reading Arduino GPIO status on Raspberry GPIO

Other Hardware General Electronics
1

I want to connect Arduino GPIO 11 (5V) to Raspberry (RPi3) GPIO 12 (3.3V).
My Arduino program is setting GPIO 11 high and low at times and another program running on RPi3 should input the highs and lows. The pins will be connected by wire.

I have to get the 5v down to 3.v between the connection. There is a nice example on http://www.dummies.com/computers/raspberry-pi/connecting-the-raspberry-pi-and-the-arduino/
It has the following resistors:

RPi<-----3OOk<----Arduino
|
1K
|
v
Ground

My question is: which ground is this? Arduino's pr RPi's (probably does not matter) ?
I measure voltage between arduino and after the 1K but get 4.3V (that may be because the Arduino 5V is giving me just 4.3V).

I also have a buck converter which I could use to step voltage down. It has V+/V- IN and also V+/V- out. Am I correct in connecting the V+OUT to Arduino GPIO and V-OUT to Arduino ground and also the V+IN to RPi GPIO and V-IN to Rpi ground? That is after I've set the dial to 3.3V on the IN end of teh buck converter.

2

My question is: which ground is this?

The ground on the Pi and the Arduino must be connected together.

I also have a buck converter which I could use to step voltage down.

No do not use that for a signal, it will not respond fast enough and could cause damage to your Pi.

P.S. I wrote that book.

3

A 300k:1k resistor divider will not be useful! A 1k:2k2 divider will be - you only need a 2/3rds reduction,
not a factor of 300

4

Yes the link actually says 300R not 300K. That will work but a 510R will be better.