Gas-sens_Rs-485

a Compact, cheap gas sensor with RS-485 connectivity

Concept:

24V supplied, Gas detector with half-duplex RS-485 interface. For industrial or home use and application ready product, containing a current or voltage output for reference measurment. a sinlge LED used for showing cominucation or Error state. The small footprint makes it ready for most plastic enclosures. the PCB can be hand-solderd with a bit of skill. Use cases can be in Garage systems (linked with the vents and gas alarm system), hobbyist workshop or any other room such as Office spaces and schools.

Components

A STM32G031K8 is the logical unit. Is is cheap and readily available. The very most components use standart fooprints. Thus, they are easily exchangeble in case of a shortage. All parts are SMD, except for the Sensors, DCDC converter and Connectors. All PCB components can be sourced by Mouser, except for the Gas Sensor. For developement purposes (TAG v0.1) a gas sensor type MQ-X were used (MQ-7 and MQ-135) TAG v0.2 makes use of the nemoto sensor, that uses low current and is more gas specific. The I2C conection gives a possibility, to further extend with more sensors, such as the Sensirion SHT4X.

Programming

Hardware is developed in KiCad 7.0.

everything is programmed on STM Cube IDE. The Sensor and trim potentiometers are read-in using DMA. The max and minimum values of the potentiometers can be set by the Modbus. the specific address are found in the excel-table "Modbus-memorymap" in the software folder.

Simulation

Low pass filter for the PWM (from uC to OpAmp to voltag eand current output) has been simulated using LTSpice.

next steps

• PWM filter simulations
• build and test prototype
• Prgram IC for analog measurment and analog output
• Find better sensor supplier and include it on second version PCB -> nemoto sensors
• implement Modbus via RS-485
• create Bill of material
• Make second physical prototype