Hi Larry,
To add to your graphic, there is no purple wire as the top rail is fed by 3.3v and bottom rail has the external power ranging from 12V to 0V
For context,
This is a signal from my electrical power company. 12V mean I am on a high $kw/h and low means I am paying less for electricity. I want to monitor that state and do other things with the signal, i.e. trigger a relay, send me an e-mail etc. etc...