arduino turns on through I/O

My arduino uno just started turning on when i plug 5v into IO and gnd to gnd......y?
Am trying to put input into one of the pins, but it just turns arduino on.
Nothing else connected except 5v and it does the same thing. Please help!

Yes it will, it is called parasitic powering and can damage your arduino.
Never put a powered input on an unpowered chip. That is a universal rule with electronics.

Ok i never thought it was possible lol
So if i have the board powered the input pin is still not working

If there is any chance of inputs being high while the board is powered off you need
to add about 10k+ resistor in series to limit the current and avoid burning out the input
protection diodes.

Pushing a lot of current into an input protection diode is the most likely way to send
a CMOS chip into latch-up, which can cook the whole chip quite quickly if enough current
is available.

CMOS outputs are also protected with diodes so don't back-power through them either!

The normal function of the diodes is to protect against small static discharges when the
circuitry is handled by people wearing nylon(!). They cannot sustain large currents.

Alright now i am a little confused. Well from so far my board seems fine and i won't put 5v into pins without board on. It seems like its working thx

What i have is 3 servos controlled by arduino and using raspberry pi as input to arduino io pins. I need to get a 3v3 to 5v converter for gpio to arduino input. Using this as a sentry gun for airsoft with laser and webcam. Thanks!

I need to get a 3v3 to 5v converter for gpio to arduino input.

It should work without although it is slightly under the specified voltage to be recognised as a voltage high.
Two solutions:-

  1. Use the arduino’s analogue inputs to get the signals.
  2. Use a simple transistor to boost the voltage. (this will invert the signal but you can easily cope with this in the software )

Actually the ATmega's input thresholds are 0.3 and 0.6Vcc, ie 1.5 and 3.0V, so 3.3V
signals will be recognised as HIGH, but with reduced noise margin and lower speeds
when significant stray capacitance is involved