Hello, I have used the button as input in the PLC and it successed, but I want to ask could I use the sensor replace the button?
Like use a light sensor (output logic high when there is enough light) to control the red LED.
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A light sensor is probably an analog input. You can use it as long as you make sure it is electrically compatible with your board and that you wire it up to one analog input in your board.

The analog inputs are accessible with the located vars %IW0, %IW1, %IW2...

The best way to test it is to open Radzio Modbus, connect it to the OpenPLC and ask it to read Input Registers. Once you wire up your analog sensor, you will see some numbers comming up in one of the registers.
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