GPIO/Electronics question

[tigermisu]

Hello! I am a total begginer to both eletronics and raspberry pi, and I have 2 questions.

First.

Is there any difference between pluggin the LED circuit into the -3v pin and the grd pin? arent both of them the same? If not, what should I use the grd one for?

Second, I built this basic circuit:



I coded this in python, it counts in binary to 7.

It works great, there is just some odd thing that makes me curious:

The red LED is much more brighter than the other 2 when counting, and if I happen to touch the red led when the circuit is off (Or even if I hold the positive side out of the breadboard while leaving the negative one going to the resistance and then ground) the led turns on very dimly. I replaced the red one with another and this behavior didn't occur.

Why is this?

[drgeoff]

There is no -3 volt pin. There are no negative voltage rails in a RPi.

The +3.3 pin is not the same as ground (GND).

[klricks]

The 'T' adapter is poorly labeled. The 3V3 (-) and 5V (-) should be labeled GND.
LED's 'of a different color' do not have the same voltage rating. To get the same brightness from each would require different resistor values.

[tigermisu]

So, it would be perfectly equal if I connect it to a -3v/-5v thing to a grd pin?

2 more questions:

1) What is default GPIO output? 3v or 5v? And how do I change from one to the other?

2) What are the pins with special labels? (TXD, UART, I2C)

[P_Monty]

The gpio pins a 3.3v. You can't change to 5v, if you need 5v then you'll have to use the gpio pin to drive a transistor or relay...

[klricks]

So, it would be perfectly equal if I connect it to a -3v/-5v thing to a grd pin?

Yes, they are all the same.

1) What is default GPIO output? 3v or 5v? And how do I change from one to the other?

They use 3.3V logic levels ONLY! The use of 5V devices connected to GPIO inputs would require a level shifter circuit or in some cases a simple voltage divider using 2 resistors can be used.

2) What are the pins with special labels? (TXD, UART, I2C)

The special designations are for different default functions for example TXD is transmit data (out) and RXD is receive data (In) for serial devices.