The app controls 2 DC motors connected to pins 10 and 11 of the Arduino. The speed control is using PWM signal that vary digitally the voltage to the motor.
The app sends to HC-05 2 bytes every 200mS, each byte represents the value of the slider, 0-127. The Arduino generates PWM that relates to the slider position. Comments in the code explain the detail operations.
Software for Arduino and Android can be downloaded from GitHub:
APK file for Android app needs to be transferred to the device and installed. The app is in Basic for Android (B4A) free program, the code is included.
The 8 pins PDIP buffers is a mosfet driver. IC's that can be used are: TC4423, TC4424, TC4426, TC4427, MC33152, MC34152. These are very similar and pins are identical.