Schematic:

Telephone interface with Ring Detector realized with the TS117 that is also occupying to couple the line (HOOK). A particular attention has been given to the protections. After a first GAS discharger primary protection (towards EARTH), we have a faster secondary protection TRISIL and then the classics 2 DIODE in anti-parallel. The rele, the optisolator and the TS117 guarantee also the necessary isolation to the several tests of homologation.

In some SAFETY fairs I have seen phone dialers of the respectable --- (I will not say the name) with homologation certificate, when, giving a glance (as a good planner) to the card, they were obvious a BUNDLE of tracks passing under the otpo isolators.....excuse the vent!

The PULSE command, at last, takes us to the times of the product realization.... Besides DTMF, the composition of the telephone number, happens also for impulse (PULSE)

In this page of the schematic you can notice the connection with the 7 SEGMENTS DISPLAY through the serial registry 4094, then all the analogical part that takes care of the RECORDING/REPRODUCTION of the audio signal (the alarm message). This part has been realized with an integrated ISD DAST that, through the node (R30,R31) brings the signal to a first filter (U5B) and then to a second filter with the Vref slightly moved towards the bottom in order to increase dynamics (we are working with the feedings of the op-amp from 0 to 5V).
To the node (R30,R31) ends also the DTMF signal coming from the EFG71891 generator.
While the signal coming from the BOURNS transformer is being filtered by U5C and "squared" by U5D for being presented in the correct shape to the Micro PIC, through the signal TONE/IN.

Here we have the PIC Microcontroller, the NOT volatile memory EEPROM 24C01 where the phone numbers will be saved and the various settings. Finally the keyboard 4X4 (16 keys) read to matrix.