+----------------------------------------------------------+
|                  Peter's minimalistic                    |
|                "Two-Color LED" Solution                  |
|           (compatible with existing stuff)               |
+----------------------------------------------------------+
                                                            .
                                                            .
                                                            .
                    +-----+------o HD (Pin 4)               .
                    |     |           Op-Panel              .
                   +++    |            Cable                .
               47K | |    |                                 .
                   | |    |                                 .
                   +++    |                                 .
                    | B   |                                 .
                 +--+--+  |                                 .
               E | 2SA |  |                                 .
          o------+ 608 |  |                                 .
        +5VDC    |(PNP)|  |                                 .
  from existing  +--+--+  |                                 .
      LED         C |     |                                 .
                    o     o                                 .
                    |     |                                 .
             Green \T/   \T/  Red                           .
              LED  ---   ---  LED                           .
                    |     |                                 .
                    +-----+                                 .
                    |                                       .
                    o                                       .
                    |                                       .
                   +++                                      .
              330R | | existing                             .
                   | | resistor on                          .
                   +++ panel card                           .
                    |                                       .
                    |                                       .
          o---------+  GND                                  .
                                                            .
------------------------------------------------------------

                * Principle of Operation *


You need to unsolder the existing green LED.

The +5VDC contact needs to be fed to the above circuit.

Only the common cathode of the bi-color LED gets soldered
back in the place where the original LED cathode was.

The two LED-parts use the existing 330 Ohms resistor in
common. The PNP transistor gets minus-potential over the
47K resistor and the green LED lights up. Even when the 
HD-pin does not feed minus to the resistor (tri-states)
the GND connection is given over the red LED part and the
current-limiting resistor, so that the green LED will 
light under any circumstance.

Once the HD-pin gets positive (on HD operation) the 
positive +5V pulse will turn Base of the transistor
positive and the green LED goes blank - the red LED is
then on and indicates HD activity.

The type of the PNP-transistor is uncritical.
Any type will do. I used this 2SA608 while I had it ...

In case you want to return to original condition you
only need to remove the circuitry and resolder the
single green LED.

Pretty easy, eh ?


------------------------------------------------------------
     (C) 1999 by Peter H. Wendt ( )
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