I wanted to add some sensors to a Raspberry Pi but I was quickly running out of GPIO pins, luckily I had some spare Arduinos so decided to experiment with plugged one into the Pi. I got the Arduino set up with the sensors and connected it via USB to the Raspberry Pi.
To get the readings in Node-RED install Node-RED serialport, restart Node-RED and select the serial port node. To set it up you need to put in the serial port you are using. My settings looked like the following image.
Source: Adafruit – Arduino