Re: who can solve my problem
Hi ajaykumar,
Ring detector
The ringing signal is 70-90V RMS 20-25Hz.
Ring detector basically detects whether the incoming signal ac signal signal magnitude is more than some set threshold level.
One circuit idea is to apply the ringing ac signal to a rectifier and convert into dc.
Compare with a reference.
Don't forget to isolate the rectier from line DC voltage by decoupling capacitor.
(Open circuit telephone line voltage can be as high as 50V DC)
DTMF decoder
I have used MT8870 DTMF decoder chip.The cicuit diagram provided by the chip manufacturer works fine.
Link MT8870 data sheet.
http://www.chipdocs.com/datasheets/datasheet-pdf/Mitel-Semiconductor/MT8870.html
Off hook detector.
The exchange detects a loop current of 13.5mA to 16mA in off hook condition.
Less than 7.5mA is considered as on hook condition.
So by measuring the current through the telephone line we can conclude whether the circuit is in OFF HOOK condition or not.
Unlike ring detector and DTMF detector off hook circuitry is not tested.
Do reply me if you want more info on these circuits.
Regards,
Sudhir