piclist 2002\01\30\174052a >
Thread: Telephone ring and busy indication?
picon face BY : Russell McMahon email (remove spam text)

> Does anyone know of the simplest way to detect the following 2 condition
> on a telephone line?
> 1.      When it is ringing
> 2.      When it is busy/off hook.
> My output from the above circuit needs to be logic 1 when the conditions
> are true and logic 0 when they are false.
> The circuit must also conform to the proper specs for interfacing to the
> phone line with regards to line isolation.

General guidelines.

When a telephone connected to a normal analog exchange is ringing an AC
signal is imposed on one wire relative to the other.
This causes the polarity to reverse on negative ringing peaks. Detecting
such a reversal will indicate ringing. You could cheat somewhat and simply
detect a non positive signal.

When the phone is off hook the DC voltage across the line will drop to a
substantially lower level than when "on hook" Actual value depends on feed
bridge resistance, line resistance and telephone resistance. (The 1st and
the last may be generated electronically by eg a current source and current
sink  rather than true resistances).

As a first attempt you could have two devices (transistors, gates etc) which
turn off when the applied voltage falls below a preset threshold. The off
hook detector determines when the line voltage falls to a lower level and
the ring detector determines when the voltage falls to zero or below zero
during ringing.

       Russell McMahon

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