Sensirion SEN55 not found by I2CScan

[martingrassl]

Hi,

I have a Sensirion SEN55 wired like described in the data sheet and connected to my D1 Mini VCC, GND and D1 (SCL) and D2(SDA). Anyhow, the I2C interface looks like it can't recognize the sensor.
I installed tasmota-sensors and I compiled my own build, and both show the driver enabled for 76:

stat/d1-mini/d1_mini_003/RESULT = {"I2CDriver":"3,5,7,47,76"}

Should the fan start when it gets powered already? I tried to use an external power supply as well to find out if the D1 Mini provides not enough power, but without any success. In the data sheet I read that by default the sensor starts in idle mode and is waiting for commands, but this should not be the case in Tasmota, right?

I2C in general is working as I can connect successfully a 2004 display with I2C backpack and send text to it.

Thanks,
Martin

[arendst]

What is the result from command I2cScan?

With VCC you mean +5V?
You did connect the SEL pin of the SEN55 to Gnd?

Chk if SetOption156 is 0 (no passive mode)

[arendst]

You could try to add pull up resisters of 10k on the i2c lines. Alternarively add another i2c device with pullup resistors.

As long as i2cscan doesn't report a device all else fails.

[martingrassl]

I soldered now and added the pull up resistors, but without success... Maybe the sensor is broken

[martingrassl]

Anyhow, thanks for your quick support!

[sfromis]

Well, do make triple sure that your wiring is entirely correct and solid connections.

[martingrassl]

OMG - I could scream... I looked at the wiring like 10 times or more and was sure I connected correctly - what I didn't recognize was that the data sheet is showing the sensor bottom up and my cable had the wires in the opposite order - pin1 of the sensor was in my cable pin 6 and so on... sometimes you have to take a break then you see the things with new eyes...

Thanks a lot for helping me out - now the fan is running, the sensor shows values, everything is fine 💯 I hope I didn't break anything in the sensor.

@arendst thanks a lot for your great work! With Tasmota building smart home sensors is really easy and effortless (if you don't make mistakes in the hardware :))

[martingrassl]

@arendst is there any way to get two I2C buses working in Tasmota? I would like to output the data of the sen55 by an 2004 LCD display, but can't get both working at the same time. Either the display is working or the sen55, but if I connect both it fails. I suspect it is because of the pull-ups which interfere somehow. I tried to connect the display to 3,3V or 5V, to remove the pull-ups of the sen55 as the LCD backpack already have some, but everything failed.

If I connect both to the same bus the I2CScan detects both, but the SEN55 doesn't start the fan and send any value... alone it works fine.

If it is not working, I still can show my values via MQTT, but a display next to the sensor would be nice... and one idea is to connect the display to e.g. D5 and D6 and have SEN55 at D1 and D2 so that they can run independently. Or is this a bad idea?

[martingrassl]

that's a pity that I can't use them. Then I'll look for an ESP32. But then my setup should work?

[arendst]

Yep. An ESP32 or ESP32-S3 would do. They have two I2C busses.

[martingrassl]

OK thank you for your great support and thanks for trying to help me!

[martingrassl]

Hi @arendst , sorry to ask again - I got today my esp32-s3 zeros (hope that they are the right ones) and connect everything together. The console shows now this output:

00:00:00.250 CMD: Fall back to serial port, no SOF packet detected on USB port
00:00:00.251 HDW: ESP32-S3 v0.2 (PSRAM)
00:00:00.267 UFS: FlashFS mounted with 304 kB free
00:00:00.273 CFG: Loaded from File, Count 20
00:00:00.336 QPC: Count 1
00:00:00.338 I2C: Bus1 using GPIO04(SCL) and GPIO03(SDA)
00:00:00.339 I2C: Bus2 using GPIO02(SCL) and GPIO01(SDA)
00:00:00.363 BRY: Berry initialized, RAM used 3766 bytes
00:00:00.372 Project tasmota - aq_az2 Version 15.3.0(release-tasmota32)-3.3.7(2026-02-19T13:57:18)
00:00:00.479 I2C: HIH found at 0x27
00:00:01.765 WIF: Connecting to AP1 Smart-MyHomeIsMyCastle Channel 11 BSSId EA:DE:27:66:FA:76 in mode HT20 as Air-Quality-2512...
00:00:03.162 WIF: Connected
00:00:03.366 HTP: Web server active on Air-Quality-2512 with IP address 192.168.4.196
22:44:12.364 MQT: Attempting connection...
22:44:12.390 MQT: Connected
22:44:12.392 MQT: tele/Air-Quality/aq_az2/LWT = Online (retained)
22:44:12.393 MQT: cmnd/Air-Quality/aq_az2/POWER = 
22:44:12.396 MQT: tele/Air-Quality/aq_az2/INFO1 = {"Info1":{"Module":"ESP32S3","Version":"15.3.0(release-tasmota32)","FallbackTopic":"cmnd/DVES_F0A9D0_fb/","GroupTopic":"cmnd/tasmotas/aq_az2/"}}
22:44:12.431 MQT: tele/Air-Quality/aq_az2/INFO3 = {"Info3":{"RestartReason":"Software reset CPU","BootCount":10}}
22:44:12.444 MQT: stat/Air-Quality/aq_az2/RESULT = {"POWER":"ON"}
22:44:12.445 MQT: stat/Air-Quality/aq_az2/POWER = ON
22:44:15.667 QPC: Reset
22:44:16.702 MQT: tele/Air-Quality/aq_az2/STATE = {"Time":"2026-02-19T22:44:16","Uptime":"0T00:00:08","UptimeSec":8,"Heap":250,"SleepMode":"Dynamic","Sleep":50,"LoadAvg":20,"MqttCount":1,"Berry":{"HeapUsed":4,"Objects":50},"POWER":"ON","Dimmer":68,"Color":"AD006D","HSBColor":"322,100,68","Channel":[68,0,43],"Scheme":0,"Width":1,"Fade":"OFF","Speed":1,"LedTable":"ON","Wifi":{"AP":1,"SSId":"Smart-MyHomeIsMyCastle","BSSId":"EA:DE:27:66:FA:76","Channel":11,"Mode":"HT20","RSSI":64,"Signal":-68,"LinkCount":1,"Downtime":"0T00:00:03"},"Hostname":"Air-Quality-2512","IPAddress":"192.168.4.196"}
22:44:16.749 MQT: tele/Air-Quality/aq_az2/SENSOR = {"Time":"2026-02-19T22:44:16","HIH":{"Temperature":-29.0,"Humidity":26.7,"DewPoint":-42.3},"TempUnit":"C"}
22:45:00.756 CMD: I2CScan
22:45:00.788 MQT: stat/Air-Quality/aq_az2/RESULT = {"I2CScan":"Device(s) found on bus1 at 0x27"}
22:45:00.806 MQT: stat/Air-Quality/aq_az2/RESULT = {"I2CScan":"Device(s) found on bus2 at 0x69"}

what you can see the sen55 is not recognized during boot, but only when running I2CScan. In my own build including display enabled I can set the text in the display, so this is working fine (please ignore the HIH here I'm using the build from GH). However, the sen55 doesn't start.

If I connect the sen5x alone and configure the same ports as SDA/SCL 1 it as well doesn't start, if I connect my D1 mini to the same wires it works fine.

Any idea what I could be doing wrong? According to the pinout I can use any GPIO for I2C...

[sfromis]

For readable logs, do use triple-backtick lines before/after, instead of only single backtick.

If the SEN55 is not recognized at boot, make sure that the driver is included in the output of the command I2CDriver, number 76.

BTW. Do you have a Honeywell HIH sensor on I2C address 0x27? If not, better disable that driver with command I2CDriver36 0

[martingrassl]

@sfromis , ok sorry, will use the right formatting in future. Now I only configured one bus and connected it like in my D1 mini and yes, 76 is enabled. The HIH is only shown when I don't enable display, then the display is identified as HIH and shows weird data - but when I enable display then everything works fine. Just the sen55 is not working:

00:00:00.250 CMD: Fall back to serial port, no SOF packet detected on USB port
00:00:00.251 HDW: ESP32-S3 v0.2 (PSRAM)
00:00:00.261 UFS: FlashFS mounted with 304 kB free
00:00:00.266 CFG: Loaded from File, Count 33
00:00:00.271 QPC: Count 1
00:00:00.273 I2C: Bus1 using GPIO03(SCL) and GPIO02(SDA)
00:00:00.295 BRY: Berry initialized, RAM used 3766 bytes
00:00:00.304 Project tasmota - aq_az2 Version 15.2.0.6(tasmota32)-3.3.7(2026-02-19T21:56:57)
00:00:01.725 WIF: Connecting to AP1 Smart-MyHomeIsMyCastle Channel 11 BSSId EA:DE:27:66:FA:76 in mode HT20 as Air-Quality-2512...
00:00:03.126 WIF: Connected
00:00:03.330 HTP: Web server active on Air-Quality-2512 with IP address 192.168.4.196
08:47:23.327 MQT: Attempting connection...
08:47:23.483 MQT: Connected
08:47:23.485 MQT: tele/Air-Quality/aq_az2/LWT = Online (retained)
08:47:23.487 MQT: cmnd/Air-Quality/aq_az2/POWER = 
08:47:23.490 MQT: tele/Air-Quality/aq_az2/INFO1 = {"Info1":{"Module":"ESP32S3","Version":"15.2.0.6(tasmota32)","FallbackTopic":"cmnd/DVES_F0A9D0_fb/","GroupTopic":"cmnd/tasmotas/aq_az2/"}}
08:47:23.524 MQT: tele/Air-Quality/aq_az2/INFO3 = {"Info3":{"RestartReason":"Vbat power on reset","BootCount":16}}
08:47:23.537 MQT: stat/Air-Quality/aq_az2/RESULT = {"POWER":"ON"}
08:47:23.540 MQT: stat/Air-Quality/aq_az2/POWER = ON
08:47:26.752 QPC: Reset
08:47:27.755 MQT: tele/Air-Quality/aq_az2/STATE = {"Time":"2026-02-20T08:47:27","Uptime":"0T00:00:09","UptimeSec":9,"Heap":252,"SleepMode":"Dynamic","Sleep":50,"LoadAvg":19,"MqttCount":1,"Berry":{"HeapUsed":4,"Objects":50},"POWER":"ON","Dimmer":68,"Color":"AD006D","HSBColor":"322,100,68","Channel":[68,0,43],"Scheme":0,"Width":1,"Fade":"OFF","Speed":1,"LedTable":"ON","Wifi":{"AP":1,"SSId":"Smart-MyHomeIsMyCastle","BSSId":"EA:DE:27:66:FA:76","Channel":11,"Mode":"HT20","RSSI":90,"Signal":-55,"LinkCount":1,"Downtime":"0T00:00:03"},"Hostname":"Air-Quality-2512","IPAddress":"192.168.4.196"}
08:48:35.279 CMD: I2CScan
08:48:35.312 MQT: stat/Air-Quality/aq_az2/RESULT = {"I2CScan":"Device(s) found on bus1 at 0x69"}
08:48:46.044 CMD: I2CDriver
08:48:46.048 MQT: stat/Air-Quality/aq_az2/RESULT = {"I2CDriver":"3,5,7,8,9,10,11,12,13,14,15,17,18,20,24,29,31,36,41,42,44,46,47,48,58,62,65,69,76,77,82,89,97"}

this is my configuration now:

and this is the GPIO view if it helps:

I don't know why the four pins are marked as high as I don't have anything connected to 1 and 4.

I'm using and external power supply to avoid that the ESP has not enough power, but the GND is connected to both.

[martingrassl]

I got it working only one more time. It looks like it is a timing issue or something else. I increased the log level and now I get from S5X a "reset error 268", I don't know if this helps somehow:

00:00:00.250 CMD: Fall back to serial port, no SOF packet detected on USB port
00:00:00.251 HDW: ESP32-S3 v0.2 (PSRAM)
00:00:00.271 UFS: FlashFS mounted with 304 kB free
00:00:00.279 CFG: Loaded from File, Count 38
00:00:00.284 QPC: Count 1
00:00:00.285 CFG: CR 437/699, Busy 0
00:00:00.287 I2C: Bus1 using GPIO02(SCL) and GPIO01(SDA)
00:00:00.290 TFS: File 'mcp23x.dat' not found
00:00:00.290 MCP: Invalid template
00:00:00.291 LED: RMT gpio 21
00:00:00.293 ROT: Mode 1
00:00:00.297 BRY: GC from 3680 to 3480 bytes, objects freed 4/20 (in 0 ms) - slots from 36/61 to 28/61
00:00:00.310 CFG: No '*.autoconf' file found
00:00:00.313 BRY: GC from 5332 to 3766 bytes, objects freed 8/37 (in 0 ms) - slots from 57/76 to 37/76
00:00:00.313 BRY: Berry initialized, RAM used 3766 bytes
00:00:00.319 BRY: No 'preinit.be'
00:00:00.321 SRC: Restart
00:00:00.324 Project tasmota - Tasmota Version 15.3.0.1(tasmota32)-3.3.7(2026-02-22T22:43:32)
00:00:00.324 DOM: Support 1 Device(s), 0 Button(s), 0 Switch(es) and 11 Sensors
00:00:00.337 CFG: Domoticz loaded from file
00:00:00.337 ETH: No ETH MDC and ETH MDIO GPIO defined
00:00:00.349 SHT: Sensor did not ACK command
00:00:00.791 S5X: Reset error 268
00:00:00.810 BRY: No 'autoexec.be'
00:00:00.093 WIF: Checking connection...

[martingrassl]

I finally got it working. I implemented to reset the sensor in a loop 3 times and according to my logs it always fails the first time, but is successful the second time.

    if (!Settings->flag6.sen5x_passive_mode) {  // SetOption156 - (Sen5x) Run in passive mode when there is another I2C master (e.g. Ikea Vindstyrka), i.e. do not set up Sen5x sensor, higher polling interval

      for (uint16_t attempt = 1; attempt <= maxRetries; attempt++) {

        if (!Sen5xError("Reset", sen5x->deviceReset())) {
            AddLog(LOG_LEVEL_DEBUG, PSTR("S5X: reset success"));
            break; 
        } else {
          AddLog(LOG_LEVEL_DEBUG, PSTR("S5X: reset failed %d x"), attempt);

          if (attempt < maxRetries) {
              delay(300); 
          } else {
              AddLog(LOG_LEVEL_DEBUG, PSTR("S5X: all reset attempts failed"));
              continue;
          }
        }
      }

      delay(1100);                              // Wait 1 second for sensors to start recording + 100ms for reset command
      if (Sen5xError("Measurement", sen5x->startMeasurement())) {
        continue;
      } else {
      }
    }

Somehow I can't push to GH, otherwise I would raise a PR.

I can now show the values on my 2004 display, however I had to change the rows and columns in the settings. Without this change the row 4 was unreachable for the DisplayText command:

However, I'm happy now with the solution and would like to thank you again that you took your time to help me.

[sfromis]

You should be able to change those display parameters using the various DisplayXxxx commands, like DisplayCols and DisplayRows
https://tasmota.github.io/docs/Commands/#displays
You can see all of the settings with the command Display

[martingrassl]

Yes I tried, but without success. The documentation says that DisplayCols and DisplayRows is only useful for display mode !=0

[sfromis]

Whatever happens, those two fields are simply the same as modified using the commands. And the lines you are changing are only to set the defaults when resetting all settings to firmware defaults, not after that.

Tasmota/tasmota/tasmota_support/settings.ino

Lines 1366 to 1367 in c6f6b9f

	Settings->display_rows = 2;
	Settings->display_cols[0] = 16;

Thus, I suspect that "something else" is going on in your setup.

[arendst]

In the meantime I was startled by your issues and did a rewrite of the I2C Wire lib for ESP8266. The dev branch now default supports 2 busses on ESP8266. I also updated the LCD library to support the second bus. And finally I've added a second probe for the Sen55 on init as you experienced to be succesful.

All in all it should now work on your ESP8266 as you initially expected.

Rgrds, Theo.

[martingrassl]

Wow great, thanks a lot! I'll try it soon