Help design ring detector, off hook & DTMF decoder for p

[ajaykumar988]

who can solve my problem

i need to design circuits for the following in my telephone project

Ring Detector
off hook
DTMF decoder

Please help me through whatever means.Thanks in advance

 

[aandavan]

Re: who can solve my problem

hi,


Use 555 timer circuit for ring detection and a relay for hook switch and m8870 for DTMF decoding. The circuits look so simple to interface. Moreover for hook switch you have to know the telephone circuit.

 

[Sudhir.K.A]

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

 

[ajaykumar988]

Re: who can solve my problem

Firstly thankx Mr.sudhir
Could you please give me detailed circuit diagram and description at my email address
collective.ajay at gmail.com

 

[Sudhir.K.A]

who can solve my problem

hi ajay,
I might have a soft copy of project report.If i get it i will send it to your mail id.

Otherwise i will send scanned circuits diagrams to your mail id.

regards,
Sudhir

 

[ajaykumar988]

who can solve my problem

dear Mr.sudhir
could you send it quickly.i desperately need them to complete the project in time.
my email add
collective.ajay (at) gmail.com

 

[siongboon]

Re: who can solve my problem

Some ring detector and DTMF circuit that I have found previously,
**broken link removed**