4 connector LED glows with skin resistance but not with current from ARDUINO UNO.

I searched a lot here and in Google but i don’t know what to really look for in search terms.
I have some basic knowledge about electronics but i am pretty new to LEDs.

i got a batch of white 4 legs LEDs i tried to get them to work with Arduino but i don’t know the required forward current.
I tried to use the standard values 3.3V with 220 Ohm. But the only way i can get it to light up is with connection to Arduino GND on the one site and my finger on the other side.

This is the image of the LEDs:

the interesting thing is that it does not matter what direction i wire, it lights up when i touch it.

My Question:

  1. what is the voltage/current from skin → GND (0V) ?
  2. how can i find out the proper driving current ?
  3. does anyone know the specs of these type of LEDs ?

(deleted)

the interesting thing is that it does not matter what direction i wire, it lights up when i touch it.

That is understandable because you pick up AC interference. You obviously have never touched an oscilloscope probe. You can pick up to 50 volts from your body but at a very high impedance.

The interesting thing is why you can not get them to work when you connect them to ground, through a resistor and then to an arduino pin.
They do have to be the right way round then and you have to set the arduino pin to be an output in your sketch.

Are these tricolour LEDs?
Are these opto-isolators?
Why have they got 4 pins when normal monochrome LEDs need only two?

@spycatcher: i got them together with other samples, so i don’t have a link or specs

@Grumpy_Mike:
ok so the voltage could be up to 50V but the current must be tiny < 5mA
i read somewhere that the skin has a resistance of 100k Ohm ?
That said it doesn’t matter what resistor i put in-between it always lights up the same.

from the light level it looks very dim though, maybe 10% (guess) from the full brightness.
Its dim but clearly visible.

the Leds are pure white. And i believe that 2 legs on each side are the same and just
for stability or easier mounting. so you got 4 legs but 2 connections basically.

here two photos to show the effect.

Henry_Best:
Are these tricolour LEDs?
Are these opto-isolators?
Why have they got 4 pins when normal monochrome LEDs need only two?

They are called "Pirahna"-LED's

http://imall.iteadstudio.com/media/wysiwyg/Products/Discrete_LED/LedPiranhaScheme.jpg

The 4 pins are for heatsinking.

// Per.

To take pictures of your Arduino assemblies, do so in full daylight (in order to permit the auto-focus on the camera to operate as well as to provide adequate illumination to reduce "grain" and prevent the LEDs from flaring out the whole photograph) and most importantly, use the camera at full (real) zoom and at least a metre away. A proper camera with 7 times zoom or better is important, a phone camera generally very poor. Crop the picture and size it to no more than 1024 pixels in any dimension.

The link Zapro provides is very useful. The cathode is (and usually is) the terminal corresponding to the large chunk of metal; this goes to Arduino ground and you connect the anode to 5V through your resistor.

If that does not appear to work, get out your multimeter (or another LED) and test your connecting wires - they have not infrequently been found to be faulty. The LED is clearly extremely efficient, able to light from a very tiny current.

here two photos to show the effect.

No they don’t because you can’t see where the wires go.

ok so the voltage could be up to 50V but the current must be tiny < 5mA

Yes 1mA is a massive amount of current for this effect.

i read somewhere that the skin has a resistance of 100k Ohm ?

It is not really a factor at all in this case. The skin resistance is between two points, you have not got two points here.

That said it doesn’t matter what resistor i put in-between it always lights up the same.

No it won’t matter. Maybe if you put a 10M resistor in line it won’t glow with your fingers but it is irrelevant for the effect you are seeing.
Have you ever come across a neon screwdriver? It is the same sort of effect.

They are called “Pirahna”-LED’s

No they are called “Pirahna”-LEDs.

LED is plural, it does not own a Pirahna.

@Paul: thanks for heads up on the photography, i will do better photos next time, i was in a hurry and the Arduino setup is not really relevant here, as i only use 2 cables and connect to GND on arduino ... i think the photos where sufficient to show the effect of the skin current lighting the LEDs. If there is more complicated things to show i will use better quality images. Though i did resize to 1200x which doesn't hurt i am sure.

@Zapro: ah thakns a lot that brought me on the right track :slight_smile:

i believe it are those or very similar:

http://www.pcboard.ca/kits/leds/piranha.html
LED Color - White, Voltage: 3.0v-3.4v, Current: 20mA

http://lighthouseleds.com/5mm-piranha-led-white-ultra-bright-12-000-mcd.html

There is one with added components:

SPECIFICATION
Voltage: +3.3-5V
Pin Definition :1 - control 2 - Power 3 --- GND
Weight: 5g
Color: White

Skin resistivity varies over many orders of magnitude depending on the
moisture level and voltage - ohms law doesn't apply to living tissue once
there's enough voltage to cause damage.

Modern high-brightness LEDs are 10000+ times brighter than the original
devices, a few uA can cause visible glow in subdued ambient lighting.

White LEDs typically require 3.1 to 4.0V forward voltage - trying to power
them from 3.3V is a gamble.

ok, i got them to glow with Arduino connection too, but same dim as with skin contact.

i found a interesting article here, where they talk about 100mA current per LED:

So i guess they are high-power LEDs that need a higher current.

@Grumpy_Mike:
haha, ok i am willing to learn. My purpose was to show the strength of LED glow compared to the Arduino LEDs,.
and if you guys want to be super correct its called: Piranha ( not Pirahna) LEDs. :smiley:

Interesting is that with skin contact it does not matter in what direction i connect the LED.
Does that mean the skin has a AC power while the Arduino has DC ?
Or where does the effect come from ?

i made some better schemes of what i tried so far:

scheme1.jpg

scheme2.jpg

Does that mean the skin has a AC power

No the skin has no power at all.

You are basically a bag of salt water and are picking up electromagnetic signals AC from the mains, other equipment and radio transmitters. You are acting as an antenna and channeling this signal into your LED. Normally this can be used as a voltage input to an amplifier for making a receiver but here you are using it to light an LED because only a tiny current is apparently needed to light up these LEDs.

You will get the same effect if you replace "you" with a long wire.

Grumpy_Mike:

They are called "Pirahna"-LED's

No they are called "Pirahna"-LEDs.

LED is plural, it does not own a Pirahna.

Sorry, there was no real english education at my special class-school. All written english i know, is self-taught.

// Per.

I want to hear what happens when you use the same connections - same wires and all - to light up another LED.

Something is clearly missing here.

All written english i know, is self-taught.

It is a lot better than my Danish. :wink:

@Paul: i tried a couple of different cone LEDs: RGB, Blue, Yellow, Green, Red ... they light up just fine.

So my conclusion is that these Piranha LEDs need a much bigger current than Arduino provides, or ?

I just find it strange that 1mA (skin) and 20mA from Arduino seem to make no difference at all.

It should be no problem driving the Pirahna's from your Arduino.

I have driven 3 of them from an Attiny85 with 220 ohm series resistors (Color-wash-thingy) without any problem.

// Per.

So my conclusion is that these Piranha LEDs need a much bigger current than Arduino provides,

No that is totally wrong.

You have become distracted for your real problem.

you said:-

i got them to glow with Arduino connection too, but same dim as with skin contact.

You need to do some voltage measurements.
What value are you feeding the resistor and LED?
What value do you get across the LED?
How have you wired them up?

Grumpy_Mike:
How have you wired them up?

I can't help thinking that he must have simply wired them backwards every time!