A detail about them is that they use 3.3v vs. the 5v the Arduino uses. There are many adapter boards that not only bring out the small set of pins needed and include a 3.3v voltage regulator. Here is one example:
|Not quite enough - this design will exceed the BCM specs...|
While researching adding Bluetooth directly to the stand-alone Alternator Regulator (to be used to make configuration simpler) I decided to use the base module and add in my own 3.3v regulator.
But a key question was what about the signal levels? Some web pages point out that the signal levels also need to be converted from 5v to 3.3v, but it seems almost without exception all the small 'adapter boards' only change the VCC...
So - these modules use the BC417 "BlueCore" chip as this basis. Made by Cambridge Silicon Radio, I simply downloaded a datasheet for the chip. And sure enough: Listed under MAX conditions, V-in for Signals are limited to Vdd+0.4v In other words, all those simple Adapter boards are over-stressing the Bluetooth chip.
So I added a resistor dividers to the two signals heading to the HC-05 module (RX and Key) to drop down the 5V signals from the Arduino to the needed 3.3v