There are monitors which only have three BNC inputs
and which use composite synchronization (‘sync on green’). This circuit
has been designed with these types of monitor in mind. As can be seen,
the circuit has been kept very simple, but it still gives a reasonable
performance. The principle of operation is very straightforward. The RGB
signals from the VGA connector are fed to three BNC connectors via
AC-coupling capacitors. These have been added to stop any direct current
from entering the VGA card. A pull-up resistor on the green output
provides a DC offset, while a transistor (a BS170 MOSFET) can switch
this output to ground. It is possible to get synchronisation problems
when the display is extremely bright, with a maximum green component.
In
this case the value of R2 should be reduced a little, but this has the
side effect that the brightness noticeably decreases and the load on the
graphics card increases. To keep the colour balance the same, the
resistors for the other two colors (R1 en R3) have to be changed to the
same value as R2. An EXOR gate from IC1 (74HC86) combines the separate
V-sync and H-sync signals into a composite sync signal. Since the sync
in DOS-modes is often inverted compared to the modes commonly used by
Windows, the output of IC1a is inverted by IC1b. JP1 can then by used to
select the correct operating mode. This jumper can be replaced by a
small two-way switch, if required.
This
switch should be mounted directly onto the PCB, as any connecting wires
will cause a lot of interference. The PCB has been kept as compact as
possible, so the circuit can be mounted in a small metal (earthed!)
enclosure. With a monitor connected the current consumption will be in
the region of 30 mA. A 78L05 voltage regulator provides a stable 5 V,
making it possible to use any type of mains adapter, as long as it
supplies at least 9 V. Diode D2 provides protection against a reverse
polarity.
LED D1 indicates when the supply is
present. The circuit should be powered up before connecting it to an
active VGA output, as otherwise the sync signals will feed the circuit
via the internal protection diodes of IC1, which can be noticed by a
dimly lit LED. This is something best avoided.
Resistors: R1,R2,R3 = 470Ω
R4 = 100Ω
R5 = 3kΩ3
Capacitors:
C1,C3,C5 = 47µF 25V radial
C2,C4,C6,C7,C10 = 100nF ceramic
C8 = 4µF7 63V radial
C9 = 100µF 25V radial
Semiconductors:
D1 = LED, high-efficiency
D2 = 1N4002
T1 = BS170
IC1 = 74HC86
IC2 = 78L05
Miscellaneous:
JP1 = 3-way pinheader with jumper
K1 = 15-way VGA socket (female), PCB mount (angled pins)
K2,K3,K4 = BNC socket (female), PCB mount, 75Ω .
