TELEPHONE_RING_SIGNAL_DETECTOR

Published:2009/6/24 3:45:00 Author:Jessie | From:SeekIC

Discrirrtinating between telephone-ring signals on a phone line can be accomplished by using dedicated ICs, such as AT&T's LB1006AB or Texas Instruments' TMS1520A. However, if the system already is using a microcontroller, those dedicated chips can be replaced with simpler hardware and a few bytes of code.Looking at the setup, the ringing voltage pulses the optoisolator's LED, which, in turn, pulses the low-asserted RD line to the microcontroller. The firmware analyzes the pulses to determine whether a valid ringing signal is present.The frequency limits of a valid signal are 20 to 80 Hz, which is modulated 2 seconds on and 4 sec-onds off (with distinctive ringing, though, this cadence can vary). Therefore, the simplest analysis is to count down at least 20 pulses of RD in 1 second.The routine could be expanded to determine what type of ring signal is present in a distinctive ring setting. Such a system could switch the phone line to various output jacks. As a result, several phone devices could use the same line without first picking up the line to determine if it's avoice, fax, or data call.



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