Code Explanation; Extension Practice - Keyestudio Ks0428 Manual

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Then click"Write", open serial monitor to view if there is a "0"
signal

(7) Code Explanation

Serial.available() : The current rest characters when return to
buffer area. Generally, this function is used to judge if there is data
in buffer. When Serial.available()>0, it means that serial receives the
data and can be read
Serial.read()：Read a data of a Byte in buffer of serial port, for
instance, device sends data to Arduino via serial port, then we
could read data by "Serial.read()"

(8) Extension Practice

We could send a command via cellphone to turn on and off a LED.
D10 is connected to a LED, as shown below:
128