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[General] Detecting the telephone line presence without loading the telephone line voltage

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ThirunavukkarasuThoondi

ThirunavukkarasuThoondi

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Hi
we doing some Telephone dialer projects. in our projects we have change over the telephone line from two different telephone lines why because if one line is failed we have switch over to the next line. (which one is available)
we have to detect the availability of lines with out affecting their performance . How we can manage this give a suggestion
Thanks
 

Refer to international PSTN standards for allowable quiescent current respectively load resistance in on-hook state, e.g. ETSI EN300001. You'll arrive at about 1 MOhm for mains powered and 100 kOhm for line powered equipment. That's sufficient to simple make simple detection circuits with sensitive opto couplers or similar.
 
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