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Re: LEDs and relays in Autos

Jonathan Luthje
 

Jayson,
A "real" brake light (at least in this country) leaves the parking /
tail lights on simultaineously with the brake light, so the easiest way out
of it would be to just hook them up without any switching circuitry
whatsoever - load 1 to "switched", load 2 to "permanently on", if you really
wanted to switch one off and the other one on, then I would suggest that you
use a small-ish SPDT relay to switch from one to the other - hook up the
common contact to +12V, the normally closed contact to Load 2, the normally
open contact to Load 1, and the coil to "Switched +12V".

Relays are a lot more robust than transistors, especially in automotive
applications, where voltage spikes are common due to high loads on starting
/ charging etc.

Also, make sure you include current limiting resistors in your design for
the LEDs, as vehicle system voltage can be quite high during charging (up to
16V), which wouldn't do you LED's much good!

Regards,


Jon

----- Original Message -----
From: "Jayson" <woodrufj@...>
To: <Electronics_101@...>
Sent: Thursday, August 30, 2001 11:15 AM
Subject: [Electronics_101] Re: LEDs and relays in Autos


Well I'm doing something similar. I've planed out to have the "A" lit
with normal leds, while cluster "B" is super bright leds. Now i'm
trying to think of a circuit that turns A off when B goes on, check
out my previous post. This way it'll go 10% to 100% bright, more like
a real brake light.

Thanks

Jay W

--- In Electronics_101@y..., Larry Hendry <hendrysr@y...> wrote:
I'll try this again with an actual answer this time. :)

--- Jayson Woodruff <woodrufj@y...> wrote:
There are two problems I have. 1.) Both the front and
rear need to have both a low light and bright light
setting. Low for lights on, bright for blinker or
brake operation. The voltage sources I have to work
with are two 12V lines. One operates with lights on
and both are active with blinker/brake operation.
Does anyone have a better idea?
Since you are dealing with brightness caused by LED clusters, what
about making your cluster (your arrangement of series / parallel
combinations) in two parts. Not two discrete parts, but two
integrated parts. All LEDs would be the same. But, one 12 volt line
would light 50% (or some other appropriate %) of the LEDs. the
second
12 volt line would light the other 50%. So you r cluster has a "A"
and "B" light with the LED arranged like this:
ABABABABA
BABABABAB
ABABABABA

Obviously, when the second line comes on, the total brightness will
increase.

Larry H


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