This app is a Bluetooth serial plugin for Tasker. It allows you to send (but not to receive) messages over Bluetooth serial to your paired devices, for example to control your Arduino. Messages can be regular strings or binary messages, and can be dynamically set using variable names as well.

Note: this app is complete free (open-source, no ads), so even though I will attempt to resolve issues, I cannot guarantee timely updates.

Source code: https://github.com/giech/arduinotaskerbluetooth
Relevant blog post: https://ilias.giechaskiel.com/posts/bluetooth_serial/index.html