Bluetooth Communication
The HC-05 bluetooth module takes a serial (USART) input and transmits any incoming data to a paired phone. This is compatible with any microcontroller with a hardware (or emulated) serial port. On Arduinos, this is accessed by using 'Serial' in your code.
The HC-05 modules are cheap to buy - generally under £5.
Sending data over bluetooth takes time and power. To minimise these on the eChook a very compact packeting system was used. Each sensor update is sent in a packet of data 5 bytes long. Each byte looks like this:
[{][id][data_1][data_2][}]
The [ ] indicate each byte.
{} - These braces indicate the start and finish of a packet - this allows for basic error detection as the receiver knows to expect a '}' 4 bytes after the '{'. When that is detected, anything within is likely to be data.
id - This is the data identifier. It is a single character that can be looked up in a table to identify what sensor the data in the packet is from.
data 1 and 2 - These two bytes contain the value from the sensor.
Encoding the data into 2 bytes required limiting the data that is sent. To achieve this any value below 127 is sent to an accuracy of 2 decimal points, and any value above 127 is sent as an integer.
If the value being sent is below 127 the data1 byte contains the integer portion of the value and the data2 value contains two decimal points. If the value is over 127 the data1 byte contains the hundreds and thousands portion and the data2 value contains the tens and units.