Microcontroller projects
LoRa P2P (LoRa without WAN)
last updated: 2021-03-25
under heavy construction :)
Introduction
LoRa is the first possibility to reach all devices in a house by radio without repeater (long range). The second cool feature is the minimal power needed to do so. We can power our devices with batteries for a long time. The disadvantage is the reduction of data that can be processed, but e.g. temperature control must not be that very quick.
You can find more about the theory and programming of LoRa and LoRaWAN in the following tutorial:
There you can find also code to access the Things Network via LoRaWAN.
Here we will use radio communication via LoRa with our own sender and LoRa gateway forwarding the information via MQTT.
LoRa P2P low power
One of the goals is to use all my heating thermostats with LoRa. The first step was to dive a little deeper into sleep modes, so that the thermostat and other devices could function on battery for al long time (min. 1 year). This information can be found in the tips and tricks section and is needed to understand some the following content:
The LoRa chips (boards)
There are 2 major chips on the market for cheap LoRa solutions with Arduino.
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First we have the radio chip SX1276 from SEMTECH (q: Wer hats erfunden? a: les Français). This chip is accessible via the breakout board RFM95W from hoperf. The chip has no WAN stack implemented, so we need a library providing this if we want to use the chip for LoRaWAN. The SX1276 communicates via the SPI interface with the microcontroller. An IRQ helps to wake up from sleep.
Second we may use the chip RN2483 from Microchip. It has an implemented WAN stack that has to be deactivated for LoRa P2P. This chips uses a classical serial connection to communicate with the microcontroller.
P2P with the SX1276
The chip is accessible via the breakout board RFM95W from hoperf. We can use the cool LoRa library from Sandeep Mistry. Information about the library can be found in the corresponding LoRa Application Programming Interface (API).
LoRa sender and receiver with DS18B20 temperature sensor on a breadboard
LoRa sender with DS18B20 temperature sensor on a breadboard
To use a minimum on energy we run the mega328 chip (Arduino) on internal RC with a maximum of 4 MHz. To do so we have to burn an alternative bootloader. Then we can add the LoRa chip (SX1276) by using a breakout board RFM95W from hoperf. The board needs the SPI interface (MOSI, MISO, SCK, SS), a Reset (input) and an Interrupt DIO0, to signalize if data is received.
LoRa sender with Promini LoRa board running on batteries
The next step was to use a PCB and I found the TTGO promini lora board from LILYGO which was really cheap and contained an Arduino (mega328p) and the SX1276 chip. I use the 868 MHz version (Europe) and the board came in version 2. Pictures showed the older version, and there was no circuit available. To understand the board, I had to draw the circuit and to reduce the power consumption to strip some components from the board.
The first LoRa sender will be an Arduino mega328p driven temperature and humidity sensor, running on 2 AAA alkaline batteries.
P2P with the RN2483
The RN2483 from Microchip has an implemented WAN stack. So this chip is predominated to be used with LoRaWAN. But... The chip is accessed by a classical serial interface facilitating the wiring. And my hacked thermostat (HR20 from Honeywell) that I want to use with LoRa has a serial interface, so perhaps I can pimp the thermostat without even opening the housing.
To use the chip in LoRa P2P mode the WAN stack has to be deactivated. Even as it's only normal serial commands a library would made things easier. As I didn't find a library, I wrote one.
