[biotechee]

Although this doesn't specifically pertain to my 968, I am wondering how to wire up an LED in my daily driver. Basically, I want to illuminate a switch in my car which is currently (surprise!) not illuminated. The switch has the necessary opening/clear area to accept lighting, but did not come from the factory that way.



So, I know I need to buy an LED and a resistor, but I really don't know too much else than that. To top it off, the local Radio Shack folks don't have a clue!



Can anybody lend some insight?



- What type of LED?

- What size resistor?

- What are the wiring connections I need to make? (I assume one connection to the 12V+ and then the other to ground?



Thanks in advance!



P.S. No- this is not mood lighting or "ricing" my car- the factory sunroof switch in my Jetta is not illuminated and it should be!

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[Greimann]

Valuing a resistor to drive an LED just requires some simple math and 3 bits of information. First the Formula:



R =(Vs -Vf) / (current in milliamps)*1000



Where:

R = resistance in ohms

Vs = supply voltage (in a car, use 13 volts)

Vf = Forward voltage of the resistor. This information should be on the data sheet that comes with the LED

current = the rated draw in milliamps, again, from the data sheet



EXAMPLE: You have an LED that says the Vf is 1.5V and the draw is 20ma, what resistor is required to drive it when supply voltage is 13V?



R = (13 - 1.5) / 20 * 1000

R = 575 ohms





Now deciding on which one to use may require some experimentation. It depends on how much light you need. The high output LEDs that are used in flashlights and brake lights might be too much for your little switch. These high output ones are typically rated at 1000 to 5000 MCD (look at the data sheet) The lower output ones are less than 1000 MCD. Then there is also the question of color. The choices are bewildering.



I would suggest starting with a white one at 1000 MCD and see how that works. If it is too bright, you can always add a higher value resistor. If it too dim get a higher MCD value, don't go too low on the resistor or the life of the LED will be short.





Here is where I like to get mine: Jameco High Output LEDs

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[sat968]

Try this:



http://www.luxeonstar.com/resistor-calculator.php

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[biotechee]

Gentlemen-



Thank you! I appreciate the information. I was finally able to find a write from someone who actually did it using a Radio Shack LED with integral resistor. I installed it and it works just fine.... The only remaining thing to do is hard wire it.



The calculations will undoubtably come in handy in the future!!!

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[biotechee]

Dave,



I think the formula you wrote was grammatically incomplete....



I created an Excel calculation to do this for me and when I set up the equation as this:



R=(Vs-Vf)/(mA*1000)



I got 0.000575 Ohms.... that didn't seem right from the decimal place perspective so I set it up this way:



R=[(Vs-Vf)/(mA)]*1000



and I got 575 Ohms. So I have assume the multiplication step (*1000) is the last step in the equation.



Would you confirm this for me? it is just not clear from how you typed it where the multiplication step occurs.



Thanks again!



Now that I am armed with all this new electrical power, I'll go LED-up my washer nozzles on the 968..... J/K!!! <img src="/forum/images/smilies/968/laugh.gif" class="smilie" alt="" />

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[biotechee]

Alright... another question....



Armed with the knowledge on sizing resistors and what the deal is with LED's, could one now switch the gauge panel to LED lighting and increase the brightness??? (in the 968)



I would imagine it would be fairly simple..., no?

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[Anchorman]

Bio, pardon me for an off-topic question, but - are you going to keep working on the 968, or will you be driving it someday? We have to have another brew meet in New Hope when the weather warms up.

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[biotechee]

Jim-



I so desparately want to get the car in tip top shape!!! But I am nervous about what will/can happen by taking it to someone. I know of two places that I feel somewhat comfortable about, but that is only via word of mouth, not first-hand experience. Since I do not have the tech ability, space, nor tools, I am at the mercy of others on this one. Sooner or later, the car will go in and will get fixed.



This Spring, I plan to continue with the small things (steering wheel, etc.). I may not drive it, but at least I will continue to straighten up the details, slowly but surely.



Perhaps next time we can try something a little closer to you? Or maybe in Lambertville?



How is your son doing? Well I hope....

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[Greimann]

[quote name='biotechee' date='Mar 1 2005, 12:48 PM']Dave,



I think the formula you wrote was grammatically incomplete....



I created an Excel calculation to do this for me and when I set up the equation as this:



R=(Vs-Vf)/(mA*1000)



[right][post="1303"]<{POST_SNAPBACK}>[/post][/right][/quote]



The second pair of quotes is where you went wrong. In my original equasion there is no second pair and if you do the calcualtion from left to right, it comes out OK.

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[biotechee]

Ahh- gotcha!



I still need to put the thingies (can't spell paranthesis!) around the section as shown in my second equation for it to work in Excel I believe.... Since Excel does orders of operation thusly:



* first

/ second

+ third

- fourth



I think. Either way, it all works out in the end and I really do appreciate the formula! I may end up getting a second interior switch just to try a different LED (now that I know how to size the resistor!!!).



It's really a good thing that I'm in the biology field!

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[Anchorman]

I'll suggest that, instead of being nervous about where you bring the car, get it to my OPC and you'll be worry-free. It may not be the least expensive route (in terms of first cost) but they know this series of cars and are very honest and capable. There aren't a lot of dealers that I'd say that about, but in this case I have no hesistancy - and, for anyone that wants to know, I'm talking about Princeton Porsche.

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