Bluetooth Control
One very useful sensor that works with Arduinos and has well documented examples on the net is the HC05. This article describes the essence of the system by which an app on a mobile phone can connect to a bluetooth module and communicate information to an arduino. There are many things you can then do once this basic system is working - in this example I am just turning on lights or relays.
Note: The module can be powered from 3.6 to 6 volts. However, the logic voltage level of the data pins is 3.3V. So, the line between the Arduino TX (Transmit Pin, which has 5V output) and the Bluetooth module RX (Receive Pin, which supports only 3.3V) needs to be connected through a voltage divider in order not to burn the module. On the other hand, the line between the Bluetooth module TX pin and the Arduino RX pin can be connected directly because the 3.3V signal from the Bluetooth module is enough to be accepted as a high logic at the Arduino Board. This can be seen on diagram above copied from the Mechatronics site.
The net has many well documented examples - see two below.
https://www.instructables.com/id/Arduino-AND-Bluetooth-HC-05-Connecting-easily/
https://howtomechatronics.com/tutorials/arduino/arduino-and-hc-05-bluetooth-module-tutorial/
I decided to make a simple Android App using MIT Appinventor to interface with the bluetooth circuit.
Below is a circuit diagram and Android App. the front screens and the designer code. Even without experience in using AppInventor you should be able to see the connection between the buttons pressed and the alpha character that is sent to the HC05. Further down this screen you can see the Arduino code that receives these characters and issues instructions through data pins to action and external circuit.
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separate power inputs and battery switch
This is a general purpose box to connect with the Android app. Just 3 outputs with leds to check and relays to switch whatever devices are required.
