Hummingbird Duo sensors run at 5V.
Hummingbird Bit sensors run at 3.3V. All sensors packaged with Hummingbird Bit are 5V tolerant, so they are all backwards compatible with Hummingbird Duo controllers.
The Hummingbird Duo distance and sound sensors do not operate at 3.3V, so they do not work with Hummingbird Bit. Hummingbird Duo light, temperature, and dial sensors are compatible with the Hummingbird Bit.