References: Rotary Encoders, Touch Sensors, and Audio Input
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Rotary encoder - Wikipedia - Explains quadrature encoding and how two phased signals reveal direction and steps. Directly supports this chapter's encoder-reading logic.
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Capacitive sensing - Wikipedia - Describes how touch sensors detect a finger by measuring capacitance changes. Reinforces the chapter's touch-input section.
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Fast Fourier transform - Wikipedia - Covers the algorithm that converts an audio sample buffer into frequency components. Background for the chapter's audio-input and spectrum work.
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Make: Sensors - Tero Karvinen, Kimmo Karvinen & Ville Valtokari - Maker Media - Hands-on projects with encoders, touch, and sound input that parallel the chapter's hardware.
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Practical Electronics for Inventors (4th Edition) - Paul Scherz & Simon Monk - McGraw-Hill - Reference on the signal conditioning and input electronics behind the sensors in this chapter.
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Rotary Encoder Tutorial - Last Minute Engineers - Step-by-step guide to reading an encoder with polling and interrupts. Reinforces the chapter's CLK/DT direction logic.
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Adafruit MPR121 Capacitive Touch Sensor - Adafruit - Guide to a 12-channel capacitive touch breakout over I2C. Directly supports the chapter's touch-sensing examples.
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machine.Pin - MicroPython - Reference for the interrupt-driven pin reading used to track encoder rotation. Connects the chapter to working MicroPython code.
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machine.ADC - MicroPython - Reference for sampling an analog microphone signal, the first step in audio input. Supports the chapter's audio-capture section.
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An Interactive Guide to the Fourier Transform - BetterExplained - An intuitive, visual explanation of how signals break into frequencies. Makes the chapter's FFT concept approachable for beginners.