Come up with a brilliant solution .......

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I work in the trailer manufacturing business.
We use LED tail-light clusters.
The modern truck/tractors utilise a computer system to perform diagnostics
on various systems within the rig. One of these is the lighting system.
A pulse is sent to each tail light, the system monitoring the current. If
there is current flow, it is assumed the incandescent globes are OK.

Unfortunately, the LED lamps draw such a small amount of current that the
computer does not see a "filament" and flags a major fault on the truck
dashboard. In addition to the error message, the system continues to send
curent pulss in the forlorn hope that things at the rear of the rig have
improved. This causes the entire suite  of LED lamps to flash like a
low-class disco!

There are ways around it. Some manufacturers have placed incandescent globes
in parallel with the LED lamps, others have used high wattage resistors.
Neither is acceptable for obvious reasons.

There must be a way to "tell" the computer that the LED lamps are fine by
emulating the current drawn by an incandescent globe, without using the
solutions noted above.

Do any of you outstandingly brilliant contributors have any novel and
innovative ideas?????



Re: Come up with a brilliant solution .......



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The only ways I can see to "emulate" current draw is to either:

(a) actually draw it (resistor, incandescent bulb or similar, as you mention);

(b) modify the sense circuitry for greater sensitivity; or

(c) interpose a "box" that senses LED array current and fudges the pooter input.

If the current sense circuitry simply senses volt drop across a sense resistor,
you'll need to increase it.  If it uses a transformer approach with an open
toroid, wind more turns on it.  This assumes that the circuitry can be modified
(legality, access, information availability).

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Nope, only basics.

Re: Come up with a brilliant solution .......


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Does the manufacturer offer an updated ROM for your model?

Cheers,
Nicholas Sherlock

Re: Come up with a brilliant solution .......


No. Manufacturers of the truck/tractors offer no assistance. They will not
permit any tinkering with their computer systems - will void the warranty -
and have not come up with any viable solution themselves!


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Re: Come up with a brilliant solution .......


Maybe I am missing something here - it seems simple.  Power transistor thru
resister from active side of tail lamp supply to earth - resistor set so
required current is drawn when transistor is biased on - transister base fed
thru zener so conducts when pulse arrives.  Pulse sent, transistor conducts,
computer thinks happy thoughts.

David - or is this just too easy (and cheap - $5 worth of bits at the most)

Nicholas Sherlock wrote:

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Re: Come up with a brilliant solution .......


On Fri, 23 Sep 2005 07:37:54 GMT, quietguy
keyboard and composed:

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Maybe I'm missing something, but wouldn't your transistor be on
whenever the lights are on?

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

Re: Come up with a brilliant solution .......


Since the computer sends a pulse to check the lights this would have to be
higher than the normal 'alight' voltage of the taillights etc.  So you would set
the zener going to the base of the transistor a little higher than the 'alight'
voltage - eg the tranister only turns on when the pulse is sent

David

Franc Zabkar wrote:

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Re: Come up with a brilliant solution .......



message
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How do you know this? And why do you think it has to be so?

Ken

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'alight'
thru
so
base fed
conducts,
most)
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Re: Come up with a brilliant solution .......


I don't know 'this' - just guessing.  And since the computer measures current
draw, it seems unlikely it is an RF or Audio pulse, but rather a DC pulse.  And
to superimpose a DC pulse onto a 24v DC line, it seemed logical that it would
have to be higher than 24v in order to be detected.

But of course I am just 'supposing' - is that a problem for you?

David

Ken Taylor wrote:

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Re: Come up with a brilliant solution .......


It's not a problem to me, it's just plain wrong. It's not the way lamp
current sensing is done.

Ken

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Re: Come up with a brilliant solution .......


Ken, since you say you have the inside knowledge here it would be helpful if
you could let us know how it is done, so we could be more hopeful to the OP

David

Ken Taylor wrote:

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Re: Come up with a brilliant solution .......


message
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Here's what you wrote:
<<
I don't know 'this' - just guessing.  And since the computer measures
current
draw, it seems unlikely it is an RF or Audio pulse, but rather a DC pulse.
And
to superimpose a DC pulse onto a 24v DC line, it seemed logical that it
would
have to be higher than 24v in order to be detected.
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No offence, but you don't know what you're on about. If reading that back
again doesn't tell you that, then don't let it bother you.

The OP has plenty of good (and some mediocre) advice to go on, including the
non-trivial nature of what he wants to do.

Cheers.

Ken




Re: Come up with a brilliant solution .......


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One of the best lessons I learnt at tech was KISS (Keep It Simple Stupid)
The answer to this problem has been answered already in this thread. Let the
computer do the fancy pulse stuff, the only thing it is looking for is a
load. Not return pulses, not higher than supply voltages, just a substitute
load. Give it a load which fools it into thinking it has a working globe and
it will be happy.



Re: Come up with a brilliant solution .......



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Except now you don't know when the LED lamp array has failed, so the
diagnostic is useless.



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A simple comparitor cct using an op amp. Everyone wants to turn this into a
major project.



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the
lamp
back
including
Stupid)
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Let
a
Per light? Sheesh, who's making it a major project?

Ken



Re: Come up with a brilliant solution .......



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I have it down to one comparitor and 1 transistor plus a few passives.
I reckon i can get it down to at least 3 transistors, perhaps 2 and no
comparitors. Mind you. thats in spice, not in the real world, and
thats with a few glasses of red wine too....Dang its been a while
since I used spice...

Re: Come up with a brilliant solution .......


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Spice your wine...? Cool!  :-)

All that electronics so you can use LED's in a trailer light - I think this
project is in serious need of management oversight. :-)

Cheers.

Ken



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You could get even trickier, use 1 4bit uP and a transistor. Even
better if the uP has an ADC on board..

Re: Come up with a brilliant solution .......


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to
way
measures
DC
that
that
thread.
looking
just a
has a
the
this

There are some neat networked chips that fit into an RJ45 socket - now if
they just had wireless......

Ken



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