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sensors - Connecting two Arduinos via I2C while I2C Pins A4/A5 are already in use - Arduino Stack Exchange
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Amazon.com: CQRobot Ocean: MCP23017 IO Expansion Board Compatible with Raspberry Pi/Micro:bit/Arduino/STM32 Motherboard. I2C Interface, Expands 16 I/O Pins, Up to 8 Expansion Boards Can be Used Simultaneously. : Electronics
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