness every time.  Now, if you have already thought of this, then good thinking, however if you never considered that as being a possible problem then you really shouldn't be doing this!

I too agree with the above posts after consideration. I admire the DIY gusto but if not just for the reasons stated above alone,I feel using an aftermarket replacment is the way to go.

Now the diagram I posted only shows one LED but that was done for simplicity in the drawing, I was under the assumption that he wished to build an array. Regardless,with asking what resistor values to use for brightness, there are too many factors to consider to post a reliable suggestion.

I don't think it's a kick in the nuts, it's solid advice.  We all appreciate your DIY attitude, but I don't think you are realize the liability involved.

In fact, if you install your brake lights you make, and they fail and someone hits you, you could be held personally liable.

Plus, beyond the words of caution, your questions were answered.

Please don't take any of our suggestions that way. It was only out of concern for the safety of you and others as well as legal issues.

As a matter of fact I think you should try to build them but do so as a "science project" keeping it small and the costs down. Test different value resistors, brightness levels, if it can stand up to harsh environments, moisture, vibration etc.

Just try to avoid the temptation of using it on pubic roads. Maybe you can make a set for your car and swap em out at shows and meets etc and then put the legal ones back in for the trip. Then you can really go nuts and make them flash patterns, change colors or anything your imagination can come up with.  And if it's too bright at a car show that is a good thing. On public roads however it really can be a problem.

Best of luck.

Good answer. I have a question however.

Lets assume the voltage will never rise above 14 as if the regulator in the vehicle went bad and sky rocketed the voltage, there would be alot more to worry about then burned out LED's.

But that aside, if resistors are being used, and the LEDS would have no reason to draw more current then required for them to operate, would there really be a risk of them blowing?

I understand why a regulator should be used in this case as it would not be a good thing to have the tail/brake lights fluctuating in brightness. But if the LED's are rated for 12v, and resistor values were used that under any but the most extreme circumstance would prevent over 12v from reaching them, is this really a concern?

Just curious, not looking to dabate, I am no LED expert but do use them often and never really recall seeing one blow out.

It's always good practice to control the system in every aspect against what 'might' happen.  It's not uncommon for the battery voltage to rise and fall above 'normal' (13.8vdc - 14.4vdc) during normal operation. 

LEDs that are rated at 12vdc generally have resistors built in to them.  These resistors should provide enough protection against voltage changes (I would imagine they actually size them for 16 or so volts to be on the safe side).

However, when buying bulk LEDs (like I assume this guy is going to do) they don't come with any resistors at all.  When sizing the resistor you need to know your MAX voltage and the maximum current you want to run through them.  If you calculate your max voltage to be 16vdc you won't have to worry about 15vdc spikes.  If you calculate your voltage around 12vdc you are asking for trouble.

The other side of this is the guy doesn't know how bright he wants to run the LEDs.  He might buy 12kmcd LEDs but only run them at 6kmcd.  That will provide some headroom as well. 

I guess it all goes back to my post earlier saying that the electrical environment found in a car is absolutely horrible, and when designing electronics for cars you need to take that in to account and build in as much protection as possible/feasable for your application.

95poopscort wrote:

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yeah cuz i will be running a resistor to the leds. so the voltage shouldnt peak through the leds anyways right?

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It's not the voltage that will hurt them, it's the extra current that comes along with it.

Ok I see that point. Ohms law. If the voltage increases, so will current. If the voltage doubles, so will current.

Bulk LEDS without resistors are probably rated at 2volts so I understand the concern.

Thanks again.

OK, with 1 set of 6 seried LEDs let's say you want to run 10mA to each LED.  You would want to set your resistor up to limit current to 60mA.  You are using 14.8VDC as your source voltage. 

R=V/I

R=14.8/0.060

R= 246.667 ohms

The closeset 'standard' resistor is 240 ohms

Now let's say you're driving down the road and shut your A/C off.  The quick load change shoots your battery voltage to 16.0vdc.  That same resistor is now going to flow 67mA, overdriving your LEDs by 10% a piece. 

Depending on how hard you are driving your LEDs this may be ok, but it will cause the LEDs light output to flucuate.

Now, the same system, with a 7805 from Radio shack (1A 5vdc voltage regulator) would give you a 5vdc system voltage that is going to be constant under all conditions (as long as battery voltage is between 7-30ish vdc).  You would want the same current, 60mA and would calculate your new resistor to be:

R=V/I

R=5/0.060

R = 83.3 ohms

82 ohms is a standard value, so that is what you would use on each 'leg'.

The 7805 is capable of running up to 1A of current, three legs would be about 180mA, well within it's ability, but when using it with a load over 100mA you should use a heat sink.

You are left with the same amount of power (watts) going to the LEDs, but now you are using a regulated power supply that will provide constant voltage/current to the load.

I don't know if this will help or not, or how much it has to do with brightness per say, but for determining the resistor values for a particular led use this simple formula:

supply V -  led V / I led

So for example if you were doing a setup where fluctations were not a concern....

supply would be 12...led v would be 2...12 - 2 = 10. So you would divide 10 by the current and get the proper resistor value.

With bulk leds the voltage and current ratings should be given upon purchase or labeled on the package/box they come in.

This formula works for us when using 2v bulk leds for switches etc, things like that used inside the car.  However if you use a higher voltage number for supply (let's say 16) you would get a higher resistor. This would prolong the life of the led but at the expense of some brightness. The greater the resistance, obviosly the less the brightness, but the more current will be limited extending the life.

The regulator seems to make sense for tailights, safety and reliabitly are a major factor.

Good luck with it.

Yes you will depending on the voltage of the regulator.

You did the calculations for 16, 15 and 14.8 volts but if the regulator(s) lets say are 5 volts, you will need to recalculate for that taking brightness levels into consideration as well.

Instead of going through all that..just connect the tach wire. Heres how...

06 Ford heavy duty tach:

Notes: On diesel vehicles the tach wire is lt. GREEN / WHITE in a 5 wire harness held up with white tape behind the parking brake release.

Can also use the DEI 454T tach signal generator.

I don't know..it depends on if it's a 5 volt regulator or not. I was just using 5v as an example.

another thing to take in to consideration if you werent' aware. You mentioned you will wire some leds in parallel some in series.

Now the leds that are wired in parallel will share the same voltage but use more total current. what I would do is on all the leds in parallel use a resistor on each one. With the leds wired in series, the voltage will divide but current will remain the same (the current has nowhere else to go (branch) in a series circuit. what you can do for those is add up the voltage of each led you use in each series array and apply that to the formula I posted, and use just one resistor. As you link each LED in series each will recieve less voltage as you add more so in turn will require a lower value resistor in proportion

If you wire enough leds in series you may not even need a resistor..too many and they might never be bright enough. so as you see there are quite a few factors to consider. Hard to give specific values for anything really until you have everything you need along with all of it's ratings, and THEN start to experiment and take it from there.

wow im sorry bout that mess. what it boils down to is this. im coming from my 5 volt regulator. i have 8 set of 2 leds with a 33 resistor for each set.