[Citizen86]

Hi guys and gals,

I tried a super simple DIY project, basically 3 LED's powered by a 9V battery... I followed this diagram:

So I connected the battery positive to the resistor, resistor to + LED 1, - LED 1 to + LED2, - LED2 to + LED3, - LED3 to - battery. Simple, right?

Lights are not turning on. I used my multimeter to try and see what's up, and the strange thing is that using the DCV 20 setting on my MM, it shows ~9 volts in the 1st LED, but negative 9 volts through the 2nd, and positive 9 volts through the 3rd. Does this mean that the 2nd LED is reversed? The spec sheet and even looking at the LED, the small part of the metal inside the LED is positive, that's how it is connected...

Does anybody have any idea why my LED's refuse to turn on? Thanks.

 

[TzeYang]

what is the value of the resistor?

 

[Citizen86]

270 ohm, 1/4 watt. I am using infrared LED's with a rating of 1.3V and 20ma. If my calculations are correct....

(9v - 3.9v) = 5.1, 5.1 / 0.020 = 255 ohm resistor.

 

[Emooze]

Never had any experience with diodes in series. And the LED's won't be the same brightness if you get it working dude to voltage drops across each one.

Try putting the LED's in parallel, might need 2 more resistors for that though.

Edit: What are you using to tell if the LED's are on?

 

[Citizen86]

I'm trying to set up this program called Freetrack. It's a head tracking program.

Basically I'm using a modified webcam that picks up light, but easily picks up IR light. I know it works because I am using my Xbox remote which it picks up VERY easily.

 

[obobskivich]

tried reversing the LED just for grins? (would take like ~3 seconds)

will a single LED drive in the circut? (remove the other two)
if yes, test each one individually (given that these probably cost you what? like pennies, odds of failure are there (these aren't exactly "precision machined" (no LED really is afaik))

 

[Logistic]

These LEDs are rated at 1.3V, you shouldn't have 9V across them. Are you sure you connected them in series like that?

 

[obobskivich]

Quote:

Originally Posted by Logistic /img/forum/go_quote.gif These LEDs are rated at 1.3V, you shouldn't have 9V across them. Are you sure you connected them in series like that?

from what I read, he's getting 9V across the first one, and -9V across the second one

which would be 18V across them, somehow (I have no idea how you'd actually get 18V from a 9V battery with a resistor and LEDs in series, unless you measured wrong, or I'm missing something (either misread or misunderstood what I read))

 

[Citizen86]

Quote:

Originally Posted by Logistic /img/forum/go_quote.gif These LEDs are rated at 1.3V, you shouldn't have 9V across them. Are you sure you connected them in series like that?

Battery positive to resistor to + LED1, right?

@obobskivich, I might try switching the middle LED. I'm a noob with all this, but I have checked the connections over 10 times since last night and everything appears correct, unless I somehow got an LED with a switched positive negative.

The measuring I'm not completely sure if I'm doing it correctly. I own a multimeter very similar to this one, it's a rather economic version:
Steren | Catálogo
To test the volts, I set it to DCV 20, touch the red to positive, and black to negative on an LED, right?

 

[obobskivich]

positive on what?

I'm just not sure how you're getting like 18V between the resistor and LED 2, unless you've got things flipped around or wonky somewhere

honestly I'd just pull the wonky one out, and try again, you did buy more than 3 total LEDs right?

 

[Citizen86]

Yeah, I'll give a different LED a try.

 

[8xOverMsOctober]

Quote:

Originally Posted by Citizen86 /img/forum/go_quote.gif Does anybody have any idea why my LED's refuse to turn on? Thanks.

Quote:

Originally Posted by Citizen86 /img/forum/go_quote.gif 270 ohm, 1/4 watt. I am using infrared LED's with a rating of 1.3V and 20ma. If my calculations are correct.... (9v - 3.9v) = 5.1, 5.1 / 0.020 = 255 ohm resistor.

???

If you're using infared LED's then you won't actually see them "on". The only way that you'll be able to "see" them on is by measuring voltage drops across them. You do know that infrared is a spectrum of light that humans can't see, right?

Just make sure that you're dropping 9V across the whole circuit, and 1.3V across each LED, and ~5.1V across the resistor and you'll be fine...

Don't know whey you would be dropping 9V across each one unless you had them hooked up in parallel w/o the resistor...
Edit: and use the diode setting on your MM to check if the LED's are hooked up the proper way. If you hooked them up backwards then you probably fried at least one.

If you actually want to see your LED's on then you'll need to use regular LED's.

 

[DKJones96]

Since it is IR I doubt the lens is frosted so look inside. The negative side of the LED is the small 'bowl' shaped part of the emitter.

Oh, and ANY digital camera will be able to see the infrared light from those. I use my camera phone all the time.

 

[mriguy2]

Hi... The circuit is correct and should work. The polarity of the battery is critical. In your drawing, the battery must be connected with the negative terminal at the bottom (to the cathode of the bottom LED). Your calculation of the resistor's value is correct. As sown correctly in your drawing, all of the LEDs must be connected with the polarity shown. If one or more is backwards, then none of them will light. In fact, if one of them is backwards, it is at risk of failing to to inverse voltage,which most LEDs don't like. To correlate which physical leg of the diode is the cathode (the bar of the triangle on you drawing), look on the package if purchased at radio shack or equiv. It is usually shown as a function of lead length and/or a visible internal structure.

No offense, but your voltage measurements are goofy. Any Red or Green LED that has a voltage over 2 volts across it, is either installed backwards or it has failed, by definition. As you suggest, a normally lit LED has about 1.3 volts across it, which is for the most part independent of how much current it is handling

 

[mriguy2]

Good point about the possibility that it is an IR diode.

By the way, did you guys know that most cell phone cameras do not have an IR filter in front of the sensor. As such, they are very good at "seeing" IR diode emissions. Try it on your remote controls, while pretending to take a picture of them!