The problem
The client needed a humidity sensor for use in grain storage silos and an industrial packaging line that could integrate with their existing RS485 / Modbus RTU infrastructure. Commercial sensors in this price range either lacked on-device calibration or required a laptop and proprietary software to set the Modbus slave address — impractical in a silo environment.
What we built
We designed a compact PCB around a calibrated capacitive humidity and temperature sensor element, an RS485 transceiver with 15 kV ESD protection, a small OLED display and a rotary encoder with push-button.
The on-device UI lets operators adjust the calibration offset and set the Modbus slave ID without any external tools. Turning the encoder scrolls through menu items; pressing it confirms the selection. Settings are stored in non-volatile memory and survive power cycling.
Firmware implements the Modbus RTU slave protocol. The sensor exposes humidity (0.1% RH resolution), temperature (0.1°C resolution) and device status as standard holding registers, readable by any Modbus master.
PCB dimensions and mounting holes were specified to fit the client's existing DIN enclosure. The board operates from 9–30 V DC supplied over the RS485 pair's shield line.
Outcome
The sensors are installed in wheat storage silos and on a packaging line, where they have been read continuously by the client's existing SCADA system without modification to the master configuration. Field calibration has been performed by the client's maintenance staff using only the on-device UI.