1A2 Multi-Line Phone Control Card

1A2 board
1A2 Multi-Line Telephone Control Board (REV B).
Supports 2 lines, 4 extensions, standard 1A2 features for ringing, Intercom with dial-pad buzzing.


This board was a personal weekend project in the mid 2000's that turned into a full on serious endeavor that ended up as a somewhat professional board. This board controls old 1A2 multi-line business telephones that were made available by the AT&T / Bell Telephone company during the 1960s through the mid 1980's. It can operate their standard features, interfacing them to modern phone lines, either PSTN (Publicly Switched Telephone Networks), or the more popular modern VoIP systems offered by most phone and cable providers, which usually include an interface box for regular phones.

Basically this is a "single board KSU" (Key System Unit), similar in function to what the original AT&T 551a KSU “shoebox” does, but small, light, and with some added features. Uses a simple 12v DC/1.5A power supply (wall wart) to operate all features of the phones, including lamps, buzzers, and ringers. The board provides standard 50 pin Amphenol (or “Telco 25 pair”) connectors on board, allowing phones to either be directly plugged in (for demos), or to long Male-to-Female CAT3 extension cables for an actual home or office setup with remote extensions.

This board provides the usual 1A2 features, plus some extras:

Just add a 12v supply, some 1a2 phones, attach to POTS compatible phone lines, and it should all work.

Here's a short 3 minute video demonstrating the board's features in action, showing 2 phone lines and 4 AT&T 1A2 telephones:


Why do this? To bone up on analog circuit design. It was a design goal to NOT use computers on this project (no PIC chips, Arduino, Raspberry Pi, etc), and use only chips/techniques available from around the 1980s. Presented below are the schematics, board design, circuit description, and user's manual, so that one can print and build these boards on their own like I did.

I heavily referenced the Bell System Practices manuals (BSP), books on telephone design, line card patents, and various sources on the web for old phone equipment. Also studied vintage equipment from the period these phones were popular; an AT&T 551a KSU, 400D line cards, ComKey 416 phones. When I ran into trouble with analog design issues, I often conspired with a friend well versed in EE and analog circuit design who kept me on the right track.

Here's a shot of an early prototyping board circuit, along with REV A and REV B versions of the board:



Here's the PCB layout, showing the silk screen and component/solder side traces in the ExpressPCB software:



Here are the schematics for REV B of this board (click to view full resolution):

..and here's a REV B circuit description which walks you through how the above schematic works.
The circuit description is mostly complete, but has a few sections that are a work in progress.

Users Manual

ExpressPCB Board File + Editing Software

Note: This REV B1 file has a fix that my original REV B printed board didn't have. As shown below, REV B (left) had the symbols for the "BELL CALL" diodes (yellow) on the silk screen reversed. This was fixed in REV B1 by rotating the symbols without changing any of the trace wiring (right). This fix is in the .pcb file above.
REV B - diodes in wrong direction
REV B1 - diodes in proper direction
- - - Work In Progress - - -

For questions or if you find mistakes in any of the above, contact erco@seriss.com.