These boards are an easy "single circuit board" solution to managing multiline "1A2"
business phones popular in the 1950's - 1980's in the United States and other countries,
collectively called "key systems".
The current version of this board is REV-H.
Older board revisions are, in progression of new-to-old:
Click here for a complete history of previous board revisions.
Latest to oldest:
REV-H (Sep 2019)
This revision removed JP1 and JP2 as clearance for an on-board 5mm
x 20mm glass fuse. Shrank component footprints for C5-C7, R19-R24,
R25-R27 so that resistor leads can be folded against the package.
Added extra trace clearance around JP3/JP4 pads for T1/R1 and T1
and B8. Added unused pin on SECONDARY (JP4) to drive unused inputs
on CPU1 and CPU2 in case firmware needs to know if the current board
is configured as a secondary (unused currently, may be needed in
future). See this animation
comparing changes from REV-G1 to REV-H.
These were printed by WellPCB in Matte Black.
REV-G1 (Jul 2019)
This revision has an electrical fix adding 4 flyback diodes to suppress
inductor backcurrents from the buzzers to protect the TIP125 power transistors. Otherwise, changes
were made to facilitate easier board assembly: replaced the 4 surface mount resistors with through-hole
equivalents (SMD components are annoying to solder), pad sizes increased for CN9/JP3/JP4, extra resistor
pads added for different length options, spacing of resistors R25/26/28 increased for easier lead trimming
during assembly. These were printed by PCBWay in Matte Black.
REV-G (Jun 2019)
This revision has a small fix for an issue in Rev-F: when two boards
were interlinked, the lamp flashing for hold and ringing wasn't synchronized between the two
board's lines. The upper left pin on the interlink connector was changed from an extra ground
to carrying the sync signal. Also added: a 10K pullup (R28) for Q6's input, solder pads for JP3
and JP4 were enlarged slightly, surface mount resistors R9/10/11 were given more clearance
around Q2, and various traces were moved to simplify routing, but made no electrical change.
The cpu1/cpu2 signal tables on the silk screen had their columns aligned better. The first run
of these Rev-G boards were printed in Matte black, using WellPCB.
REV-F (May 2019)
This revision adds an optional ribbon cable "interlink connector" that allows two boards
to be combined with a single 30 pin ribbon cable to manage 4 lines and 8 extensions. There are two
32 pin jumpers to let the user select which board is which. To enable rotary phone support, another
PIC chip and optocoupler was added, allowing me to remove the components IC10, RN3, and Q12 through Q15 that were in REV-E.
This left room for the connectors and jumpers needed for the new Interlink feature. These boards were printed
in purple by PCBWay.
REV-E (Apr 2019)
This revision replaces about 50 discrete components
that were in REV-B with a single chip CPU ("PIC" chip). And this time I used
Sprint Layout 6.0
to construct the board (instead of ExpressPCB), which let me save out Gerber files that could be shopped around
to different board printing services, bring the cost of 3 boards down from $380 to $60 or so, greatly reducing the
board printing costs. These boards were printed in blue by PCBWay.
One of my beta testers requested I resurrect a bridging feature I'd described I had designed but not yet implemented,
allowing one to easily combine two boards to double the lines and extensions. They also asked if rotary phones could
be supported for intercom dialing. So I came up with REV-F..
REV-B (Feb 2018)
This revision was the first board I started with,
and is all analog components. I purposefully avoided using any CPUs in that design, to see if I could
use technology only available in the 1980's when these phones were still popular.
These boards were printed in green using the ExpressPCB board printing service.
But when people started asking if I could sell them this board, I realized it was too hard and expensive
to build in any quantity. So I went back to the designing board; I simplified the circuit, and used
different board layout software so that more cost effective print services could be used. The result being REV-E.