[GreenKeys] sneak peek

[John Nagle]

    OK, that looks straightforward. Bluetooth radio on the left,
four phone jacks on the right.  There are two TTY circuits,
with send and receive.  The 6-PIN DIP components vertically
down the center are presumably optoisolators.  (Expecting them
to burn out? They're the only socketed components.)

    At the lower center there's what looks like a small
switching power supply.  This is presumably generating
loop current.

    This looks like a traditional "generate loop current
and lose most of the power in the ballast resistors"
circuit.  It's not a photoflash-type charge a capacitor
and dump it to get the pull-in current, then sustain
from a low voltage supply.

    Somewhat puzzled about the resistor values.
1500 and 1200 ohms in parallel is 666 ohms.

    U3 is a microcontroller to control the
Bluetooth radio and handle data conversion. Not
sure which one.  Pins 14-22 are marked as analog
outputs, which knocks out the ATmega 32L, the
PIC 18F1455, and the ATmega 328.
Its clock crystal is directly above it.

    J9 looks like a memory card socket for
the microcontroller.  There's provision
for a USB-A socket at the lower left, but
it's just for power; the data pins aren't
connected.

    Nice.

			John Nagle