Arduino is on open-source electronics prototyping platform that lets you make interactive stuff without having a degree in electrical engineering.
For about $25 you can buy a credit card sized circuit board that has input connections (for buttons, knobs, light sensors, microphones, humidity sensors, fart detectors, Internet signals, etc) and output connections (for servo motors, LEDs, buzzers, speakers, stepper motors, vibrators, etc). You write programs on your computer to tell the Arduino how to process the input signals and how to activate the output components. This program is uploaded to the Arduino's microprocessor, making it self-contained.
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An interesting link found among my daily reading