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Send serial commands to your project from any device with a headphone jack that supports HTML5 and JavaScript. Does potentially require a few components to be added to your project. Designed for reliable results by default, but allows many adjustments. Uses 9600 8N1 by default. The baud rate is easily changed. The library is heavily commented to assist with any tweaking you want to do.
For schematics of how to connect to your MCU/MPU, see...