Phone line status indicator
This simple circuit comes in very handy to check wheater a phone line is free, engaged or receiving a ring signal.
I used to service and install remotely read Time of use kWh meters all over the place which were read by cellphone or landline.
The reason I designed it was that after the installation a reading check was carried out and I was waiting for the meter to be read and never was sure if anything was in progress.
As you already know is that a phone line has a standard no load voltage of around 50 Volts DC.
So to set up the circuit the VU meter needs to read about ¾ schale.
R1 requires a value of around 400 Kilo ohms ¼ Watts. You may have to experiment with different values between 270 and 470 k ohms. as movements vary between 100 and 250 µA.
The bridge rectifier is to avoid having to swap the phone wires over in case the polarity is the other way around.
How it works. If the line is engaged the voltage is about 8 volts dc and the meter will give a low reading. If the ring signal is present it will give a high reading (ring voltage around 90 volts ac). and the no load voltage will give a ¾ schale reading.
I have 3 of these meters connected permanently to my phoneline at home and the loading on the phoneline is negligible.
One near the computer, the other two built in standard house phones.
It comes in very handy to see if the line is engaged to the internet while in the other part of the house.