How can I run HC-05 on 8MHz Pro Mini? I run an Arduino Pro Mini on ATMega328 3.3V 8MHz, and I am unable to send data over HC-05 breakout board on this frequency, and on 5V 16Mhz, it has no issues.

The Arduino code is down below:

#include &lt;SoftwareSerial.h&gt;
#include &lt;Wire.h&gt;
SoftwareSerial BT(10, 11);
byte value;
unsigned long pmillis_bt = 0;

void setup() {
  Serial.begin(19200);
  BT.begin(9600);
}
///////////////////main///////////////////

void loop () {
  if (millis() - pmillis_bt &gt;= 100) {
    byte Data[3];
    byte cmmd[20];
    int insize;
    char str[256];
    int i = analogRead(A0);
    value = BT.read();
    Data[0] = 'a';
    Data[1] = i / 256;
    Data[2] = i % 256;
    sprintf(str, "%d", i, Data[0], Data[1], Data[2]);
    Serial.println(str);
    if (value == 49) {
      for (int j = 0; j &lt; 3; j++)
        BT.write(Data[j]);
      value = 0;
    }
    pmillis_bt = millis();
  }
}
Basically Arduino Uno Is working On 5v Voltage Level so you can Directly connect the HC-05 Bluetooth module to it using it 4 lines VCC,TX,RX,GND.

But in Pro mini 3.3 V level due to you need to Generic 3.3V To 5V 2 Channel Logic Level Converter Bi-Directional Module

using this you can easily convert the vlotage level.

if you have to make using simple component also using 

SparkFun Logic Level Converter - Bi-Directional