BLE Moisture and Humidity Monitor/Logger

Damp wall

For a client engaged in building refurbishment & restoration, a device was required to monitor & log the progress of drying-out after water damage and damp problems.

The client had a PCB design using a resistance probe to measure material moisture level, along with a Texas Instruments HDC2010 humidity & temperature sensor, a Fujitsu FRAM for non-volatile data logging, an RGB LED to give a visual indication of the drying progress, and a BLE module based on a Nordic Semiconductor nRF52832 to communicate with a smart phone.

The Client approached us to first bring up and debug the hardware, and then develop the firmware for the product.

The test firmware was started by just blinking an LED on a Nordic nRF52832-DK development kit as a known-working reference, then moved to the first prototype hardware, then features were added to test each part of the hardware.

Having tested the first prototype hardware, and agreed necessary changes, we proceeded with the product firmware development.

A slightly unusual requirement was for the device to continue broadcasting some data in Bluetooth advertisements while connected to a phone. This is not a use case directly supported by the Nordic SDK, so required some digging into the inner workings of the stack.

The device was battery powered, so we had to pay careful attention to energy consumption to achieve the goal of 1 year battery life.

It was initially intended to model the BLE communications on the Nordic Thingy:52, but this turned out to be not ideal for the phone app developers; so a set of custom Services and Characteristics was agreed, and implemented from scratch.

The product was finally checked against a commercial moisture meter (costing an order of magnitude more!) and found to give acceptable results.

STM32 LoRa Discovery Board

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