Bluetooth Arduino Data Type Error

First thing, you have to decide what data format the phone application will use for sending the data to the Arduino: define a “language” of sorts. Then you have to program both the phone and the Arduino to speak that same language.

Unless the data throughput is critical, I strongly advise you to use a text-based format, where each “message” is followed by a dedicated character that signals the end of the message. Popular choices for the message terminator are '\n' (ASCII line feed, numeric value 0x0a = 10), '\r' (carriage return = 0x0d = 13) and ';' (semicolon = 0x3b = 59). The semicolon is often used as a second-level separator, to split a message into smaller fields.

Here is a simple example that reads one-line messages, using a line feed as a delimiter:

void loop() {
    if (bluetoothSerial.available() > 0) {
        String message = bluetoothSerial.readStringUntil('\n');
        Serial.println(message);
    }
}

If you want to send the data as binary, then message framing becomes a tricky problem. Since every possible byte value can legitimately appear within the message (any byte can appear within a float), you cannot simply reserve one byte to serve as a terminator. This is why, unless you are an advanced programmer, I strongly encourage you to send whatever data you want to send as text. Text also makes debugging much easier.

Finally, I have to warn you that the String class is not friendly to the memory of your Arduino, and can lead to instabilities if used extensively. If you have time, I encourage you to study the robust way to read text from the serial port: see the article Reading Serial on the Arduino, by Majenko.

I'm guessing the line arrayOne=bluetoothSerial.read(); is converting the received value to a string (character array of ASCII charachters terminated with the '\0' character). Then, you copy that string to another string variable. If you want to split the string you should:

• define a starting character > 0
• and/or: define an ending character < the total string size.

See this link for more info.

You could try the following. I'm sure it's far from perfect but it might get you a bit further.

void setup()
{
    bluetoothSerial.begin(9600);
    Serial.begin(9600);
}

void loop()
{
    if (bluetoothSerial.available)
    {
        delay(100); // wait a little for the serial buffer to saturate. This value should be as short as possible

        // Save the buffer bytes as the float (Big Endian!)
        double received_float = (bluetoothSerial.read() << 24) | (bluetoothSerial.read() << 16) | (bluetoothSerial.read() << 8) | (bluetoothSerial.read());
        Serial.println(received_float);
    }
}

Some conciderations:

• The nr of bytes a float occupies may differ on your phone and your Arduino. The example above uses a 32-bit float
• If possible you'd want to start the transmission with a sequency of starting characters. That way, you can be sure to read the proper value.