Cecil,

Here's the deal.

I will assume the low intensity tail light and the high intensity brake
light share the same LEDs. In a typical system there will be 10's to
even 100's of LEDs. The required light output from the tail light
function and brake light function is quite different. I have seen
numbers from ranging from 7:1 up to 15:1. Let's say that the brake light
requires 10X the output required from the tail light.

Contrary to what was posted earlier in this thread, the light output
from LEDs is roughly linear with forward drive current over several
orders of magnitude. This is true up to nearly the point where the
device explodes from overheating.

So why not just change the DC current to get the varying output required?

The problem is device matching. As you well know all semiconductor
devices are subject to significant performance variation depending on
exact process conditions, materials, phase of the moon, or whatever.
LEDs are typically specified at one nominal current, with a nominal
light output.

When used at the nominal conditions a batch of LEDs can be selected to
match light output performance so that no obvious visual artifacts
appear. However, if these same matched LEDs are driven at significantly
different conditions, the light output will have wider variation,
perhaps detectable to the ordinary observer.

Therefore the recommended method for reducing light intensity from a
matched array of LEDs is to operate at nominal electrical conditions,
but at a reduced duty cycle.

I do not know the details of the vehicle you observed, but it is likely
that the brake light function uses DC or a high duty cycle pulse, while
the tail light uses a low duty cycle pulsed activation.

There are other issues such as temperature variations and dealing with
non-functioning LEDs, but that is enough for now.

73,
Gene
W4SZ




Cecil Moore wrote:


It appeared that they were all pulsed at once. Perhaps I should
have used the word "pulsed" to avoid confusion in the RF analog
(non-digital) world.