Another option for VDO gauge illumination - now with photos

@ScottAndrews Looks pretty simple, I agree. If I'm understanding you correctly, the rheostat in its standard state in our vehicles is decreasing the voltage flowing between input and output to decrease the brightness of the light. The LM134 is limitng the current, in contrast, thereby allowing modulation of the LED brightness. Your diagram shows that the 12V from the battery goes to the LM134, that output goes to the rheostat but then it appears that both the output that went through rheostat as well as direct out of LM134 are joined to go to LED lamps. So I guess the rheostat is not a 0-100 possibility as it is now but is modulating the total above and beyond that allowed by the LM134?
Also, given that I'd consider myself a beginner-intermediate macro-electrician (i,.e., cars and houses) but a complete novice at micro-electrics (circuit boards et al), is something like the LM134 available in an already-built fashion so someone with my skill level could try it out without having to put together a whole circuit board?
 
I am not aware of any pre built broads for the LM 134. There may be some small LED drivers available that you could patch in. For example, here's one on eBay. You would need to unsolder the "adjustable resistor, and attach wires that would go to the car switch rheostat. Like this:
LED Driver Board
Screenshot 2025-12-06 at 8.07.45 AM.png
 
Ordered. Soldering I can do. I figure if @coupesport can spend a few bucks to experiment with some different bulbs to help guide the rest of us, I can spend $8 and some time to try to rig this up.
 
e9Leveque said:
Ordered. Soldering I can do. I figure if @coupesport can spend a few bucks to experiment with some different bulbs to help guide the rest of us, I can spend $8 and some time to try to rig this up.
Click to expand...
Do you have a digital multimeter? If so, measure the resistance of the adjustable resistor. There will (probably) be three pins in a row. Measure the resistance value between the outer pins. If they are arranged in a triangle, then try the various combinations until you get the highest one. If there are two combos that measure the highest, move the adjustment slot and test again.

Then check the resistance of the light switch rheostat (it is functionally the same as the adjustable resistor on the board. However, it may have a different total resistance). Depending on what the difference between the resistors is, you may need to add an additional resistor. This will either be in series or in parallel depending onthe difference between the resistances of the variable resistors.

For example, suppose the one on the board is 1000 ohms, and the one in the car is 500. You will need to add 500 ohms in series withthe car rheostat. This will mean that the LEDS may not dime all the way off, butt hat's probably OK.

If the rheostat in the car is, say 2000 ohms, then you will need to add a 2000 ohm resistor in parallel across the two outer contacts of the rheostat (or solder the resistor onto the board. This will cause the car rheostat to vary between 0 and 1000 ohms instead of 0 to 2000.

Check the board. It is likely that the wiper contact for the variable resistor (usually the middle contact) is connected acted directly too one of the other contacts. In that case run the center wire to one of those two spots.

Post PM if you have a questions.

Scott
 
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Thanks Scott. I do have a digital multimeter and that amount of soldering and assessment is within my skill set. I’ll let you know what I discover once the part arrives.
 
@coupesport When you had first posted this thread I had ordered the lights you'd indicated since I was planning on cleaning gauge faces etc. I had ordered some from superbrightleds which were a 4-light forward-emitting BA9S base, but I don't see that listed anymore in your writeup. Was that bulb for the telltale lights which you now left as the 4W Osram bulbs?
Also, cleaning the gauge faces makes a big difference. I had to have my clock rebuilt so of course it got cleaned, and with the new LEDs now I'm having to clean all of their interior glass because there's 50 years of some dust which looks quite a bit different than the freshly cleaned one
 
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