In Reply to: Need help to implement and design Alarm sensors circuit using microntroller and telephon line posted by Khalifa on 02/06/02 at 2:42 AM:
To interface to the telephone line you need a hybrid or DAA (Digital Access Arrangement), which performs the two to four wire conversion. To dial out, you need a DTMF generator (however, by carefully choosing your microcontroller's clock, you can generate the DTMF by SW, if you are so inclined). There are many published apps. for these around. However, before jumping into this, define your requirements fully:
For example, if the controller is just to report some go/no-go conditions to the remote phone, then you can probably use the DTMF itself for signalling (with an DTMF decoder at the other end). You can use synthesized voice also. If more substantial data reporting is required, you are much better off using an external modem, or modem module, on both ends. That also solves your phone line interfacing problem too. It also solves the problem of what if the phone is not answering or the line is busy or there is no dial tone etc.