There are few ways to connect Serial Gateway to Raspberry pi. The easiest one is via USB port to raspberry pi. Due to the size of raspberry pi zero i prefer to have small PCB which will serve me purpose of gateway, zone and boiler controller. This can reduce the cost of having multi zone smart heating at the same time.
Connecting Serial Gateway on Raspberry Pi Zero GPIO
Raspberry pi GPIO are 3.3v tolerant but we have to use Arduino 5v version so we need bi-directional logic level converter to connect Serial Gateway on Raspberry Pi Zero GPIO. Million dollar question is why not use Arduino at 8MHz and remove the need of bi-direction logic lever convert? I made prototype Serial Gateway on Raspberry Pi with Arduino at 8MHz it but for some reason it only receive messages and I wasn’t able to transmit messages to zone controller and boiler controller, why so? I have no idea, may be you can log ticket on MySensors forum to investigate this further and enlighten us here as well.
Serial gateway is connected to 5v supply over micro USB and 5v feed to raspberry pi zero over GPIO. There is onboard voltage regulator HT73333 to supply power to nRF24L01 PA LNA 2.4GHz RF Wireless Transceiver.
DS18B20 Temperature Sensors
Serial Gateway on Raspberry Pi Zero GPIO comes with footprint on PCB for One-Wire DS18B20 digital temperature sensors connected to Raspberry pi GPIO with 4.7K resister between DQ and VDD. You must follow steps to enable 1-Wire DS18B20 temperature sensor on raspberry pi GPIO as descried here DS18B20 Temperature Sensor with Raspberry Pi
SPI and FTDI
SPI and FTDI headers on side of the PCB to burn bootloader and MySensors Serial gateway sketch to Arduino.
Enable Serial Port on GPIO
The Raspberry Pi GPIO serial port configuration has changed and also with the Raspberry PI 3. By Default serial port is disabled and on RPI 3 Bluetooth made it even made it more confusing to enable Serial Port on GPIO. You must enable serial port on GPIO.
Roadmap for Serial Gateway
I m still trying to make plan and trying to make room for mSATA or M2 SSD onboard to have decent quality storage onboard for PiHome to run from, if you have idea and want to include and suggest please feel free.