Hello Guys,

I am working on an 8051 based secure telephone set, and have come across a problem. I hope you people advice me with your knowledge and experience. Although the question may seem off topic, but I request you to give your valuable ideas/advice.

Question:

What is the accurate and cheap solution to detect the presence of a parallel telephone set on the telephone line while my secure phone is already connected (in off hook state) to telephone line.

What I have learnt from my experiments is that observing the line volatge does not solve the problem, since there is no appreciable change in line voltage.

Also I know that any measurement ( V or I) should be relative and not absolute, i.e volatge/current varies from line to line, so there is no rule of thumb.

Your ideas/advices will be highly appreciated.

Regards

IJAZ