[SOLVED] Si7021 / Gy-21 with pro mini 3.3v

  • I have hooked up a GY-21 module as per the instructions to a pro mini 3.3V (Previously working with DHT22 sensor.

    I just get "not recognized" message in the serial monitor. I have metered out A4 and A5 pins on my clone board and they are connected to the main IC.

    This page http://www.guillier.org/blog/2015/07/sht21htu21d-humidity-and-temperature-sensor/ seems indicate that this board can be used on both 3.3 and 5V arduinos.

    Am I missing something?


  • Mod

    @grumpazoid this page https://www.mysensors.org/build/humidity_si7021 is completely dedicated to your sensor.
    Did you follow this guide?

  • @Yveaux That link seems to take me to the DHT22 guide....but yes I followed the Si7021 guide.

  • Mod

    @grumpazoid Damn phones... Anyway, corrected the link.
    So, I assume you get the error message 'Sensor not detected!' as the sensor.begin() call fails.
    You also triple checked A4 runs to SDA and SCL to A5.

    What Arduino board are you using?
    Are you sure the sensor is functioning correctly?

  • @Yveaux I have just tested on a second board. This time a 5V arduino uno. Same result - as you say at "while (not sensor.begin())" call.
    This board has SCL and SDA pins as well as A4 and A5. I have tried both.
    Looks like a duff sensor then 😞

  • Having spent some more time fault finding, it seemed that the logic level converter chip had been soldered on wonky and one of the pins was not connected.
    Ive unfortunately damaged the PCB in an attempt to fix so a new one has been ordered from another chinese seller.

    Having now had time to read futher about the GY-21 is appears it will work with both 3.3 and 5V microcontrollers.

    Interesting to see there is a GY-21P board that includes a pressure sensor. I would be interested to hear from anyone who has used this.

    Edit 9/10/19 Just an update on this.
    I now have new sensors on order Si7021 (both 3.3 and 5v boards) and also a BME280 which has temp, pressure and humidity.

  • Hi, I have used a couple of
    this without any problem.

    The supplier is a chinese store, so the shipping time is quite long, but the price is very low.

    Not yet tested the version with pressure sensor.

  • I have now received 2 new Si7021 ebay boards. One 3.3V and one 5V. Both happily working straight away.
    I have incorporated one into one of my motion sensors so I now get motion, battery level, room temp and humidity.
    At first my pro mini would not sleep and just kept looping in the serial monitor - but as soon as I removed the FTDI board it started to behave? Somehow the PIR kept getting triggered. maybe too much going on with everything connected on the 3.3V line.

    Anyway very happy with the finished sensor.

  • Mod

    @grumpazoid Good to hear and thanks for reporting back 👍

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