rear stop/tail light from led's

[toot]

i'm no electronic engineer, so bear with me.
I'm trying to build a rear light using led's, but need help. I'm trying to run 34 red, 1.85v, 1000 mcd and 60 degree veiw angle on a 12v system. so i am running in batches of 7 so no resistors are needed to run on the 12v, right or wrong???
Thats the easy part, what i want to do now is use each led for a tail light (half brightness) and also a stop light (full brightness). I assume that running a strong enough resistor in line will make the led dimmer but how do i connect the brake switch circuit into the same line to make the led brighter when braking??
help !!!!!!!!!!!

 

[Lewish]

OK, here goes a partial answer. 7 in series is too many. Your 1.85 volts is only a nominal value and varies with temperature. Use 6 in series and a current limiting resistor. For half-brightness you use one size resistor, and for full-brightness, you use a different one of half the resistance of the first. If you look at your present wiring scheme, you will see that there is a tail-light wire and then there is a brake-light wire.

On the other hand, why don't you just go buy one of those that is readily available? By the time you get thru trying to seal it against the weather, making a mounting, etc., I don't think it is worth the effort.

 

[toot]

thanks for the mail lewish.
i am aware there are two wires, both are positive, but the led only has two connections one + and one - so i can't just join them together with different size resistors as there would be problems with shorting. won't there ??? don't i need a diode or relay to join them??
i'll do the 6 and the resistor though.

 

[toot]

sorry, fogot to say, i wish i could just buy one that would go straight on but i have only a very small space left to put a light that finding something the shape needed is impossible. unless you know a good supply????????

 

[Lewish]

Let me know what size and shape you need. I know of quite a few sources.
The two resistors are connected to the + input for the LEDs. The other end of each resistor goes to the appropriate wire, either brake or tail.
Have you seen the newest LED tail-lights. They use only 9 LEDs instead of the 30+ that the older designs used. Each LED is much larger in diameter, about 2X as large, and quite a bit brighter. I have one at my desk here, and with 30ma of current, I can see a red spot on the ceiling, in a fully lit engineering office. These are so bright they will hurt your eyes.
Fewer is almost always better. They are made by Kingbright. Part number L793SRC/E.

 

[toot]

yer thanks.if you mail me at toot@toot99.freeserve.co.uk i have photos of what i have now that i could send you.
the size of the light now is about 15 cm wide by 2 cm thick in a rectangle shape.
i know you can get much brighter than the ones i have chosen but i find you lose out on veiw angle. i need atleast a 60 degree angle of veiw.

 

[Lewish]

I don't know of anyone who makes them that narrow. Length is about right.

Just make sure you have one "tail" and one "brake" resistor for each string of 6 LEDs.

 

[a31ford]

as far as the "2 wires" thing, here's the catch, resistors in the + side NOT neg. ( LEDS don't care as long as current is limited), for the tail, brake thing, use 2 resistors, and 2 diodes (one of ea. in series with each other, eg: tail wire, diode, "dim" resistor, leds, same for the "brake", simply connect the last resistor wire(s) to the + of the leds), the diodes MUST point the same direction (Anode vs. Cathode) (Stripe towards the leds ??) can't remember, it's to early in the morning......

 

[IRstuff]

Sorry for entering so late, but why not just buy one? There are direct LED 1157 and other replacements. They're awfully expensive, but are already form, fit and function compatible. 1157 is dual filament brake/tail light, so presumably direct replacement does the same job.

http://www.lightlens.com/coloredbulbs.htm

http://www.led-r-us.com/

http://www.xtremewhite.com/ORD-Ligh...ail~and~accessories~bulbs-No-4664-1-1-1-1.cfm

TTFN

 

[47gandalf]

And if you still want to do it yourself, I offer this suggestion:

Why not create the brightness difference by pulsing the LEDs with differing frequencies for STOP and TAIL functions. As long as both frequencies are not too low, the human eye will never notice that they are not constantly on. But the perceived brightness will vary with pulse frequency. And you'll only have one wire going to the LED from the control circuit.

I think the STOP lamp intensity is more than 2 times the TAIL lamp by federal edict.

And as was said elsewhere, you always need some circuit element to limit the forward current of conducting diodes, including LEDs.

 

[underling99]

Hello, Check out

http://hometown.aol.com/cruzzerkid/customrodtronics.html

They may have something you can use.