History of the Link Designs
The Original 2 Phone Link Design is the updated version of the very first Link circuit that I designed for Silicon Chip magazine here in Oz, back in 1996. It was a bit hardware heavy, (nd therefore expensive for what it would do) having two heavy duty relays and two by two transistor oscillators, for ring tone and ring impulsing. These items have been replaced with optocouplers and an NE 556 dual timer chip, as per the rest of the Link cicuits. It provides for basic communications between two phones only, and there aren't even any numbers to dial.
This being the first cab off the rank back in 96, there soon followed some requests for more phone extensions, and so the second Link design was 'born' and published in SC mag in June 1997.This version needed pulse dialling to access more than two handsets, and this was where the 'fun' really began. Trying to use phones that had been designed back in 1964, with garden variety '$2.00' off the shelf electronic chips and 50 cent components, proved a headache for me at first. After I learned a few tricks (mainly correct biasing, filtering and timing techniques) the rest became rather simple.
After that second success, the need for an outside line materialised, and this was accomplished by using the 'recall' buttons (usually used by PABXs to access the outside line or special functions,) built into those old 800 series dial phone handsets here in Oz. While this would have worked for some, I couldn't guarantee to a worldwide Internet audience that the wiring would be the same in handsets availabe to them, and so another approach was needed. Remote control applications were also a consideration, ande were more easily achieved with DTMF than with dial pulsing techniques.
Dialling a special access code (like a '0') to get the outside line obviated the need for an extra hard wired button, and saved on wiring, reducing the arrangements in the more advanced pulse dial Link from six wires for each handset, (two for speech and dialling, two for ringing the buzzers and two for the recall button) down to two wires for the next DTMF version, the A2B+1.
Modifying this circuit again for internal use only, has produced the Link 4+0, which will be the last in the series (for now, anyway.) there are crosspoint matrix CMOS chips that can be used in conjunction with PIC processor chips to go beyond the limitations of these simplistic circuits, but I'll have to leave that area to more knowlegable people that I.
Thanks once again Mike, for helping me get these designs on the Net, and I do hope that many people out there in Cyberspace will use them for their own needs, without all the hassles I went through to get this far.