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How does the multifunction dimming control work?

 
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  • 3 Post By rsrocket1
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Old 09 Oct 2016, 01:59 am
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Default How does the multifunction dimming control work?

Sounds like a stupid intro to this thread, however I'm thinking of modifying the internal electrical function of the dimmable lights around our car.

Firstly, all of the small grain bulbs and tiny incandescents are being replaced with LEDs. The forward voltage (voltage before the LED starts conducting and lighting up) tends to be high for the blue LEDs I'm using, making the first setting or two a little pointless. I either want to alter the stepping of the dimmable bus at the source, or even replace it with something automated by Arduino. That is, so it's possible to have "breathing" LEDs around the entire car or a completely different managed switch off after locking. Things like that.

I'm yet to get ahold of a Haynes manual for our PT so I don't have a complete (or even partial) electrical schematic, so this might be a lot simpler than I am expecting. I suspect that it's something embedded deeply in the ECM, however that can be worked around.
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2001 2.0l PT Cruiser Touring Edition
Mods: New gear knob, blue LED re-lighting, token chrome bits.
215'000km (55.9% distance to the moon, 71.7% of a second at light speed, Usain Bolt running for 200 days)

"People demand freedom of speech as a compensation for the freedom of thought which they seldom use" - Søren Kierkegaard
"What can be asserted without evidence can be dismissed without evidence" - Christopher Hitchens
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Old 04 Nov 2016, 11:09 am
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Default Re: How does the multifunction dimming control work?

The dimmer is just a potentiometer with one leg tied to ground and the low side of the lamps tied to the center tap. The high side of the instrument lights look like they are tied to the headlamp switch output. But it also looks like the panel lamp dimmer also controls all interior lights that are dimmed by that dial (triangle F on the second page) so you have to scale the wattage of that pot appropriately too.

Coincidentally, I just replaced the MFS on my 2009 base PT yesterday. My son said the turn signal lever would not stay in place at a signal so he had to hold it there and yesterday morning it snapped off .

$76 and 15 minutes later that evening, it was replaced. I made him watch the Youtube videos and had him replace it. It was about as simple as possible. 2 Phillips screws deep in the lower shell which held the upper shell, a single Torx screw which held it to the steering column and a single Phillips screw holding the MFS assembly to the main assembly. 2 wire connectors and that was it.

If I can remember to do so, I'll dig into the trash can this evening to see if I can determine the resistance value of the dimmer pot.

For dimming with an Arduino, are you thinking of PWM the LED's? They should respond fast enough and there are quite a few PWM LED dimmers out there.
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Old 04 Nov 2016, 01:42 pm
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Default Re: How does the multifunction dimming control work?

Thanks for the info - I got ahold of the Haynes manual (Chilton in the US?) so I should have those diagrams now. I can't say I've looked back into this one since the original post, however I am thinking that it might be as simple as adding in a bias resistor in series with the potentiometer or something along those lines. The pot is likely (if I thinking correctly) a switched one that shorts out to full/zero resistance when engaged. I'll have to take a closer look at that.
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2001 2.0l PT Cruiser Touring Edition
Mods: New gear knob, blue LED re-lighting, token chrome bits.
215'000km (55.9% distance to the moon, 71.7% of a second at light speed, Usain Bolt running for 200 days)

"People demand freedom of speech as a compensation for the freedom of thought which they seldom use" - Søren Kierkegaard
"What can be asserted without evidence can be dismissed without evidence" - Christopher Hitchens
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Old 04 Nov 2016, 04:11 pm
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Default Re: How does the multifunction dimming control work?

This may be a mute point now. But if anyone is interested in how the dimmer in a PT Cruiser multifunction switch works, Here's a photo.

Instead of using a rheostat like car's once used for this, there's a circuit board inside the multifunction switch stalk directly underneath the ring that the driver uses to dim the dash lights. The circuit board has a number of contact plates on it. Each plate is soldiered to an individual micro resistor. When the ring is turned, a copper brush moves to different contact plates. And then the resister soldiered to the plate where the brush is provides the selected voltage that determines the brightness of the dash light bulbs.

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Old 06 Nov 2016, 03:00 pm
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Default Re: How does the multifunction dimming control work?

Great work. I suspect that it works as a cross between a multi-position single pole switch and a potentiometer. More than likely the series resistance of the resistors from the end of the chain to the position of the "wiper" (switch position/pole) are used to define the voltage of the panel lamps driver circuit. It's better than a rheostat because they're super lossy. There's probably a MOSFET or some other device that supplies the grunt based on how much this small circuit tells it to.

I'll have to have a look and suck through my teeth a bit at it, whilst drinking a cup of coffee. This isn't keyboard work.
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2001 2.0l PT Cruiser Touring Edition
Mods: New gear knob, blue LED re-lighting, token chrome bits.
215'000km (55.9% distance to the moon, 71.7% of a second at light speed, Usain Bolt running for 200 days)

"People demand freedom of speech as a compensation for the freedom of thought which they seldom use" - Søren Kierkegaard
"What can be asserted without evidence can be dismissed without evidence" - Christopher Hitchens
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Old 07 Nov 2016, 09:20 am
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Default Re: How does the multifunction dimming control work?

Remember, if you are going to use an Arduino or other microcontroller to dim LED's, you do it with PWM, not current control unless you use a DAC to control a power transistor or FET. If you use PWM, you'll need to convert all the lights it controls to LED's rather than incandescents.
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Old 07 Nov 2016, 10:15 am
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Default Re: How does the multifunction dimming control work?

Of course, you can also just use dimmable LED bulbs designed for incandescent replacement.

http://www.ebay.com/itm/2X-USA-12v-P...VXqmu-&vxp=mtr

http://www.ebay.com/itm/Fits-Toyota-...aeK27w&vxp=mtr
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Old 07 Nov 2016, 12:11 pm
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Default Re: How does the multifunction dimming control work?

I'd rather not. I chose very specific wavelengths of blue for colour coding around the car. This is even going to extend to the point of modifying bulb bases and fitting these blue LEDs in there. The forward voltage of the LEDs and apparent brightness per setting is the only reason I may need to modify the values around the control. The batch I've got isn't bad though, since LEDs vary from bin to bin and illuminate at the lowest setting. Once I have the immediate engine issues (serpentine tensioner, engine mounts) sorted out I'll spend more time looking into this. It might be that current resistor selection is more useful for incandescents as a more linear dim-to-bright.

Whilst I'm not wanting to take this thread off-topic (I think you pretty much nailed the "how it works" bit already!), here's the datasheet for the LEDs I'm using:
http://uk.rs-online.com/webdocs/139d...6b8139d765.pdf
__________________
2001 2.0l PT Cruiser Touring Edition
Mods: New gear knob, blue LED re-lighting, token chrome bits.
215'000km (55.9% distance to the moon, 71.7% of a second at light speed, Usain Bolt running for 200 days)

"People demand freedom of speech as a compensation for the freedom of thought which they seldom use" - Søren Kierkegaard
"What can be asserted without evidence can be dismissed without evidence" - Christopher Hitchens

Last edited by Prostheta; 07 Nov 2016 at 12:14 pm.
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Old 07 Nov 2016, 12:59 pm
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Default Re: How does the multifunction dimming control work?

You do know that the eye is about 15 times more sensitive to yellow/green at 550 nm than it is to blue light at 470 nm, right?
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Old 07 Nov 2016, 02:38 pm
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Default Re: How does the multifunction dimming control work?

I do, yes. I don't like the yellow-green colouration of the displays or the VFD in the overhead console. In my humble opinion, the PT Cruiser is a great car that is a bit locked "in its time" by those bits and pieces. If I could change the colour of the odometer along with the cluster bulbs, I would. I might even go as far as to seeing whether a live OBD2 read can replace the dials and readouts themselves one day. That's the kind of crazy I do.

So yeah, I'm on a bit of a long term mission to eradicate the cheap plastics (big job) and old tech from the car. Starting with the bulbs.
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2001 2.0l PT Cruiser Touring Edition
Mods: New gear knob, blue LED re-lighting, token chrome bits.
215'000km (55.9% distance to the moon, 71.7% of a second at light speed, Usain Bolt running for 200 days)

"People demand freedom of speech as a compensation for the freedom of thought which they seldom use" - Søren Kierkegaard
"What can be asserted without evidence can be dismissed without evidence" - Christopher Hitchens
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