VECTREX 3D GOGGLE ELECTRONICS DESCRIPTION

If you have any comments or questions about this file or the 
schematic, please direct them to zacketh@ibm.net

                                 Zack Ethridge
                                 12/26/95

The 3D goggles are fairly simple, mechanically.  They consist of
an approximately 1"x2" circuit board which controls a small dc
motor.  This motor turns a color wheel which is opaque over half
of it's area and red, green, blue on the other half.  As the
color wheel spins it alternately covers each eye which allows the
Vectrex to produce the 3D effect.  As each of the 3 different
colors go past the eye, the Vectrex draws the object on the
screen which will be seen as that color.  Simple but effective.

The Vectrex sends a stream of pulses which controls the speed of
the motor using Pulse Width Modulation.  The feedback from the
goggles is the pulse from the index hole.  The Vectrex alters the
negative width of the pulses it outputs until the index pulse it
receives is approximately 27 Hz.

There are 3 circuits on the circuit board.  See 3DGOGGLE.GIF or
3DGOGGLE.BMP for the schematic.

Circuit component values are as follows:
R1 - 3.3K     R6 - 3.3K    C1 - 47 uFd  10v
R2 - 1K       R7 - 220K    C2 - .1 uFd
R3 - 150      R8 - 3.3K    C3 - .1 uFd
R4 - 27K      R9 - 1M      C4 - .1 uFd
R5 - 47K     R10 - 10
Q1 - PNP driver transistor
     (Radio Shack 276-1604 [2N3906] works here)
Q2 - Photo transistor
     (Radio Shack 276-145A works here)
D1 - generic diode (1N94 should work)
D2 - generic LED (red)
U1 - LM393 5v dual opamp
M1 - small pm dc motor (no gearing)

One circuit consists of C4, R9, D1, U1a, R6, R1, R2, Q1, C3, C1,
C2, R10, and M1.  This circuit takes the pulse from the Vectrex
and uses it to drive the motor.  The wider the negative input
pulse, the faster the motor spins.   

The next circuit consists of R3 and D2.  This circuit is the LED
which shines through the index hole in the color wheel.

The last circuit consists of Q2, R4, R5, U1b, R7, and R8.  This
circuit senses the index pulse light through the color wheel and
shapes it into a negative going pulse output to the Vectrex. 

That's all there is.  The Vectrex does the speed correction, and
uses the index pulse to time the screen draws to get both the
color and 3D effects.   

The Vectrex uses 4 lines to connect to the 3D goggles.  Looking
at the end of the goggle connector the pins are numbered like
this:
        1 2 3 4 5
         9 8 7 6.  

Pin 8 is the ground.  Pin 7 is the +5vdc.  Pin 3 is the signal
in, from the Vectrex.  Pin 2 is the signal out, from the goggles.

Both the signal in and the signal out are negative going pulses. 
During the Narrow Escape game, the pulse repetition time for both
pulses is 37.5 msec.  The signal out pulse width is 1.05 msec. 
The signal in pulse width bounces between 12 and 16 msec.

SIGNAL OUT:  a - 1.05 msec 
             b - 37.5 msec

           :---------b---------:
           :-a-:               :
+5vdc______     ___________     ________
           |   |           |   | 
           |   |           |   |
Gnd        |___|           |___|


SIGNAL IN    a - 12-16 msec
             b - 37.5 msec

           :-----------b-----------:
           :--a--:                 :
+5vdc______       ___________       ________
           |     |           |     | 
           |     |           |     |
Gnd        |_____|           |_____|


During the startup sequence at the beginning of the game the signal in 
pulse train seems to be very high and have about a 50% duty cycle.  I 
believe this happens in order that the motor will get up to speed quickly.
Once the wheel is spinning near normal speed, the pulse train changes 
to the normal signal, apparently synchronized to the output pulse train.