Hardware for Talking Through 800 Years Old Walls Using LoRa

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Part 2: The Hardware you Need to Control a Distributed Heating System Through Meter-thick Walls You are Not Allowed to Drill

In part 1 of this series we talked about how to retrofit a temperature control system into an 800-years-old heritage-protected monastery without drilling and running new cables. The solution turned out to be the LoRa (Long Range) radio transmission technology that trades transmission speed for extreme transmission range, enabling us to wirelessly talk to stations behind several thick walls over 100 meters away.

The Constraints

• No changes to the heritage protected building were allowed.
• Retrofit budget was limited, ruling out changes to existing cabling.
• It needed to be retrofitted to existing underfloor heating piping and valves.
• It needed remote per-room temperature control.

The Hardware Structure

Let’s refresh our memory and look at the proposed system structure:

The Hardware Parts

Note: I listed all the hardware parts I used with links to the manufacturer web pages. These are just for your information and I’m not earning any income through these.

Temperature Control Stations

Most of the rooms of the monastery have a large central table containing the electrical installation, and underfloor heating piping with valves, but some had only a small cubicle. I was going to have to install the control stations for these rooms there.

The designer chose to build these tables and cubicles from thick iron plate with laser/plasma cut decorations on the sides. It looks wonderful and they are extremely durable, but it meant I could not hide the entire control station inside — the iron plates were effectively a Faraday cage and prevented any radio communication inside. This meant I had to use control stations made from two parts — the radio communication part on the outside wall, and the power supply box with relays on the inside.

After fruitlessly searching for a suitable off-the-shelf product I decided to build these myself from individual modules:

The Wireless Station

The wireless station was the heart of the station, containing:

1. A Seeeduino XIAO motherboard, providing connectivity for everything else and a small OLED display for installation and diagnostic purposes, and holding:
2. A Seeeduino XIAO microcontroller board, the heart of the control station. It controls the relays and the display, reads the data from the temperature/humidity sensor, and handles the communication with the LoRa board.
3. A Grove LoRa-E5 transceiver board, handling all the wireless tasks.
4. A Grove AHT20 I2C board used to sense the humidity and temperature.
5. The LoRa antenna for the transceiver board. The transceiver actually has it’s own little antenna, but that was not enough, so I used a proper antenna, screwed to an SMA connector with a pigtail cable and an I-PEX connector for the transceiver board.
6. The wiring block.

I had to design a housing for it as well, which I 3D-printed out of flame retardant PETG. It has some holes for the sensors and the diagnostic display. Given that it was installed in a public space, I designed it to be extra sturdy and it also features a holder making it harder to unscrew or break off the antenna.

This was mounted on the outside of the iron tables and connected, using a 6-wire cable, to the power supply and relays board inside.

The Power Supply and Relays board

The wireless station, being publicly accessible, was strictly low-voltage (5V powered) for safety and it needed a power supply and relays to control the 230VAC underfloor heating control valves, so I needed to put all the high-voltage parts on a separate board, safely installed inside the table. It contains:

• A generic 4-relay board with built-in logic-level relay drivers
• A generic 5-watt 230VAC to 5VDC safety switching power supply, powering all the electronics.
• A fuse
• The low-voltage wiring block (above, for the 6-wire cable coming from the wireless station)
• The high-voltage wiring block (below, used to connect the station to 230VAC power and to wire the 230VAC valve actuators)

This board is safely tucked inside the table/cubicle and contains some components that get warm, so I designed a lighter housing. The cover has ventilation holes for cooling, but provides touch protection.

Wireless temperature sensors

After some research, I decided to use an off-the-shelf component, the RAK WisNode Sense Home (RAK7204), for the remaining wireless temperature, humidity and air quality sensors.

These wireless sensors are an elegant solution, running for anywhere from one to several years (depending on signal quality) on one included lithium primary battery. They also feature an automotive air quality sensor, giving us an additional data to monitor, e.g. for ventilation.

I glued some strong permanent magnets on their backs and simply stuck them to a less visible/reachable metal part in the room I wanted to monitor.

The monastery has quite a few rooms and corridors, so I installed 11 of these in all the places not covered with the control stations.

The LoRa Gateway

As explained in the part 1 of this series, using a public LoRa network was not an option for several reasons. This required installing a LoRa Gateway.

After some research, I again went for a solution from RAK, the WisGate Edge Lite (RAK7268). It has since been superseded, but the product on the link is mostly the same.

It only needs network and power. I found that the easiest way to interface with it was using the built-in MQTT server, but we will cover that in the software part of the series.

I installed it in a high place to get the best possible coverage and connected it to the same network switch the application server running Home Assistant was connected to.

The Home Assistant Server

The Home Assistant Server is running on a Raspberry Pi 4 SBC. For storage I used an SSD though an USB-SATA interface cable that allows me to store several years worth of data and backups. It is connected to the same switch as the LoRa Gateway.

Conclusion

In part 1 of this series I described this solution in a general way, but this part is my attempt to clarify by showing you the hardware I used.

As you can see, the off-the-shelf parts are all pretty low-cost components, easy to buy and install. The harder part was to select and integrate the custom wireless control stations, but I was forced to do this due to not finding the right component off the shelf. Still, using the rich plethora of modules available, it is not a big deal to assemble an usable system.

The wireless station design was heavily influenced by the fact it is a one-off project. I took proper care of electrical safety and reliability, but with less than 10 stations built, I did not have to think too much about the manufacturability of it.

The system still runs three years later with the original hardware and has turned out to be very reliable. The only maintenance needed is to replace the wireless sensor batteries as they wear out. In the three years so far, I only had to replace 5 batteries, making this a minimal, once-a-year effort.

The system needs some minimal software maintenance as well, but we will talk about that in part 3.

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