[urq] urQ/CGT/4k instrument cluster bulbs: some LEDs bulbs works for less $$

urq urq at pacbell.net
Wed Nov 7 20:50:38 PST 2007

It would be interesting to see the current vs. voltage plots for the LED
bulbs to understand this better, but in general a LED's intensity is related
to the current flowing through the device where the incandescent bulbs
intensity is related to voltage.  There is a way to make both types of
lights dim similarly, and that would involve the modulation of the
illumination signal so that the intensity is controlled by the duty cycle
(on vs. off ratio) of the signal.  I did a quick web search and found some
cool ideas, but nothing specific to the requirements of dimming a mixture of
incandescent and LEDs in an automotive app.  It should be pretty easy to
build a circuit to do this that would fit into the standard form factor of
an automotive relay.  The interesting thing is that as long as you are
simply switching the power to the lights on and off you don't have the power
dissipation issues you have with a rheostat.  

I think I'll sketch something up quickly to see how it looks ... a simple
microcontroller like a PIC and a high current FET should do the majority of
the work ... 

BTW, I have tried red LEDs to illuminate the dash and think that a broader
spectrum source like the warm white LED you mention would provide a color
much more like the factory bulbs ... the filter they use is not a pure red.
Honestly, I didn't care much for the LED red color ... maybe an "orange" LED
might do the trick, but if you have a broad spectrum source like white you
can filter it as you like ... 

Steve B
San José, CA (USA)

-----Original Message-----

As in life in general, the answer is : yes and no...

The LED bulbs do dim a bit, but their response is much faster than
incandescent bulbs. They disappear very quickly so adjustment is finicky. I
am speaking of the 2 main bulbs, because the switches LEDs are so dim
already, you don't want them to light less.

For the sake of knowledge of the other listers: the urQ/CGT/4000 instrument
panel use only 2 wedge bulbs (standard type 194, but with a little
magnifying lens on the tip) to project the whole 3 watts into a red tube
that distribute the light to the IC from above. These bulbs are 18$ each
here in Canada and we were seeking a longer lasting, cheaper alternative,
hence the LED bulbs.


-----Message d'origine-----
De : Martin Pajak [mailto:martin at quattro.ca] 

Hi Louis-Alain,

Great idea.
Only question I have is, how does this work with the light rheostat?
Does it dim/brighten with the rest of the dash or stay one intensity?


> Louis-Alain Richard
> Well Lino, I have an answer as to install LED bulbs into an urQ/CGT/4000
> instrument panel main lighting.
> The $2.79 "6-LEDs 194-type warm white" units work almost as fine as the
> expensive 857-919-040 OEM incandescent bulbs. Light amount is similar, and
> the color is nearly as red as the OEM bulb. The cheaper "4-LED 194-type
> cool
> white" doesn't, as they are not powerful enough, and cool white give a
> purple tint to the dashboard lighting. So I guess the "WLED-WHP Pure White
> High Power " single LED bulb will do the same (I did not order these).
> You want warm-white bulbs, or better, red-bulbs for the main lighting. I
> should've read this page BEFORE ordering my bulbs:
> http://superbrightleds.com/carbulb-notes.htm.
> Also, I ordered 74-type single-LED red bulbs for the switches, and they
> work
> wonderfully. You must open the switches and weld them in place (don't
> forget
> these are diodes so they have a + and a -) but the result is superb. And
> they will outlast the car...

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