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dash Lighting help needed

I switched to LEDs as per many post in the forum, but need some advice/answers on a couple of points

what and where do I place a resister or "?" to allow the LEDS to be adjusted for brightness..right now turning that dinner just turns them off and on

does the headlight switch have any lighting for the pull knob

hw does one reach the center dash switches ( defroster/fog lights/cig lighter ) to change lamps and or see why they dont appear to light up in the dark>...i would think they should be lit to see to use in the dark?
Since the Radio is below the switches..do I need to remove blower cover and go from behind to get to actual lighting circuits?


pictures as always appreciated, especially on question 1 and where to plave the watchamacallit to control LED dimming

thanks

Last edited by 7783911; 04-24-2019 at 04:03 AM..
Old 04-22-2019, 07:20 PM
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Not sure about all of your questions but I didn't think LED's could be dimmed. I thought they were either on or off.
I could be wrong and if so I'm sure someone will enlighten us.
Old 04-23-2019, 01:23 AM
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My led kit from a board member here dim just fine. All I did was replace the bulbs and go.
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Old 04-23-2019, 04:10 AM
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Originally Posted by Duc Hunter View Post
My led kit from a board member here dim just fine. All I did was replace the bulbs and go.
That's very interesting. I'll have a better look at that once I get the engine back in and running
Old 04-23-2019, 06:08 AM
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I dont get any dimming transition, just on or off. I thought my lamps were from CarMagic but may not be (years ago) so ordered new set and will try again, may add optional dimmer control as maybe the switch itself is a bit aged at 40+ years. I thought I had read about some use of a transistor to better control power control on that circuit but from sounds of others, this isnt necessary so i will try new lamps as start point
Old 04-23-2019, 06:54 AM
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My lighting LEDs (eTarga's kit, IIRC) dim on the rheostat until they fall below the voltage they operate at - and then they just shut off. Which is somewhere just below 2/3 rds the operating range on the rheostat. And still brighter just before they shut down than the original bulbs would have been (at full bright)...

I used to like to dim the instruments to be like watch dials on unlit roads. So, as much as I love the brighter, more even lighting from LEDs, I noticed this.

Sounds like your LEDs require a higher operating forward voltage. So just shutting down with no noticeable dimming effect.

In order to actually dim LEDs properly, you need an LED dimmer. Which actually pulses them (in their operating voltage range) so that your brain perceives them to be "dimmer". More dark periods == "dimmer". If the device flashes fast enough (eg enough pulses per second), you can't detect the flicker.

These dimmers use PWM (Pulse Width Modulation); and are pretty widely available. You'd ideally want one that took a 0-12V variable voltage (like the output of the rheostat on the headlight switch) and converted it to the correct pulse profile for the LED bulbs in the instruments.

Something like this: https://www.amazon.com/Lighting-Controller-Incandescent-Aircraft-Interior/dp/B001CM74R2

The same folks also do one with a remote pot: https://www.amazon.com/dp/B015JXAOMQ/ref=psdc_15734181_t1_B001CM74R2 - connect the headlight switch rheostat in place of their remote. If both potentiometers happened to have identical values (range), that circuit would then Just Work.

If the values were too different, it shouldn't even be very hard to design a similar circuit compatible with the stock headlight switch rheostat as the control input.

Wanted to do this years ago, but a long way down the list...
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Old 04-23-2019, 08:37 AM
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let me correct my description of dimming as it may allude to switch problems. The dimming when turned results in on off on off on off..very dependent on rotated position, meaning the lamps will be on full at any given position of the dimmer and off at others. So sounds like the contacts for the dimming mechanism are failing? I will re route black/blue outbound power (58a) to a CarMagic dimmer and then to a fuse and take the switch out of the equation as much as possible or should i place the fuse ahead of the dimmer?
Old 04-23-2019, 09:59 AM
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Dimming

I think what's going on here is the rheostat windings are making and breaking as you turn the knob. With incandescents that's not noticeable but LED's turn off and on so quickly you see it.
Although PWM is the best way I did convert my 968 years ago and it did dim reasonably well but it was a later (93) headlight switch.
I do not have any dimming in my '88 after conversion and I get the flashing when turning the knob as well. I was going to just try a wire-wound resistor to vary the old rheostat value. I haven't tested the rheostat yet to see what value it is though.
Old 04-23-2019, 11:00 AM
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sounds identical, the external dimmer is just an easy way for me to go for relatively low cost and effort, besides not risking destroying a 600$ light switch! (sum of part greatly exceed the whole)
Old 04-23-2019, 11:34 AM
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Hmm- what does the rheostat do with bulbs in?

Reason I ask, after I got the car (and long before I put LEDs in the instruments), I spent an embarrassingly long time trying to get the instrument illumination working.

Until I happened to knock the rheostat knob a little by accident and immediately realized it had a "dead" spot where the instruments lights didn't work at all, and that's where it had been set to the entire time I'm been sitting there with a multimeter....
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Old 04-23-2019, 01:02 PM
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i am still hoping some0ne will provide me a brilliant method to reach the back of the defog and foglight switches blocked by radio and just about everything else i can see in there
Old 04-24-2019, 04:02 AM
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Quote:
Originally Posted by 7783911 View Post
i am still hoping some0ne will provide me a brilliant method to reach the back of the defog and foglight switches blocked by radio and just about everything else i can see in there


I searched in vain everywhere for an easy way to do this. I gave up and went in thru the frunk. What a pain.
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Old 04-24-2019, 07:26 AM
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that was my guess..i hoped for a silver bullet but examining radio and amp mounted under dash plus location of switches..knew this was not going to be easy...thanks for confirming worst fears :-)
Old 04-24-2019, 08:12 AM
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Unless the rheostat is bad with open spots, usually when changing to LEDs for dash illumination, the rheostat seems to make the LEDs go off or on with not much dimming in between. The reason is the LEDs are a very light load (high resistance) compared to incandescent bulbs (much lower resistance).

The rheostat is just an adjustable resistor in series with the bulbs. Turn for more resistance and the bulb current drops and you have less light. The bulbs and rheostat are designed work with each other.

I'll make up some numbers to show what happens. Real numbers are likely different...

For example, say the rheostat resistance goes from 0 (full on) to 400 ohm (full off) and the bulb's resistance is 100 ohm. Use 10V battery (for simplicity instead of 12V). Full on current (V/R) = 10/(0 + 100) = 100ma. Full off current (V/R) = 10/(400 + 100) = 20ma. Big change in current and light output.

Now put in LEDs with 2000 ohm resistance. Full on current = (V/R) = 10/(0 + 2000) = 5ma. Full off current (V/R) = 10/(400 + 2000) ~ 4ma. Not much difference.

The one way to fix this as mentioned before is to use PWM (pulse width modulation) but that requires some interface unit. Another way is to add a ballast resistor to the illumination network like is done for turnsignals when LEDs are installed to fool the turnsignal flasher. Don't need that low of a resistance as for turnsignals.
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Old 04-24-2019, 11:31 AM
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ah..an electrical expert jumps in...

whtat you say makes sense but does contradict other folks observations re actually being able to variable the dimness of their dash lamps without mentioning any additional resisters?

I will contact CarMagic and see what he has to say

ps..i have your products in all of my external lights..fantastic products!
Old 04-24-2019, 12:16 PM
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ah..an electrical expert jumps in...

whtat you say makes sense but does contradict other folks observations re actually being able to variable the dimness of their dash lamps without mentioning any additional resisters?

I will contact CarMagic and see what he has to say

ps..i have your products in all of my external lights..fantastic products!
Engelbert has good products. I have his headlights on my 930. I've not done much with interior lights. Interested to see what he has to say.



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Old 04-24-2019, 01:25 PM
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Spoke

That is next winters project, I have a scoop setup with H4 beams but want to convert..i see you have scoops...was any modification required to adapt his lamps?
Old 04-24-2019, 01:26 PM
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Spoke

That is next winters project, I have a scoop setup with H4 beams but want to convert..i see you have scoops...was any modification required to adapt his lamps?
The only thing that has to change is the frame that holds the lamp has to be slightly bent outward in the rear since the light housing is a bit larger than a standard bulb.

The light has a daytime running light feature that was a piece of cake to implement. An extra wire on the light was easily spliced into the light picking up the running light power that is in the headlight bucket going to the front turnsignal and sidemarker.






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Old 04-24-2019, 04:28 PM
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Originally Posted by spoke View Post
The one way to fix this as mentioned before is to use PWM (pulse width modulation) but that requires some interface unit. Another way is to add a ballast resistor to the illumination network like is done for turnsignals when LEDs are installed to fool the turnsignal flasher. Don't need that low of a resistance as for turnsignals.
Unlike incandescent bulbs, LEDs only work within a certain voltage range. Below that range, they don't light up. Like, at all.

Their "best" voltage range varies (and quite widely) according to the semiconductor used (relates to the color).

LEDs for auto use come packaged with a resistor already - because their operating voltage range isnt' anywhere close to 12V; visible spectrum LEDs want anywhere from 1.9V (with 2.8-3.1V being more typical) to a ceiling of 4.0, 4.4V max.

I'm not saying there isn't a magic ballast resistor value that would allow a full range of dimming on the factory rheostat - and if you find it, I'd certainly be interested to know what value you come up with...

I just think it would be easier and provide better/best results using an approach suited to the technology.
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Old 04-24-2019, 04:56 PM
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Originally Posted by 7783911 View Post
i am still hoping some0ne will provide me a brilliant method to reach the back of the defog and foglight switches blocked by radio and just about everything else i can see in there
If you mean your're trying to replace the bulbs from the rear of the switches don't panic. You pry out the front cover of the switch with a small screwdriver and the bulb is in front!
Old 04-25-2019, 08:29 AM
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