Thanks jremington for your reply.
The BT module is mounted at a breadboard and I suppose it can be fed with 5V so I am fine getting input current from the 5V arduino or from my voltage adapter for it. See http://www.ebay.de/itm/Wireless-Bluetooth-RF-Transceiver-Modul-serielle-RS232-TTL-HC-05-Fuer-Arduino-hp-/131069546110
I have read somewhere, the arduino does fine with 3.3V as input signal so I just need down leveling from arduino tx 5V to bt module rx 3.3V.
Do you agree with that?