Bertrand Selva's LoRaTube Turns PVC Piping and D-Cell Alkaline Batteries Into a Rugged LoRa Repeater
The ultra-low-cost "LoRaTube" targets, but has yet to meet, a five-year runtime from its internal batteries.
Physicist turned self-described "embedded-systems hacker" Bertrand Selva has designed a rugged yet compact LoRa repeater, with the goal to deliver more than five years of off-grid operation using only low-cost alkaline batteries.
"In France, on the 433MHz ISM [Industrial, Scientific, and Medical] band, the ANFR [Agence nationale des fréquences, the French FCC] enforces a maximum transmission power of +10mW ERP (≈+10dBm); any attempt to increase range by raising the output power is illegal," Selva explains of the need for the project. "That's why a repeater is necessary. By placing the repeater at a high point — a hill, tower, or treetop with a clear line of sight — a single module can cover several tens of kilometers without exceeding the legally allowed transmission power."
Commercial LoRa repeaters tend to use solar panels feeding internal or add-on lithium-ion batteries, which Selva rejected as making them too expensive for wide deployment and relatively fragile. His solution: the LoRaTube, a low-cost tube-shaped repeater powered by D-size alkaline batteries installed in cheap PCV piping.
"The power supply is based on 18 LR20 alkaline batteries (D size) in series," Selva explains, "housed in a compact enclosure less than 50mm in diameter. This battery choice offers several practical advantages: LR20 cells are inexpensive (less than €1 each in most supermarkets) and widely available. Each LR20 battery typically provides 12,000 to 18,000mAh, or 18 to 27Wh. With 18 batteries, the total energy amounts to approximately 486Wh, for a total cost of €13.30 (5 packs × €2.66), which translates to just ~€0.024/Wh — a ridiculously low cost compared to lithium alternatives."
Another advantage to the D-cell batteries is that they slide nicely inside cheap PCV drainage piping, allowing all wiring to be internal. Only a small number of 3D-printed parts — totaling around 120g of filament — are required to hold everything in place. Supercapacitors provide protection against brownouts at low temperature, when the custom-built LoRa radio module — powered by a Raspberry Pi Pico microcontroller board talking to a Semtech SX1262-based Ebyte E22-400T or an SX1268-based E22-400T33D LoRa transceiver — spikes its power demands during transmission.
An initial prototype has already been deployed, but a lack of power management means the batteries are draining too quickly — expiring after just two months, well short of Selva's five-year target. A second PCB design is in the works, adding FRAM-based logging alongside improved power management which cold hit 7.4 years by what Selva admits is an "optimistic estimate."
The project is documented in full on Hackaday.io.