phone answering

[theEgO]

hello..
can somone give me some insights into how a telephone answer an incoming call??
im trying to built a circuit which can do so. my cicuit can detect incoming call and the relay is suppose to switch on once a call is detected. however when a call is detected my relay will just switch on and off alternately according to the ring. the relay is suppose to close the loop of the telephone line connecting the phone line to my dtmf decoder circuit. isnt that the way to answer a call??
plzz help!!

 

[spuffock]

I use a relay to switch an 820 ohm resistor across the line. This will hold the line open and develop enough signal voltage to drive the dtmf decoder. The ring signal will stop when the load is applied. You will need to keep the relay energised until you have the dtmf information you need. A microcontroller is a good idea.

 

[ljcox]

I suggest the attached circuit.

The relay operates to the rectified ring voltage and holds to the DC from the line once the ring is tripped.

The resistance of the relay coil should not exceed 600 Ohm.

R1 is to limit the AC current to whatever operate current the relay needs (read the relay spec).

Len

 

[theEgO]

i'm sorry but could u explain how the circuit works in detail for me?
what would be the output from the bridge rectifier? is it in the form of a high or low voltage state? is the relay shown use to complete the loop load cycle at the beggining of the line?
thanx!

 

[ljcox]

The ring voltage is about 90 Volt RMS superimposed on a 48 Volt DC potential.

The capacitor blocks the DC and allows only the AC to pass. The rectifier bridge rectifies the AC voltage and operates the relay. So the value of R1 is chosen to ensure there is sufficient voltage to operate the relay.

When the relay operates, the contact bypasses the capacitor and so a DC current flows and trips the ring. The relay then holds to the DC current.

The DC current through the relay holds the call.