As stated in the question, i2cdetect isn’t able to detect the SHT21 sensor on the BBB, although plenty of posts online claim that it works fine with a Raspberry PI.
Therefore, you can ignore it and use the sht21 kernel module anyway.
The circuit remains the one shared before.
Here’s what you need to do in a nutshell:
# gain root privileges, or use `sudo` each time you're asked
sudu su
# verify that the sht21 kernel module exists
find "/lib/modules/$(uname -r)/kernel/drivers/hwmon" -name sht21.ko.xz
# load the module
modprobe sht21
# let the kernel know to which bus the sensor is connected
echo sht21 0x40 > /sys/bus/i2c/devices/i2c-2/new_device
# the sensor values should now be exposed at /sys/class/hwmon/hwmon0 (or hwmon<some other number>)
cd /sys/class/hwmon/hwmon0
# print the temperature reading
cat temp1_input
# print the humidity reading
cat humidity1_input
Meanwhile, if somebody knows why i2cdetect doesn’t work with this particular sensor on the BBB, feel free to share!