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Answering a Phone Automatically

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cmancuso

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I am try to build a circuit to automatically answer a telephone line when there is an incoming call. Has anyone had any experience doing this that can offer some insight into how to answer the line and hold it in the off-hook state? What do I need to be doing in order to switch the tip and ring into something like an audio-amplifier or other such device?
 

dr pepper

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I've done this a couple of times.
The line is normally 50vdc, either polarity and when the phone rings there is 75v ac on the line.
If you feed the line to a bridge rectifier and then to a voltage detector, when the voltage exceeds say 60v you can turn on the led in a opto isolator via a zener diode circuit to give you a ringing signal.
T o off hook the line you need to put a 600 ohm resistance accross it, you can do this with a simple tranny and resistor, but your better doing it with an isolation transformer, you can get them for the job, I robbed mine from an old modem, and another from a fax machine.
You can also detect off hook the same way, as the line volts drop to about 12v when the phone is off hook.

Look at these:

Phone Line Ring detection circuit


Telephone line audio interface circuits
 

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