Android App
If you are having problems getting the app to talk to the dropController device you need to double check that the the Android device is connecting to the Bluetooth module and that the BT module is connected to the Arduino correctly.
When the Android device establishes a connection with the Bluetooth module the LED on the module will change. The change depends on the module you are using but on most HC-06s, for example, the LED will turn on (not flashing).
If you have a connection but still cannot get it to work the problem is likely to be the connections between the BT module and the Arduino or the baud rate. Firstly make sure the BT module is set to work at 9600 baud rate. If you are not sure about how to do this see the guides at www.martyncurrey.com.
There are 2 parts to the connection when using Bluetooth.
1. The Android device connecting to the Bluetooth module, and
2. The app connecting to the dropController/Arduino.
Connection process:
1. The Android device connects to the Bluetooth module. The LED on the BT module changes.
2. The app sends a HELLO message to the drop controller and waits for a reply.
3. The dropController gets the HELLO message and turns on the waiting LED (the yellow one)
4. The dropController sends back the HELLO message.
5. The app gets the reply and changes the BT button and the screen title to connected.
If the waiting LED on the dropController is not turning on then the dropController is not getting the HELLO message. Check the connection between the Arduino RX pin and BT TX pin.
If the LED is coming on but the BT button in the app is not changing then the app is not getting the reply. Check the connection between the Arduino TX pin and the Bluetooth modules RX pin. This is the one with the voltage divider.
Also check that the correct resistors have been used and that they are the right way around.
If you are still having problems have a look at the Bluetooth guides I have on the other website
Arduino and HC-06 (ZS-040)
Arduino With HC-05 Bluetooth Module in Slave Mode