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I've done a day of research and haven't found a clear path to my problem. I thank you for taking the time to help!

Retrofit application in 1985 VW Jetta (MKII). My instrument cluster lights, the ones that actually shed light onto the gauges, are stock incandescent. Marked 12v and 1.2 watts Everything is in DC.

schematic

simulate this circuit – Schematic created using CircuitLab

Ok, in the schematic, I couldn't find a symbol identical to what is in my manual, but I believe them to be the same. To understand the Bently wiring diagram/manual: thin lines are internal connections with in components, I believe the wiper and one post of the integrated dimmer pot are internally connected. Bold lines are actual wires. VW's part number for the switch unit is 191941531A enter image description here Second part of understanding the manual is terminal I.D.'s. --Terminal 30 is 12v+ coming into switch,

--Terminal 58b is output from pot to dash illumination lights,

--Terminal 58 is output power to parking lights, more importantly it is internally supplying 12v+ to pot when the switch is turned on,

--The remaining terminals should not be of concern in this project.

I've tested the pot with my meter and ranges from 0.002 ohms up to 25 ohms, so it tests to be 25ohm pot.

I want to replace the incandescent bulbs with plain-Jane, oldschool, 20mA 5mm Red LEDs. I can simply plop in my LEDS, 3 in series, with a 470ohm resistor at the tail to ground. I can then supply 12v and have lights, easy peasy. I've factored a max voltage in choosing 470 resistor of about 14.5-15v. That should provide a safe buffer, and I don't mind if the LED isn't at "FULL" power since after all they aren't linear devices and I can't physically detect a difference between said voltages. Even if at 12v, it was slightly dimmer, so be it.

To complicate matters, I want to be able to dim these 3 small LEDS, ideally using the stock dimmer pot, which is part of the main headlight switch. But I am well aware that LEDs need their current limited, where as incandescent's can be controlled by a pot(voltage i think), and that trying to control and LED with a pot(voltage) is not the way to go. Yes, a pot, if used with an LED, will create a "hard to use" curve, where it's either on, or has an extremely low travel to get to dimming; among other problems. It doesn't work with what I have since I've tried it, where in my case they just stay on no matter what I dim the pot to.

How do I take some of the ways to limit current, control the LEDs with the stock pot, and get the desired result? I'm not sure I follow how to use buck drivers, or PVM, or other ways to tackle this. Can a 555, opamp, or LM/Txxx regulator work, and basically make a small "relay" that is controlled by the stock pot?

What I have worked out so far, but doesn't dim as desired:

schematic

simulate this circuit

schematic

simulate this circuit

I'm looking for simple. I'm not trying to "save power", or go to any extremes, just want to design something simple, easy, and that I can plop into place w/o redesigning the wheel. Thank you again!

:EDIT:

Arrow 1 is pointing out the old bulb sockets. Unfortunately the bulbs that fit in only made contact to the old style circuit foil, and not to any connector and wires. So I can't use the old bulbs,e tc. Arrow 2 is pointing out the LED bar I'm only using for testing and playing around, but the 3 red LEDs I plan to use will fit in a similar manner. Arrow 3 is showing it nicely fitted into where it needs to go. You can get a glimpse of where the wire is running to LED bar and placement

Arrow 4 shows the position of the board that will supply power to the LEDs, and a close up of the spot where I will place a screw terminal for the two wires that will power the LEDs. One may notice the spot for the resistor that will tie the LED neg to ground (through a trace on the back side that isn't visible). Connection spot

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  • \$\begingroup\$ For comparison, my old 99 Camry had a rheostat for the illumination dimming. It worked just fine with 3 leds + resistor setup, dimming from full brightness to off. What voltage are you getting after the dimmer pot is reduced? That should tell you how much dimming you should be seeing. \$\endgroup\$
    – Passerby
    Nov 25, 2018 at 2:20
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    \$\begingroup\$ Ah, the problem is that it's 25 ohms, so at full on, your leds are seeing 500 ohms, so only a small difference in current, like 1mA less so not enough to look dim. Now I'm wondering how my Camry's rheostat works... \$\endgroup\$
    – Passerby
    Nov 25, 2018 at 2:33
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    \$\begingroup\$ Do you want to continue to use the existing potentiometer for the purposes of dimming? Even if that means some "re-wiring?" Also, where does that wire go, off-image to the right (and also up), from the pot? I see that the two lights aren't the only thing coming off of it. \$\endgroup\$
    – jonk
    Nov 25, 2018 at 3:47
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    \$\begingroup\$ @JLaf Well, the problem is the load they also present. If they weren't part of the circuit in your Jetta (and I don't know if they are), and the pot was exclusively for the cluster lighting, then that would allow some options in keeping it wired in place. Given that there may be other incandescent attachments in parallel, it complicates things. Incandescents vary their resistance as they heat up. So the circuit will present a complex and varying divider to the cluster section. Eliminating that, by converting to LED, would be helpful. \$\endgroup\$
    – jonk
    Nov 25, 2018 at 4:10
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    \$\begingroup\$ @JLaf Do you have access to the (+) voltage at the cluster lights (and elsewhere the LEDs would be?) Or do you only have access to ground at those locations? \$\endgroup\$
    – jonk
    Nov 25, 2018 at 5:12

1 Answer 1

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You can't draw in "comments", so here is an idea circuit that might get you going in the right direction. You can put as many LEDs in series as you need and you may have to play around with the resistor values. Two potential problems: the pot may not be linear and you have to turn the pot in the opposite direction from where you are now to dim.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ thank you my friend! I will throw some parts onto the project board and report my results. Just to clarify, "Q1", that's a PNP transistor, right? Can you recommend tolerances, or some spec on it? That may the one part I don't have ready in my parts bin:( I'm certain it's easy and quick to find, so if I don't have one, I'll get one asap and test this out! Thanks again John. \$\endgroup\$
    – J Laf
    Nov 27, 2018 at 0:17
  • \$\begingroup\$ Q1 is a PNP. How it works (I hope): \$\endgroup\$
    – John Birckhead
    Nov 27, 2018 at 14:36
  • \$\begingroup\$ Q1 is a PNP. How it works (I hope): With the pot at 0 ohms, the base is about 0.6 volts below the 12 volt rail, so Q1 is off. You may have to adjust the values to achieve this depending on the transistor beta. When the pot is turned on all the way, the base voltage is about 1.1 volt lower than the 12volts, so the emitter is about 0.4 volts lower, meaning you have 0.4/20 or 20 ma. Turning the pot between gives you an adjustable current source. The transistor could see about 10 volts at 20 milliamps or 200 milliwatts, so just about any PNP will be ok. \$\endgroup\$
    – John Birckhead
    Nov 27, 2018 at 14:45
  • \$\begingroup\$ Thanks for helping. Well, all i could find lying around was a SMD transistor, marked 2a, which i found was a pnp. i soldered on a couple legs to throw it on my test board, found some resistors, and tried it out. well, the LED burnt out with in a second or three. I rewired with a resitor after the LED, going to ground. It just glowed like normal, but the dimmer switch did dim it by a barely visible amount. I'm gonna order a regular PCB tranistor, and try it again, but i'm thinking some values may not be right,. more testing required, and I'll report my result. THANKS AGAIN for the input. \$\endgroup\$
    – J Laf
    Nov 30, 2018 at 5:27
  • \$\begingroup\$ i'm more of a mechanical guy, and if I was using mechanical devices i can draw this analogy: I currently have one gear, say 10 tooth. It only rotates 180*, and the old component attached to it rotated where it needed to, back and forth. The new component doesn't move through the same range, so a second "step up/down" gear needs added to change the ratio of output to input. This "second gear" is what I'm looking for electrically. Electronically, I imagine a component can take my incoming weak single and amplify it's "swing", and if I understand correctly , a pnp does that -i think? \$\endgroup\$
    – J Laf
    Nov 30, 2018 at 5:31

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