The EGBT-045MS Bluetooth modules (the smaller daughter board) is a 3.3v device. The HC-05 break out board has a 3.3v regulator that allows an input voltage of 3.6v to 6v but the TX and RX pins are still 3.3v. This means you can use the 5V out from the Arduino to power the boards but you cannot connect the Arduino directly to the HC-05 RX pin.
For the HC-05 RX pin (data in) we need to convert the Arduinos 5V to 3.3v. A simple way to do this is by using a voltage divider made from a couple of resistors. In my case I use a 1K ohm resistor and a 2K ohm resistor.
As a quick guide to the voltage divider; 1K + 2K = 3K. 1K is a third of 3K so it reduces the voltage by a third.
Since the Arduino will accept 3.3 volts as HIGH you can connect the HC-05 TX pin (data out) directly to the Arduino RX pin (The 5V Arduino takes a voltage of 3V or more as HIGH).
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