AG-1621 have inbuilt two digital inputs which is having capability to detect input from 5V to 24V. Both the digital inputs can be directly accessed through gpio pins.
Testing Setup
Download the Testing Script from above mentioned link and unzip the "AG-1621_Script.zip" file in any folder. Go to "AG1621_Script" folder using "AG1621_Script/Scripts/inputs" command. If asked for password, then type "atreyo" and press enter.
blackmango@atreyo:∼$ cd AG1621_Script/Scripts/inputsRun modem start script, using:
sudo sh two_inputs.shIt starts printing output as per below screenshot (0 = connected and 1= not connected).
You can now connect a 5-30V signal source to the digital input and observe how the state changes in the terminal.
To stop running this script press ctrl + x shortcut.