Here's a pic of the history of the project..

From top to bottom, the R&D prototyping boards which I started around the time I joined this forum, so exactly 10 years. Spent years on and off (mostly off) noodling the circuit and reading up on analog/phone circuit designs, studying patents, BSPs, product schematics. Then I started designing the board's circuit layout. At some point last year the project's inertia finally picked up to where I felt ready to print some boards, which became Rev A. Made some fixes, and a month later printed Rev B which I'm happy with:

[Linked Image from seriss.com]

I've done a bit of industrial circuits before, and this was no different, albeit more challenging. In particular, I found patent #4,093,830 the most inspiring; here's my version of it as a sketchbook drawing I made early on in the project:

[Linked Image from seriss.com]

It's a very simple line card circuit, and put me on the road to using a similar pair of optocouplers/pair of relays. I also liked the circuit description. It's clever use of transistors and the rarely used pin 6 on the optos impressed me too, but I didn't use that in my design, preferring my own techniques.

To layout the board, I used ExpressPCB's free software, which I highly recommend; very easy to learn and use. And their board printing service is quite good. Using that software locks you into using their service though; you can't take a file made with that software to other board printers. But this was fine with me; their printing service seemed fine, and I don't usually switch printers once I find one I like.

One of the problems I caught in the Rev A board: I took what I thought was a clever shortcut, using negative logic to run the lamps. This involved controlling the lamps via the ground wires, and tying the lamp signals common. This seemed OK because in the 2565 phone I used for R&D, the pairs of lamp wires were isolated. But turns out that /didn't/ work for the 2564 phones, which buss the lamp ground inside the phone. So fixing that involved switching to positive logic; NPN transistors became PNP.. which changed the gate logic a bit, causing more components. So Rev B works for all the 1A2 phones I had handy; 2564,2565, and 2830. Changed a few other things too, including powering the DTMF circuit only when the intercom is in use.

I'm pretty happy with the "Rev B", but will probably do a "Rev C" just to add a feature for the PowerDSINE ring generator, so it can be added in such a way that it's only powered when there's ringing, and completely off when the system is idle, so idle power consumption is low.

My goal with this project was to use old chips that were available in the 80's. No one-board computers or software. I could have easily used a PIC chip, Arduino, or RaspberryPi to do a lot of the timing and oscillators.. but I wanted to learn more about analog electronics, so working within that design goal allowed me to immerse myself in analog a bit more than usual. Consulted with an EE friend from time to time for general guidance.

I standardized on LM339 ICs as the main component for the logic and oscillators; these are "comparitors", a flexible gate-like device that are like opamps, but tailored to this type of analog work. I used power transistors that are overrated; they don't get hot or even warm during full use. Wanted to avoid having too many relays. I used relays where it made sense, such as redirecting tip/ring paths and ring voltages. Originally I'd used solid state for ringing, but decided against it for cost and flexibility.

My goal of avoiding computers was so that one could just assemble this board and it would work; no USB programming or software. Just wire it up and go. All the "programming" is in the circuit traces, and choice of components.

Another design goal was to have the board be easy to setup phones without 66 blocks, and use for the most common use; up to two lines, up to 4 phones, with programmable ringing and intercom with simple TouchTone intercom signaling. Basically all the features one would want out of a 1a2 system for a typical small office with front office receptionist and other employees. Intercom was important I felt as a default. Basically tried to get as close to a ComKey setup, but using 1A2 phones.

Will try to follow up with a short video showing 4 phones and 2 active lines, demoing ringing + buzzing + lights. 1A2 is so simple, there's not much to show really.. but it's nice to see it all working.