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Micropython BLE and ESP32 - SwitchBot (Part 1)

SwitchBot is a device which allows a non-internet-connected device to be automated. It has a mechanical arm that can switch on or a device. At my home, I have a few remote controls that operate on weird frequencies. An example is one of my ceiling fan which operates a 300MHz remote. In order to switch on/off the fan, I have stuck a SwitchBot to the remote.








The Micropython ESP32 BLE feature has come to a stable state but the documentation is still sparse. There is a link to the Micropython APIs doc which describes the APIs and the different roles for BLE devices and the examples can be found here

I have been trying the BLE Central Role to build an interesting project to control the smart home devices. For part 1 of the series of blogpost, I shall start with SwitchBot.

I was happy for a short while and then realised that it will be better for me to switch on/off using the remote than to use a roundabout method to control my fan. I wanted to control the fan using my Openhab setup as part of my smart home setting.

There are already 2 GitHub repositories from OpenWonderlabs and nicolas-kuechler that provided Python API to control the SwitchBot.  nicolas-kuechler has a comprehensive APIs that implements the timers and encryption. In this project using EPS32 BLE and Micropython , I have ported the "pressed", "On" and "Off" capability of the SwitchBot because this is what I need at the moment. The video shows the SwitchBot in action.

BLE Handing in Micropython

In Micropython, the BLE handling is designed as interrupt events handling. To learn about the various BLE device roles, the documentation can be found here. For this article on controlling the SwitchBot, I am using Central Role (GATT Client). In this role, I am programming the ESP32 as a client to the peripheral.  

When a function is called, the results are retrieved from the interrupt handlers. To this make the code manageable in object-oriented methodology, the interrupt handler is as an object method.  In ESP32 BLE, the connection to the device is not always successful. The connection function has to cater to multiple retries and timeout. For the SwitchBot, most of the time 5 retries are good enough to establish a connection.
 
One unique thing about BLE device interfacing, in order to get some of the device attributes, the request has to be written to the device handles. For the SwitchBot, to get the status of the SwitchBot, the code has to write to a "Value" handler before the data is available to retrieve from the "Status" handler.

def getStatus(self):
    '''
    This is the subset of the implementation
    https://github.com/RoButton/switchbotpy 
    '''
    self.connect()
    if self.connected == False:
        return 9
    self.__ble.gattc_write(self.conn_handle, self.value_handle, commands["status"], 1)
    self.ops_write == False # reset the flag
    print ("write successful")

    # write successful, read back the status
    while self.ops_read == False:
        pass
    self.ops_read == False # reset the flag
    print ("Read successful")

    result = {}
    result["status"] =  self.bot_status
    result["battery"] = self.battery
    result["firmware"] = self.firmware
    return result



In the code snippet above, gattc_write has to called before gattc_read can obtained the values.  The source code can be obtained from Github.

Stay tuned to the future posts as I will write on getting the telemetry data from MiFlora and Mi Temperature and Humidity sensor and connecting to MQTT to send the data to Openhab.


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