Newbie question: Why isn't this working?

Hi there,

I'm a newbie trying to help my daughter with a science fair project involving wearable electronics. I'm barely out of the gate and I'm STUCK!!!

I've included three pictures that show an Arduino and a series circuit that (at the moment) is comprised to two LEDs sewn with conductive thread.

This first picture shows me connecting the Arduino's negative to the first LED's negative terminal, and the last stitch before the second LED to the 5-pin. As you can see, the first LED lights up as expected.

Then I touch the Arduino's negative to the first stitch below the first LED, and the positive terminal on the second LED to the 5-pin. That makes the second LED light up as expected.

So how come nothing works when I connect the Arduino's negative to the negative of the first LED, and the positive terminal on the second LED to the 5-pin?

Seems to me that should light up BOTH lights, but I'm getting nothing.

One more note ... I have the conductive thread running from the + of the first LED to the - of the second LED. That's correct, isn't it?

I know I'm missing something incredibly obvious, and you can tease me about it ... but at the moment I'm just looking for someone that can help me help my daughter! :slight_smile:

Not enough voltage to light both in SERIES?

Do these LED's have a built-in current limiting resistor?

Do you have a Part Number for these LED's?

The LED need 3.2V to work. Lilypad can only provide 5V


http://lilypadarduino.org/?p=465

Those look like the sewable ones that come with the lilypad kits.

They have a resistor on board.

The problem is that you're connecting them in series - but the voltage across them isn't high enough to light two LEDs. You must connect them in parallel.

Unlike lightbulbs, which would glow dimly in such a situation, LEDs have a threshold voltage - if you apply less than that, they don't do anything. With two in series, the applied voltage was probably less than the sum of the two threshold voltages.

mrsummitville:
Do you have a Part Number for these LED's?

These are the lights I'm using: LilyPad LED Pink (5pcs) - DEV-14010 - SparkFun Electronics

DrAzzy:
Those look like the sewable ones that come with the lilypad kits.

They have a resistor on board.

The problem is that you're connecting them in series - but the voltage across them isn't high enough to light two LEDs. You must connect them in parallel.

Unlike lightbulbs, which would glow dimly in such a situation, LEDs have a threshold voltage - if you apply less than that, they don't do anything. With two in series, the applied voltage was probably less than the sum of the two threshold voltages.

Thanks ... that sounds like the answer I need! Time to redraw our sewing diagram, I guess.

OK, newbie question part 2, please...

We're wiring/sewing this project in parallel now. We'll be using 5 pins on the Arduino, each running 4 LEDs, for a total of 20 LEDs. Can all 20 lights (4 different parallel circuits) share the same negative wire, or does each group of 4 LEDs need its own unique negative wire?

Thanks again!

Yes, it can share the same negative wire.

I just got to the part where I realized I asked a silly question. :slight_smile:

In any case, thanks for the answer ... my silliness has been confirmed!

We'll be using 5 pins on the Arduino, each running 4 LED

No! Too much current for a pin. Not sure what the value of that resistor for the LED is but the data sheet shows the 'normalized' luminous intensity is at 20ma. So, having 4 LEDs connected to one pin will draw ~80ma. Way too much for one pin. - Scotty

davehultin:
OK, newbie question part 2, please...

We're wiring/sewing this project in parallel now. We'll be using 5 pins on the Arduino, each running 4 LEDs, for a total of 20 LEDs. Can all 20 lights (4 different parallel circuits) share the same negative wire, or does each group of 4 LEDs need its own unique negative wire?

Thanks again!

Typically, each Digital I/O port is rated at 40ma Obsolute Maximum.
Each LillyPad LED uses about 18 ma.
So, good idea to connect only one LED per Digital I/O port.

Typically, each Digital I/O port is rated at 40ma Obsolute Maximum.

Sometimes, there is even an absolute maximum quoted that is current.