custom device 'PM detector': case study

custom device 'PM detector'

This device measures PM1.0, PM2.5, PM10 (μg/m³), T (°C), RH (%), AQI (0..500) and n@0.3, n@0.5, n@1.0, n@2.5, n@5.0, n@10 (number of particles). About atmospheric particulate matter see Wikipedia for info and limits and aqicn or sensor.community for actual values.

This device has a double-colored screen and can exchange data via USB. It shows, sends, and also stores the measurements.

The protocol is USB-serial, 115200:8N1.

installation notes (Windows)

The basic application is 'PM2.5'. On Windows is required a driver for CH340.

1. Download the 'PM2.5' software
2. Download the 'driver for CH340'
3. Connect the device to a USB port
4. Select START > Device Manager - you will see an "unknown USB device" with a question mark under the USB devices
5. Click > Update Driver - browse to the downloaded driver file
6. Check the Port assignment after updating the driver - COM port number
7. Launch PM2.5.EXE from the download
8. Click the last option in the top menu to change language (English)
9. Click the Equipment > Config menu and set the COM Port from that in the device manager and the baud rate to 115200
10. SAVE.

protocol reverse engeenering

Using the basic application 'PM2.5' and 'SerialMon', (by DuNovo) I captured the following 'functions':

 send to device				receive from device

 {"fun":"01","sendtime":"002"}}             {"res":"1"}
 {"fun":"02","storetime":"004"}}            {"res":"2"}  
 {"fun":"03","clock":"21-03-05 20:57:24"}}  {"res":"3"}                     (YY-MM-DD hh:mm:ss)
 {"fun":"04"}}                              {"res":"10","total":"004354"}   (start dump historical data)
    (push RT/dump historical data)          {"res":"4","y":"2021","m":"03","d":"05","h":"11","min":"53","sec":"03", 
                                    "cpm2.5":"0033","cpm1.0":"0022","cpm10":"0037",   (cpm: Condensable Particulate Matter)
                                    "apm2.5":"0036","apm1.0":"0023","apm10":"0037",   (apm: Ambient Particulate Matter)
                                    "aqi":"033","t":"19.6","r":"45"}  ...             (aqi: Air Quality Index) 
                                            {"res":"0"}                     (end historical data)
 {"fun":"05","flag":"1"}}                   {"res":"5"}                     (start(1)/stop(0) push RT data)
 {"fun":"06"}}                              {"res":"6"}                     (clear internal memory)
 {"fun":"80"}}                              {"res":"80","SendInteralTime":"000002","StoreInteralTime":"000005", 
                                     "WritePoint":"000046","ReadPoint":"000000","SendInteralFlag":"000000",}
                                            {"res":"-4"}                    (error condition, e.g. no data)

notes

• Data is like JSON data, but with some exceptions. The extra ending '}' is not required.
• The display is updated every second, the data is sent every "sendtime" sec, and stored every "storetime" sec.
• The "particle number" is shown on the display but not sent.
• The data shown on the display as PM is APM (total). Sent also CPM (Condensable PM).

custom device definitions

In a custom device, we can choose the DPs as we want. I used the following criteria:

• This custom device, like many tuya devices, will use atomic data points.
• Any SET will echo the update data, the response messages {"res":"xx"} are used as ack.
• Internal memory can be used in many ways. I have chosen to produce daily stats every hour, as well as TVR Thermostatic Valve, so I can use the same interface.
• The complete definition of all 27 DPs is on the standard documentation.

implementation

See extra/custom PM detector.

pick and execute

• Format standard input commands from tuyaDAEMON device/capability/value to required serial command. note: The input 'fake' commands have 2 extra fields: msg.to:<deviceId> and msg.infodp:<dp> added by CORE to speedup the process.
• Setup: initializes the 'tuyastatus' structure.

RT data processing

• Convert serial responses to standard tuya answer object with multiple DPs.
• The answer {"res":"10"...} enter the 'skipdata' mode (historical processing) and stop RT

start dump

• Inizialize required data structures
• Sends the command {"fun":"04"} (start dump data)

historical data processing (only in the 'skipdata' mode)

• Cumulates data to do averages.
• The answer {"res":"0"} (end data):
  • produces the output
  • terminates the 'skipdata' mode
  • restarts RT mode

timers

• At 00:00: clear data in device memory
• Every hour: create historical data

startup (init)

• Initializations with defaults: sendtime, storetime, clock, RT on

Manual commands for test.