simple LED question (start-stop switch sidetrack)

[lunarhighway]

i've got a pretty basic question but i'd rather look stupid asking something than frying something because i didn't ask.

i'm still planning my start/stop switch, work has prevented me from getting all the parts needed but i should start building it any day now. meanwhile i've worked on the theory a bit more.i'll probabky go for two push buttons with a different colored LED build into each, so that i'd can spot the buttons from the corner of my eye while watching traffic even at night and don't accidently use the wrong one. i plan to have the leds on at all times when the engines on, red and green would look cool especially since id be using it at stoplights

here's the schematic.

basically what i want to do is place the two leds (wich are part of the switches but have separate connections) in series with a resistor to keep them from burning up.

the only info i have on the leds is that they need 1.7V

here's how i calculated the resistor:



i'm assuming 0.02A is ok, but this is my first question, is it?

secondly since most resistors have a 10% tolerance i'm likely to get e slightly different resistor especially if i get one from the E12 range. also battery voltage may varry so i'm wondering how much deviation from my 0.02A is acceptable?

finally does the scematic i drew up look acceptable, or is it plain wrong?

i used to studdy electronics a long time ago in school but haven't really used it since... i sort of got lost when things got to theoretic and complex and moved onto totally different studdies afterwards, but now i've decided to give these simple practical applications a try i can pick it up again.... it's a shame how much one can forget when you don't use it every day. i find myself having to look up even the most basic things again....

anyway i'd appreciate any help i can get in making sure this works in a safe way, i might always expand on it later to improve it.

[Rick Rae]

First: Kudos for diving in and going for it.

basically what i want to do is place the two leds (wich are part of the switches but have separate connections) in series with a resistor to keep them from burning up.

Unless you drive them with a current-limited or constant current source, there has to be a resistor somewhere. You're on the right track.

the only info i have on the leds is that they need 1.7V

Well, technically that's the voltage drop across the LED; you also need to know at what current they should run. Otherwise, you can take a safe guess and go from there.

i'm assuming 0.02A is ok, but this is my first question, is it?

20mA should be safe, but the result may not be bright enough. I wouldn't count on being able to see them light up during the day, for example. It might work, but you're fighting a very high ambient light level. Ideally you want to look at the LED curves or at least have the typical and absolute maximum ratings for current to work from.

Also, be aware that the forward voltage drop as well as efficiency varies from LED to LED, especially between different colors. You may have to give each LED its own resistor to get them balanced the way you want. But it won't hurt to breadboard it the way you have it drawn and see how it works -- could be perfectly fine as is.

secondly since most resistors have a 10% tolerance i'm likely to get e slightly different resistor especially if i get one from the E12 range. also battery voltage may varry so i'm wondering how much deviation from my 0.02A is acceptable?

The closest 10% values will be 470 on the high side and 390 on the low. You can stab in the middle of those two values if you get a 5% resistor (which I think are more readily available than 10% these days).

Battery voltage will vary; count on it.

As long as you're aware that less current equals less brightness and you don't exceed the absolute maximum rating, varying the current some isn't going to hurt anything. I'd suggest derating the max a bit given how hot it can get in a car. There may be derating info on the spec sheet; if not just stay reasonably far away from the max and you'll probably be okay.

finally does the scematic i drew up look acceptable, or is it plain wrong?

I'm not quite clear on what you're doing on the shutoff (is that the fuel injector you're killing?) but in general it looks okay. The one thing I'm not sure about is, you appear to be using switched power ("after contact") to energize a relay to turn off the switched power ("contact")... that will likely make a better buzzer than it will an engine shutoff. But as I said, I'm not quite clear on what you're doing there, so it may be golden.

Do you have part numbers on the LEDed switches? Maybe we can dig up the spec sheets.

Rick

[lunarhighway]

thanks for the quick and thorough reply

i'm planning on useing this switch, probably in red,green
http://www.velleman.be/product/view/?lang=en&id=339472

as far as i can see the led has got two separate pins so i assume it's not directly linked to switch. but unless i miss something the voltage drop is the only thing given about the leds.

it's going to be fitted very low on a black dashboard inside a recess so ambient light shouldn't be a terrible problem. given my knowlege of the subject i'd be more concearned about blowing the leds or worse than that they wouldn't be bright enough.

the leds are actually the least important part of the setup so i could always fit a normal switch but than i wouldn't have learned anything

basically i'm tapping into the electric wiring after the original key contact switch. (so that the system wont do anything without the key being in the ignition and turned to "on")

the first part simply places a puch button parallel to the starter key so that ill be able to start the car at a stoplight without having to reach to the key all the time, wich is a bit uncomfortable. the new position would be direcly in front of the gear shifter.

second part is essentially the same only the other way around. my car's carburated so in order to stop it i'll just have to cut power to the ignition (wrongly marked contact on the scematic) i could do this with a direct switch but because i couldn't find a maching one that breaks a contact instead of makes on and i was/am considdering substituting the buttons for a single 3 way paddle switch (on)-off-(on) i decited to go for a relay wich i have laying around anyway.

i've been wanting to install this for a long time and i hope to finally get all the bits together tomorrow, yet i wouldn't want to blow out a fuse or worse because of a few leds

[Rick Rae]

Thanks for the clarification! Makes sense to me; maybe someone who's done a cutout switch already will chime in.

Do you have another way to direct me to the switches you're using? The link dumps me to the language specification page. I tried bringing up another product and then substituting that ID number in the URL, but that gives me a page with no product data. If I try to search on 339472 as if it were a stock number, it says "No Results."

And I leafed through the switches to see if I could spot one with LEDs -- no such luck.

Is it possible I'm getting different, only-for-export products because I'm in the U.S.? There doesn't seem to be much selection from here (only four pushbutton switches, for example).

EDIT: Got it... I had to choose Belgium rather than USA on the front page. Still came up in English, though. Stand by...

Rick

[lunarhighway]

maybe this one will work...
http://www.velleman.be/be/en/product/view/?id=339472

this is all the data they're giving on the page
Order Code: R1394B

* type: off-(on)
* cap colour: red
* illumination: LED
* ratings: 1A - 250V / 3A - 125V
* LED illumination: 1.7V
* drill Ø: 16mm
* dielectrical strength: 1500Vac
* contact resistance: 30mohm max.
* insulation resistance: 100Mohm

i haven't ordered the switches yet so i could ask about the led that's inside.
i hope that i can get all the stuff tomorrow so that i can actually start building... if i can piece this together i might be on track to put some more (maybe gas saveing) electronics in my car. (i'm contemplating a variable grill block...piece of cake in theory, just have a servo follow the needle of the temperature gauge when the temp goes too high) but that's for later, first see if i can get a couple of led's to work

[Rick Rae]

Apparently Velleman doesn't want us folks in America to have the good stuff. That switch is available for sale from Web sites scattered all through Europe and the Netherlands, but it's completely absent from Velleman USA's pages.

Anyway, every place I looked it up had exactly the same information as what was on Velleman's site. And I didn't see those switches on the drop-down list for info sheets.

So short of asking them, you may just have to experiment. As I mentioned, 20mA should be safe -- in fact it's a pretty common current for spec'ing LEDs -- but it gives you no clue how high you could safely take the current. So if the light isn't bright enough at 20mA you'll just have to decide how daring you feel.

Here is a good overview of designing for and using LEDs that gives you the typical forward voltage drops for different colors -- note that green is listed as higher than red. They comment that putting them in series with one resistor is okay, and it is -- as I mentioned, there's just a potential brightness mismatch. If the luminous efficiency of the red and green LEDs are similar, you'll be okay. If not and it bugs you, just give each LED its own resistor.

Keep us posted on how it works out?

Rick

P.S. Making the LEDs bright enough to see in the daylight might make them blinding after dark. You may need to work out a way to tie them to the dash dimmer, or at least reduce the voltage (and thus current) when your lights are on.

[lunarhighway]

thanx for the info, that's been most helpfull, some things here i wouldn't have thought of....
i'm planning to get all the stuf tomorrow, but i hope they'll have it all in stock at the shop i know they carry the entire velleman range... if they need to order it it'll be another week before i can try this. but we'll get there eventually...

i need to think about what would work best, but i suppose it shouldn't be to hard to create a setup that dims the leds slightly when the headlights are switched on. but lets try to do the basics first, the leds are just an added bonus and they'll mostly help me find the buttons at night to their daylight brightness might not be to much on an issue. they should be able to give me more data at the shop and probably there's a standard led in there anyway.
but once i get this working i'll probably want to improve it.

i'll keep you posted on the evolutions!

thanx again for the helpfull comments!

[euhero987]

Nice info you got here. And I also agree with the o.p, A really nice and wise move. One must first ask if it is unsure... Gives me alot of time not asking.. lol.. thank you o.p