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Dual Brightness for Tail/Brake using Diodes

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SirIsaac

Joined: Apr 07 2010
Posts: 2


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Post Wed Apr 07, 2010 10:49 am

Hello, I am new to the Forum, this is my first post. Hopefully I won't do or say anything too stupid icon_smile.gif With that said, here goes:
I bought some flexible LED strips to use as auxiliary brake lights. I would like to also use them as auxiliary tail lights at reduced brightness. Before posting this, I searched and read a number of posts discussing this issue, which presented various suggestions including using a PWM, separate regulated 9 Volt supply, and resistors. I found no mention of the first idea that I had, which is to just put several diodes in series in the tail light feed, dropping the voltage by 0.7 volts per diode. I think I read that 9 volts is a good dimmed level, so five diodes in series would do the job. Is there a problem with this that I am missing.? (I did have some training in electronics years ago, but don't do much with it now, so I may be missing something very obvious.)
Thanks
EricCartman

Joined: Oct 24 2008
Posts: 175
Location: Oregon


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Post Wed Apr 07, 2010 12:50 pm

There is no reason that would not work. Wire it up and see how you like it. :)

I don't think people mention that option because it's just not a common way of thinking at least for discrete parts, it seems to be used frequently in IC designs.

I think the biggest drawback of that is overall size of the implementation. Several diodes wired up in series like that can get long. More solder joints that could fail, etc.

Cost could also be another factor. Depending on where you get your parts, 5 diodes could end up costing you more than a single voltage regulator would. You can get a LM78XX voltage regulator for fairly cheap (under $1). You can get them various voltages - 5, 9, 12V, etc (hence the XX), and they are capable of 1A - which is more than you will likely need for your LEDs.

Generally, I like regulators because they are a cleaner install, cheap, and you don't have to worry about the varying voltage of your electrical system.

Good luck,
Cartman
kornholio788

Joined: May 02 2005
Posts: 9748
Location: Tosa, WI


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Post Wed Apr 07, 2010 5:27 pm

Diodes will NOT be what you are looking for. Diodes are simply a "one way street" for electricity. What you need are resistors. I Just did this mod recently. I can't remember what resistor I used. I will figure it out for you tomorrow.

Vid of mine

http://s207.photobucket.com/albums/bb18/kornhol...?action=view&current=023.flv
PwrRngr

Joined: Jul 19 2007
Posts: 4407


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Post Wed Apr 07, 2010 5:57 pm

kornholio788 wrote:
Diodes will NOT be what you are looking for. Diodes are simply a "one way street" for electricity. What you need are resistors. I Just did this mod recently. I can't remember what resistor I used. I will figure it out for you tomorrow.

Vid of mine

http://s207.photobucket.com/albums/bb18/kornhol...?action=view&current=023.flv

A diode will work if he puts enough together. Each diode will drop 0.7V. In a perfect world a diode would have any voltage drop, but in reality it does. I would use a resistor, but a bunch of diodes will work.
EricCartman

Joined: Oct 24 2008
Posts: 175
Location: Oregon


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Post Wed Apr 07, 2010 5:57 pm

kornholio788 wrote:
Diodes will NOT be what you are looking for. Diodes are simply a "one way street" for electricity.


You wanna bet? biglaugh.gif

The diode will cause a forward voltage drop of ~0.7V per diode. It would require multiple diodes to get enough of a drop to reach 9V (from 14.8 of a running vehicle), but it can be done.

Stop and think about it, the "load" in this case is a light emitting diode, so it's already a one way street. If the diode was installed backwards, it would be a non-starter. But when installed correctly, there would be a forward voltage drop (same as LEDs).

Not trying to be an ass, but it WILL work. Not really practical... but it can be done.


Cartman
kornholio788

Joined: May 02 2005
Posts: 9748
Location: Tosa, WI


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Post Wed Apr 07, 2010 7:27 pm

I stand corrected.


It will work, but not the way you want to do it.
SirIsaac

Joined: Apr 07 2010
Posts: 2


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Post Wed Apr 07, 2010 7:47 pm

Thanks for the interest and the replies.
I guess a little single chip regulator probably makes more sense. Frankly, when I first read about using a regulator, I pictured a big brick of some sort. Now that you reminded me, I sort of recall those little LM78xx things.
But I think I'll try the diodes, just to see how they work.
I am adding these lights to my motorcycle, not a car. I can see that it might become addictive. My top case sure could use some brake lights and turn signals.....
bad venge

Joined: Jul 28 2007
Posts: 3550


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Post Thu Apr 08, 2010 3:39 am

I've done it before with a resistor and a relay use the relays contact to "short" the resistor .
Running lights are the resistor in circuit , brake lights activates the relay the contacts close and the relay is jumpered out high brightness brakes , simple but requires two relays
highaltitude
Oznium Employee

Joined: Dec 07 2009
Posts: 18
Location: Pagosa Springs, Colorado Oznium warehouse


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Post Wed Apr 21, 2010 9:42 pm

Here is a circuit I saw online. I was going to draw you something similar but this one looks like it would work good. Just ignore the led part of it and replace with your strips. Pos of strip to +12 and neg of strip to Q2 For 2 4.7 inch strips half bright in tailight mode r2 would be 6 ohms passing about 100ma current and r3 could be 6 ohms as well passing about 200ma current. These values are for a pair of 4.7 strips. Could build one for each side and still spend less than 2.00. Very easy and could be done just soldering leads of components together and then heatshrinking over it. If q2 needs heatsink just mount to metal part near light or in light. However i do not think specd transistor needs a heatsink at this rating. This is fairly efficent circuit that controls current flow.


User posted image

R1______________10K 1/4W Resistor
R2______________33R 1/4W Resistor (See Notes)
R3______________15R 1/4W Resistor (See Notes)

D1___________1N5819 40V 1A Schottky-barrier Diode (See Notes)
D2--D13________LEDs High brightness, high efficiency red types (See Notes)

Q1____________BC547 45V 100mA NPN Transistor
Q2____________BC337 45V 800mA NPN Transistor

SW1____________SPST Tail Light Switch
SW2____________SPST Brake Light Switch

More info about it can be found at http://www.redcircuits.com/Page85.htm
addiction2bass

Joined: Jun 22 2003
Posts: 549
Location: Brownsburg, Indiana


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Post Sun May 02, 2010 12:30 pm

i used some resistors when i did my hitch skull last year. i just got a big pack of resistors and started lining them up to get the right dimm that i liked. then simple diode on each one to keep backwards voltage flow to my other lights.
of course it took me some time to get the right resistor code and then had to big thru the bag of resistors to get the right code that i decided on. ;)
but was dirt cheap! i just lined up afew resistors and once the led was dimmed enough i took a reading on the resitors and went for that size single or double resistor.... i think i had about a dozen of those suckers together before i got the right dimm that i liked.LOL

heres my little skull project which i wanna redo and make another!
i have afew leds that are now burned out. and i also have anew skull as well that i will actually take my time with drilling holes and such. first one i rushed and the drill of course walked around on me! :(

User posted image

this time im going to cover the leds with clear epoxy front and back to hopefully water proof em so they will last longer than a year.
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