automatic telephone off-hook

[vinodquilon]

Find the attachment of DTMF decoder (https://www.scribd.com/doc/27053839). I am using OEP1200 600/600 as the isolation transformer.
It has the property of blocking DC current through windings.
On detecting first ring voltage, BC547 will turn on line-relay to close line L2.

I have the following 2 doubts,

1. On turning on the line-relay, the resistance between the two lines L1 & L2 would be
the equivalent resistance seen on the primary of the transformer (600 ohm).
Does this resistance value works well to reduce the voltage across ring and tip lines so that
telephone will automatically turns into off-hook sate ?

2. Where can I fed-back some audio beeps to hear on the caller end after off-hook, on primary or on secondary
or on directly to L1 ?

I don't want to use separate Telephone Line Interface Modules (DAA's).

 

[betwixt]

Passing line current through the transformer is not advised. If the relay has two poles, keep one as it is and connect the other pole so it connects the resistor directly across the line wires. It isn't the ideal solution but it should work. The danger with your present design is when the relay closes, the transformer core will saturate and very little tone signal will reach the secondary side.

If you want to feed tones out of the circuit, I would seriously consider using a better clamp circuit across the line and then modulating it.

Brian.

 

[vinodquilon]

Passing line current through the transformer is not advised. If the relay has two poles, keep one as it is and connect the other pole so it connects the resistor directly across the line wires. It isn't the ideal solution but it should work. The danger with your present design is when the relay closes, the transformer core will saturate and very little tone signal will reach the secondary side.

If you want to feed tones out of the circuit, I would seriously consider using a better clamp circuit across the line and then modulating it.

Brian.

Dear Brian,
Please note my quote "I am using OEP1200 600/600 as the isolation transformer.
It has the property of blocking DC current through windings."

Audio-frequency transformers saturating when direct current flowing through the windings. So I think when the relay closes, the transformer core wouldn't saturate.

Would you explain your tone generating concept through a schematic ?