Throughout this book, you've built Arduino-based PLCswith 5V logic level outputs and control 5V based devicessuch as LEDs, speakers, and fans. But in industrial environments, you may use loads (devices) that are rated with high voltage (12V DC, 24V DC, 120V AC, or 240V AC) and high current (usually more than 1A).
An Arduino pin can supply 3.3V or 5V voltage levels with about 20-40mA of current. These voltage and current levels are useful for building prototypes to test the outputs with Arduino. You can connect various small loads such as LEDs, speakers, vibrators, and ...