Cannot get simple LED to work on Raspberry Pi

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jcarlson
 
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Cannot get simple LED to work on Raspberry Pi

Post by jcarlson »

Hello - electronics newbie here! I recently purchased the following products for my Pi (Model B)

Diffused Green 5mm LED - https://www.adafruit.com/products/298
Half-Size Breadboard - https://www.adafruit.com/products/64
Adafruit Assembled Pi Cobbler Breakout + Cable for Raspberry Pi - https://www.adafruit.com/products/914

Since I'm new to this - I'm trying to start simple - hooking up an LED to the 3.3v power so it should just turn on when the Pi has power.

I have the cobbler hooked up to the Pi and the breadboard (see picture).
Rpi LED wiring
Rpi LED wiring
IMG_0188.JPG (544.3 KiB) Viewed 671 times


The orange wire is hooked to row 1 (3v3), and the other end to row 20
The white wire is hooked to row 13 (GND), and the other end to row 25
A 370-ohm resistor is hooked up to row 25 (matching GND), and row 22
The LED has the short end (cathode?) hooked to row 22 (connected to resistor), and the long end (anode?) connected to row 20 (which is the other end of the 3v3 wire)

I would expect with this configuration, as soon as I turned on my Pi, the power should be coming from the 3v3 pin and turning on my LED. The LED never turns on.

So, I have two questions:

1) Does my wiring/configuration look correct? Would you expect the LED to turn on when the Pi turns on?
2) If my wiring is correct, how can I troubleshoot this to find what component might be faulty? I have a multimeter, but not sure what/where to check?

Thanks for any help you can provide!!!

-Justin

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zener
 
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Re: Cannot get simple LED to work on Raspberry Pi

Post by zener »

The cathode is the short pin and sometimes also the body is flat on that side. Does that LED have a flat side? Measure the voltage across the LED with a meter.

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jcarlson
 
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Re: Cannot get simple LED to work on Raspberry Pi

Post by jcarlson »

Zener wrote:The cathode is the short pin and sometimes also the body is flat on that side. Does that LED have a flat side? Measure the voltage across the LED with a meter.
The LED does not have a flat side, but I'm positive the longer end (anode) is connected to the power wire. Voltage across the pins of the LED reads between 2.5-2.6 V....so it is getting power, but just not turning on???

Anything else I should check with the multimeter?

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zener
 
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Re: Cannot get simple LED to work on Raspberry Pi

Post by zener »

It should be on. Having it in backwards won't hurt it, so just for experimentation sake turn it around and see what happens. While you have it out you can test it with the diode test function of your meter if it has one.

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jcarlson
 
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Re: Cannot get simple LED to work on Raspberry Pi

Post by jcarlson »

Zener - thanks for all the help. I feel like I must be missing something stupid here. I tried switching it around - no difference. I tried a brand new LED from the set I had, both ways with the connection, and that didn't work either.

I then figured I'd remove everything besides the breadboard/cobbler/LED from the equation. I took another brand new LED from the set, and put the anode pin of the LED directly on the breadboard corresponding to the 3v3 pin on the cobbler, and the cathode directly on the GND pin (similar to what's shown in this tutorial - the video near the end where it burns out the LED - https://learn.adafruit.com/all-about-le ... s-used-for). I got that hooked up, then turned on the Pi. I expected it to at least blink for a second before it burned out due to lack of resistor. But it did...NOTHING. I measured voltage across the pins of the LED, and got 3.2 V, which makes sense since it was being powered by 3.3 and had no resistor.

Is it possible I just have really bad luck and got a bad set of LED's? I have a pretty basic multimeter that doesn't have a diode test function (newer one on the way from Amazon that does have it), so unless anyone has any other ideas, I guess I'll wait until I get that and try out the diode test to make sure my LED's are actually working.

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zener
 
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Re: Cannot get simple LED to work on Raspberry Pi

Post by zener »

Use the ohm function and measure A to K and then K to A. Hopefully that gives us an idea.

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jcarlson
 
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Re: Cannot get simple LED to work on Raspberry Pi

Post by jcarlson »

Crummy multimeter just died on me. Will have to wait until Monday when the new one is coming from Amazon.

When you say A to K and then K to A, are you just saying to reverse where I put the leads from the multimeter on the LED? So measuring A to K means put the positive lead on the Anode side of LED, and the negative lead on the Cathode side of LED, and then vice versa for the other measurement?

The new multimeter that I ordered isn't auto-ranging. What should I "expect" resistance to be on the LED - would it be around 370 ohm, which is the same as the resistor it's connected to when everything is powered on?

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zener
 
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Re: Cannot get simple LED to work on Raspberry Pi

Post by zener »

A means Anode and K means Cathode (I don't know why but Germans are probably involved somehow...) So yes, measure it one way then the other way. What you get will depend on the meter. If the LED is good then it will measure infinite resistance K to A and "some" resistance A to K. Also when measuring A to K the LED might turn on slightly. Depends on the meter. The diode test is more reliable. With that test you would measure around 2V.

It is hard to say what is going on. Your tests seem to indicate the LED's are bad but that is fairly uncommon, unless they got blown somehow.

Another test is get a 3V battery or a couple of AA's etc, and a resistor between about 100 and 500 ohms and try to run the LED from that. If it still doesn't work then try a different LED.

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jcarlson
 
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Re: Cannot get simple LED to work on Raspberry Pi

Post by jcarlson »

Got the new multimeter today. I also picked up some new LED's from Radio Shack. The LED's still won't light up, both the ones I got from Adafruit, and the ones I bought from Radio Shack. I did the diode test on several LED's, and all lit up just fine.

I did some more playing around with the multimeter, and might have found an issue.

If I take the cobbler off, and use the multimeter directly on the 3v3 and GND pin on the Pi itself, it registers as around 3.28V. HOWEVER - If I then shut off the Pi, reconnect the cobbler to the Pi but NOT the breadboard, and then measure voltage from the 3v3 pin to GND pin on the cobbler, it only reads as 1.4V.

Is it possible I have a bad Cobbler GPIO cable that's causing my issues? Can anyone explain why I see such a drop in voltage with nothing even connected??


When I test voltage using the multimeter directly from the 3v3 pin and the GND pin on the cobbler, it only reads as 1.4V...shouldn't this be 3.3, or close to it?

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zener
 
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Re: Cannot get simple LED to work on Raspberry Pi

Post by zener »

Sounds like a bad connection. Often times a bad connection will not show up until it is under some load. In this case if it appears with no load then that is a pretty bad connection, at least it appears. You might try ohming out the cable to see if you can find some bad lines. You should get about 0 ohms from one end to the other. It is possible for these connectors to get crimped wrong. I have done it a few times.

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jcarlson
 
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Re: Cannot get simple LED to work on Raspberry Pi

Post by jcarlson »

I didn't assemble this - it was the assembled version bought from Adafruit. I just ordered some female-to-male jumper cables so I can try hooking up the Pi directly to the breadboard - taking the cobbler GPIO cable out of the picture.

Could you please explain "ohming out the cable" in a little more detail so I know how to check it?

Thanks again
-Justin

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zener
 
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Re: Cannot get simple LED to work on Raspberry Pi

Post by zener »

Pick a position on the connector (like one of the ones that you suspect is bad). Measure the resistance (ohms) from one end to the other. I am not suggesting you made the cable, but someone did. They don't all turn out perfect. If the cable tests OK then we have to keep looking. for some other cause.

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jcarlson
 
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Re: Cannot get simple LED to work on Raspberry Pi

Post by jcarlson »

Ok, that makes sense. I'll give that a try tomorrow and post back with the results. Thanks!

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jcarlson
 
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Re: Cannot get simple LED to work on Raspberry Pi

Post by jcarlson »

Yep, something is definitely wrong with the Cobbler GPIO cable. I didn't test it out yet, but I got my Female-Male jumper leads in the mail today. I hooked the 3v3 and GND pins on the Pi right up to the breadboard, wired in the LED and resistor and turned on the Pi - SUCCESS!

I'll play around some more with trying to find out what's wrong with the GPIO cable (or just see about returning it to Adafruit for an exchange), but I couldn't be happier that I finally got the stupid LED to light up!!!

Zener - Thanks for all your help on this, I definitely learned more about troubleshooting circuits!

-Justin

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