I have recently purchased the Bluetooth Modem Bluesmirf Silver breakout board and had some questions about the design. What is the purpose of the BSS138 N-Channel Logic Level Enhancement Mode Field Effect Transistors, and are they necessary when designing a surface mount with an ATMEGA328 with 3.3V input?
The transistors are included to shift logic levels from a 5v system to the 3.3v that the RN-42 module communicates at. If you will be hooking up the module to a 3.3v system, then you do not need to include these into your system. We designed the BlueSMirF and Bluetooth Mate boards to be able to interface with 3.3v-6v systems to make them more user-friendly for our customers.