LoRa Energy-Harvesting Current Monitor

The Client had an existing product for monitoring AC current, using current transformers, and reporting the measurement data over a proprietary radio link. The unit was entirely powered by energy "harvested" (or "scavenged") from the current transformers.

Seeking to broaden the potential market for the product, the client wished to investigate using a standardised radio protocol for this product.

Our brief was to investigate the feasibility of using LoRa in this application. The proprietary radio was specifically optimised for operation from harvested energy, but LoRa radios are not - so success was not a foregone conclusion!

Samples of the existing product were supplied with the proprietary radio removed, leaving just the energy harvesting and current sampling parts.

For the investigation, we used an STM32 LoRa Discovery Kit, B-L072Z-LRWAN1, from STMicroelectronics which features a muRata Type ABZ LPWA module, based on the Semtech SX1276 radio transceiver chip.

Connecting the Discovery Kit to the current sampling circuitry, and incorporating the control & reading into ST's I-CUBE-LRWAN1 LoRaWAN software to transmit the readings to a MultiTech Conduit LoRa gateway was straightforward.

As expected, more of a challenge was optimising both the software and the Discovery Kit hardware to minimise its power consumption so that it could operate from the harvested energy of the current transformers. But we were able to reach an acceptable level of performance for the Client's requirements - with a number of suggestions for further optimisation beyond the scope of this initial study.

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