FROM SCRATCH: Phone line in use indicator

Projects General Guidance
1

New Arduino user here. Be gentle :wink:

Here's the story:

I work from home. My office is in my finished basement. My daughter gets off the bus and comes down to my office (before I'm done with work) every day. Some days I have conference calls that run late. My ultimate goal is to create an LED indicator on the upstairs door that lets her know whether or not I'm on the phone.

My thoughts:

My phone knows whether the line is in use --even if it's on a totally different phone in the house. This leads me to believe there's a (hopefully a relatively simple) way to determine whether or not the line is in use.

Here's what I need:

How do I design a circuit to detect whether or not my phone line is in use? I've searched the forums here and I've yet to find any tips.

Thanks!

2

yea its silly simple, and has been a common hobby project for many years

example:
http://www.freecircuits.net/circuit-20.html

you wont really find much here as its just a handful of components. A single line phone uses two wires, typically red and green (red ring, green ground) when nothing is going on red is pretty much low, when you get a call in (ring) that joker will jump to 75-90 volts and has made me cuss many of times, and when your talking on it, its around 30-50 volts (all depending on what part of the world your in)