LED not lighting up with breadboard via 3.3V

[warry]

Hi all,

First-timer here. I recently set up a ribbon cable up to my Pi, going onto a breadboard. I've been following a beginner tutorial that came with this starter kit that involves running the 3.3V power (3V3) through a 220-ohm resistor, through the LED, finally ending with the ground (GND) pin.

My goal is to light up the LED, which to my understanding should be lighting up immediately upon plugging in the power to the Pi, if everything is set up correctly.

However, I cannot get the LED to light up.

I've triple-checked several connections, such as:

1. Making sure the ribbon cable is oriented correctly on the Pi (red line is positioned towards the SD card (outer side))
2. Making sure the ribbon cable is oriented correctly on the breadboard (only one orientation is physically possible, due to a notch in the cable)
3. Making sure the LED is oriented the right way (longer leg towards the resistor)

In addition, I have tried switching from the 3.3V pin to the #17 pin and using Python to switch on the output, but that did not work either. I have also tried another LED bulb just in case I shorted it by accident. The Pi powers on and off just fine the entire time.

Lastly, I believe that the breadboard does not have any interrupted power (the blue and red lines are continuous, with no break).

I think my next step is to buy a multimeter and confirm the electricity flow at each step.

What do you think? Is my wiring correct for now? Please see the following photo. Note that I added in purple/pink squares to more easily see where those jumpers are going.

http://i.imgur.com/EomKX4T.jpg

[KMyers]

Please check the wire on the LED. Looks to my old eyes to be in wrong row, Could be mistaken.

[drgeoff]

From #10 in the 'Common pitfalls ...' sticky:

"One of the pitfalls that beginners using a breadboard with the Pi often encounter is that there are generally two kinds of breadboards: those that have the horizontal "power strips" running through the complete length of the board, and those that have the power strips disconnected in the middle. The latter are often (but not always) recognizable by the fact that the blue and red lines on the breadboard are interrupted, as in this case:"

Perhaps this is an instance where the 3 words in brackets are relevant.

It is easy to check. Just move the plugs on the other ends of the jumper wires on the T to the other half of the breadboard.

[klricks]

What is the value of the resistor? I can't quite tell what the 3rd band color is from your photo. Looks like brown to me.
If Red Red Brown = 220 Ohm, then that is probably too much resistance. Try it with 5V and see if it lights at all.
Get the specs for the LED and Google and find an on-line LED resistor calculator.

[Burngate]

What do you think? Is my wiring correct for now?

It looks alright to me ... but something's wrong, and it's probably the wiring.

I would start by replacing the red and yellow leads with just one, going straight from the 3V3 line direct to the end of the resistor, and the same for the blue and black leads, from the GND line to the LED cathode.

You've checked the LED is the right way round, but I've known LEDs to have their legs swapped, so (just to keep me happy) turn it round to see what happens.

You've checked the ribbon's connected correctly at the Pi end - check again, and check it's not one pin displaced.

I think my next step is to buy a multimeter and confirm the electricity flow at each step.

Yes.
Yes yes yes.

[warry]

Thank you all for the very prompt replies so far. I will try all the suggestions soon, and also order a multimeter in the meantime I'm sure it will come in handy.

[rpdom]

Thank you all for the very prompt replies so far. I will try all the suggestions soon, and also order a multimeter in the meantime I'm sure it will come in handy.

Even one of the really cheap "DT-830B" digital meters are good enough for the basic stuff. I have one, as well as a decent meter for serious testing.

[warry]

Ok, I got around to checking a few things mentioned. The suggestions are italicized, with my response/test results below.

This breadboard is the type that's disconnected in the middle
> I don't think this is an issue, now that I moved all the jumpers up to the same, "close" half of the breadboard, towards the cobbler/breakout area.

3.3V and 220-ohm is too much resistance
> I don't think this is an issue, as I switched over to the 5V0 pin (with same 220-ohm resistor, LED rotated both sides) and nothing changed. An online calculator suggested a ~180-ohm resistor, which to my rudimentary understanding means that 220 would indeed be suitable.

Try to simplify the wiring
> I did this, and rotated the LED both ways, and the LED still would not turn on.

The ribbon is misplaced on the Pi side (perhaps one pin off)
> I've confirmed that it is not off by one pin, as it appears completely flush.

Now that all of the above is out of the way, I've gone and opened up a DT-830B multimeter! Thanks for the suggestion. However, I'm not fully sure I'm using it correctly. To my understanding, I should touch the black (COM) end to the ground end at all times, and touch the red end to whatever else. I'm using the 20V setting. The multimeter itself seemed to work fine when trying it on a standard AA battery.

I tried measuring the voltage by touching the black/red ends to the tops of the metal pins on the cobbler/breakout thing. Please see the photo attached. When doing this, I tried multiple combinations, each time with the black end on a GND pin, and the red end on the various 3V3 and 5V0 pins. Each time, I got just an empty 0 reading. I think that once, I saw it read approximately 5 at the 5V0 pin, but I wasn't able to reproduce this.

My amateur opinion is that something's going on with the cobbler/breakout, in that the electricity is fine on the Pi, but is getting lost on the way to the breadboard.

Am I using the multimeter incorrectly? How would you suggest measuring each step more effectively?

Link to full-size photo: http://i.imgur.com/BGl4Y9A.jpg

[rpdom]

You seem to be using the meter correctly.

The only other thing I can think of is that cable may be connected the wrong way round.

What does the meter show when you connect it to the first 3v3 pin and the one marked #21? If that shows 5v (or -5v) the cable is plugged in the wrong way on one end.

[bensimmo]

Turn the LED the other way around the 'flat' edge is negative and should face towards the 0V (aka negative), hang on just seen you've checked that anyway i's wrong in the picture. Resistor is fine by the way as 330ohms will even work.

The ribbon cable should have the red strip at the 0 and 1 Pin ends of the board, (the edge furthest away from the USB ports).
Either way as above, turn it around on the Pi (but disconnect you circuit power lines first)

You might find it easier if you actually use the other end of the cable in the T-Cobbler as normally they have that end with the cable going away from the board so it doesn't interfere. (looking at your cable it seems it does also http://viaboot.com/raspberry-pi-breakout-kit.html )

[Burngate]

I don't suppose you have access to another ribbon cable and another T-cobbler? It looks to me like one of those is faulty, and my money would be on the ribbon.

In my years of experience, I've managed quite successfully to attach the sockets to ribbon cables such that either everything is connected to everything else, or nothing's connected to anything.
Creating a T-cobbler with nothing connected would be more difficult, though not impossible.

[warry]

Okay, good news everyone! It's alive!

I took the suggestion of reversing the ribbon cable (I simply swapped the Pi end with the breadboard end). In addition, I successfully measured 5 volts from 5V0 to one of the GND pins. I set up the wiring and the red LED lit up!

I turned everything off, then tried with the 3V3, to no avail. However, my meter verifies that the 3V3 does indeed give power. Perhaps my red LED for some reason requires a lot of voltage? Either way, I'm very excited to see that I finally got this Hello World working with the 5V0.



That said, now I'm trying to continue in my starter kit manual's tutorials, using GPIO 17 to try turning the LED on and off.

Alas, this does not work for me right now. I am still using a 220-ohm resistor. gpio readall confirms that I am successfully having GPIO 17 set to OUT and a V (Value?) of 1. I've double-checked that I'm using BCM mode, plus the readall diagram checks out. In the end, my meter reads 0.0V coming from GPIO 17. And, of course, the LED itself doesn't light up.

Any ideas from here? If it's true that my red LED simply needs more voltage, what would you recommend? Do I just need to get a resistor that's < 220?

[Ferdinand]

Hi warry,

The resistor is ok. The value of the resistor may vary between 220 - 1000 ohm.

Try another ribbon cable or test each wire with your ohm multimeter.

[bensimmo]

A quick simple test to rule out the ribbon/T is to just bypass it and touch the gpio pins directly.

No harm in trying a lower resistor but it shouldn't matter.
It could be contact in the breadboard, (give them a wiggle etc or test continuity of the whole circuit).
There should be an option on the multimeter, remove the leads from the 3.3V and ground and check all the circuit. Try it both ways as the LED will only be ''continuous' in one direction (+ to -).


I was running through the gpiozero article doing a similar thing yesterday for our kids.
( https://www.raspberrypi.org/learning/ph ... th-python/ )

[tpylkko]

I have a package of red leds that on the package says 5v. It does not specify what this 5V means, that is, is this supposed to be the voltage at which to use the led, or a forward voltage or what? Nevetherless, those leds do not consistently light up with anything lower than 5V. If I plug one to 5V from Arduino without resistor it lights up normally. However, when I connect it to 3.3V it does not, it just blinks randomly. Prior to acquiring these I though that leds always operate at voltage ranges of 1.5-2.3 or something like that.

[bensimmo]

I have a package of red leds that on the package says 5v. It does not specify what this 5V means, that is, is this supposed to be the voltage at which to use the led, or a forward voltage or what? Nevetherless, those leds do not consistently light up with anything lower than 5V. If I plug one to 5V from Arduino without resistor it lights up normally. However, when I connect it to 3.3V it does not, it just blinks randomly. Prior to acquiring these I though that leds always operate at voltage ranges of 1.5-2.3 or something like that.

The only way you would have a 'real' 5V LED would be to have a built in 'resistor'.
The forward voltage of an LED is designated by its material construction. I could write more but look it up if interested.

EDIT to add e.g. http://uk.farnell.com/broadcom-limited/ ... dp/1003222? Notice the wording Integral current limiting resistor

Though I wouldn't expect to see them in a kit with instruction and then telling you to fit a resistor in front of it.

[tpylkko]

Yes. I didn't take that into account, that is, since the OP is using a starter kit, then what I suggest is unlikely to be the reason.

[Burngate]

I took the suggestion of reversing the ribbon cable (I simply swapped the Pi end with the breadboard end). In addition, I successfully measured 5 volts from 5V0 to one of the GND pins. I set up the wiring and the red LED lit up!
...
gpio readall confirms that I am successfully having GPIO 17 set to OUT and a V (Value?) of 1. I've double-checked that I'm using BCM mode, plus the readall diagram checks out. In the end, my meter reads 0.0V coming from GPIO 17. And, of course, the LED itself doesn't light up.

So swapping the ribbon end to end made a difference? That looks to me to be evidence of a faulty ribbon - or rather the connectors haven't been properly attached.

They're called insulation-displacement; each of the 40 little sockets has a pair of knife-edges that cut through the insulation to grip the copper wire.

Most times they work well, but you seem to have at least some that either missed the wire altogether or are only making intermittent contact.

Try another ribbon, or make your own (a vice is useful)

[warry]

Hello everyone,

I'd like to thank you all for being so prompt and helpful in my beginner problems. It seems that everything has been fixed (and all my scripting, wiring, etc. is working 100% as expected) after replacing the cobbler!

The new cobbler that I'm using now is from Adafruit. The old one was from Viaboot. I can't speak on either brand/company beyond this, but I will say that Adafruit has made a great first impression on me.

Enjoy the rest of your weekends!