🚀 go-pugleaf

I recently posted this to comp.sys.cbm for the Commodore 128, but just realized
that it might be useful to the general microcomputer community.  It is a 
simple circuit which accepts RGBI input (with NTSC sync, such as produced by
a PC with Color Graphics Adapter, not EGA) and produces a monochrome composite
output with 16 grey levels (green, amber, whatever) suitable for use with a
standard NTSC monochrome monitor.  Sync is not handled with great elegance, so
some monitors might show some flagging at the top, but this is not my exper-
ience.  The schematic is included below.
Sincerely,
Frank Prindle
Prindle@NADC.arpa
-------------------------------------------------------------------------------

             Digital RGBI to Monochrome (Gray Scale) Adapter (NTSC)
             ------------------------------------------------------

RGBI(DB9)
Connector
---------                             (stolen from computer or separate supply)
            * 100    DIODE                                     +5 Volts
Pin 3  R>---/\/\/\/--->|----+                                     v
                            |                              680    |
            * 220    DIODE  |                         +--/\/\/\/--+
Pin 4  G>---/\/\/\/--->|----+                         |           |
                            |                         |  2N2222   |
            * 300    DIODE  |      100         100    |     |-----+
Pin 5  B>---/\/\/\/--->|----+----/\/\/\/--+--/\/\/\/--+-----|
                                          |           |     |->---+---------->
            * 240                  47     |           |           |     Video
Pin 6  I>---/\/\/\/---------+----/\/\/\/--+           |     +-----+      Out
                            |                         |     |            To
                            |      33                 |     |    82    Monitor
                            +----/\/\/\/--+           |     +--/\/\/\/--+---->
                                          |           |                 |
                                          V           |                 V
                                        (gnd)         |               (gnd)
                         2N2222                       |
              100           |-------------------------+
Pin 8  H>---/\/\/\/---------|                         |
                            |->---+                   |
                                  |                   |  Note: + denotes a
                                  V                   |  connection; there
                                (gnd)                 |  are no crossovers
                         2N2222                       |  in this schematic.
              100           |-------------------------+
Pin 9  V>---/\/\/\/---------|
                            |->---+
                                  |
                                  V
                                (gnd)

Pin 1   >------------------------------+  Note: Carry both grounds through
                                       |  and tie both to ground to avoid
Pin 2   >------------------------------+  signal degradation.
                                       |
                                       V
                                     (gnd)

* These four resistors determine the gray scale.  As specified, they provide
  16 different gray levels, some of which are only barely different - red is
  most heavily weighted, green next, and blue is lightly weighted.  To allow
  experimentation or customization of gray scale, mount these resistors on
  a DIP socket or use 500 ohm trim-pots.

> Drawing schematics with ASCII characters is an imprecise art.  If you don't
  see three (3) transistors above, keep looking; they are there!  I believe
  the diodes I used were germanium - if you use silicon, the picture may be
  dimmer overall unless you adjust the weighting resistor values.