|During the 22 years that I’ve been a part
of the laser display industry, I have offered my knowledge and
assistance freely. One of the things that I am known for is
helping people with their scanners. Laserists and scanner
manufacturers alike have been sending scanners to me for
years, so that I can tune them for optimal performance and
make other modifications when necessary. Most often, I do this
as a free service for ILDA members.
One thing that I have noticed in recent
years is that people are having more difficulty with
connecting the various components together to make a complete
laser projector. Incorrect connections can lead to distorted
images and other problems, which are then mistakenly blamed on
poor scanner tuning or poor component performance. Incorrect
connections can also lead to a dangerous projector – for
example, one which will output a beam even when it is not
connected. For this reason, it is quite often that when I
receive a projector or scanning system to tune, I wind up
re-wiring the components so that the projector can produce
perfect images. Since so many people seem to have difficulty
in this area, I decided to write an article that shows the
proper way to connect the components together.
This article is intended to be a guide for
projector manufacturers on the best way to assemble components
to create a laser projector conforming to the ILDA Standard
Projector (ISP) specification. It may also be interesting to
many laserists, as it provides a good insight into how
projectors work and the components involved. When discussing
the connections inside a laser projector, it is best to
conceptually separate these connections into two categories:
Connections that are related to power supplies (referred to as
“Power Connections”); and connections that are related to the
ILDA DB-25 signals (referred to as “Signal Connections”). We
will discuss the power connections first.
shows the general manner in which power connections should be
made between the various components. We will discuss the
connections for the scanner power supplies first, because they
are at the top of the diagram, and also because once that
foundation is laid, the other power connections can be easily
the projector from the AC Mains Supply. An integrated switch
and fuse assembly is often used as shown. The international
color standard specifies the brown wire as the LINE (or “Hot”)
lead, and the blue wire as the NEUTRAL lead. These are
connected to the power supply LINE and NEUTRAL connections
respectively. If you look closely at the power supply, you
should see the words “Line” and “Neutral”, or letters “L” and
“N” that designate which wire goes where.
there are AC power cords that do not have polarized plugs, and
these two may become swapped at the point that the projector
plugs into the wall. However, you should still maintain a
sense of “Line” and “Neutral” throughout the laser projector,
always connecting the “Line” terminals of all power supplies
to the same (preferably brown) wire, and “Neutral” terminals
to the other (preferably blue) wire. The most important
difference between the “Line” and “Neutral” connections, is
that the “Line” connection is the one that you use when
connecting a switch and fuse in line with power supplies.
projectors whose optical output power is relatively low (for
example, a few hundred milliwatts) and projectors that have a
plastic enclosure, there may only be two AC power wires (Line
and Neutral) and the Ground wire may or may not be used. For
projectors that have a metal enclosure, a Ground wire
(international color code is green as shown) must always be
used and connected as shown.
Connections diagram shows two separate power supplies used to
generate +24V and –24V for the scanner amplifiers, but this
could be embodied as a single power supply that generates both
voltages as indicated by the light gray box. In either case,
the +24V and –24V power wires are connected from these power
supplies to the scanner amplifiers. As with the power
supplies, the scanner amplifiers may also be embodied as two
separate single-axis amplifiers, or as one dual-axis amplifier
as indicated by the light gray box around them.
important part of Figure 1 is the “Central Grounding Point”.
This is really the single most important connections concept
inside the laser projector, and the thing that many people do
not initially understand. Basically, any component inside the
laser projector that requires a ground connection should make
a “home-run” to the “Central Grounding Point”. This connection
scheme is called a “Single-Point Grounding Scheme” which is
also known as a “Star Grounding Scheme”. The “Central
Grounding Point” should be located close to the power
+24V and –24V, which can simply be connected to the scanner
amplifiers in a daisy-chain fashion as shown, the ground
connections (designated PG in the diagram for “Power Ground”)
require more careful consideration, and must each make a “home
run” type connection to the “Central Grounding Point”.
When an AC
ground connection is used (green wire on the diagram), the
easiest thing to do would be to connect it to the “Central
Grounding Point” as well. This is because, often times this
“Central Grounding Point” is eventually (through “phantom
ground connections”) connected to the optical plate within the
projector. If “phantom ground connections” can be completely
prevented, then the AC ground connection could simply be
connected to the projector chassis, but not to the central
grounding point. This provides the best performance, but
requires the most careful projector design and layout, and
close attention to detail in terms of avoiding “phantom
ground” connections. See the section on “Avoiding phantom
ground connections” later in this article.
When an AC
ground is not used, or when the AC ground is connected only to
the projector chassis and there are absolutely no “phantom
ground connections” then ILDA DB-25 pin 25 MUST be connected
to the “Central Grounding Point”.
Note that the
ILDA DB-25 pin 25 should NOT be connected to the “Central
Grounding Point” if the AC ground connection is already
connected there, otherwise there will be a ground loop formed
external to the projector, which is undesirable.
WHY USE A SINGLE-POINT GROUNDING
why it is best to use a single-point grounding scheme, take a
look at Figure 2, which shows a power supply connected to a
single-axis scanner amplifier. Each of the wires that connect
the scanner amplifier to the power supply will have some
finite amount of resistance
easy-to-understand example, let’s assume that each wire has a
resistance of 1 ohm. During strong accelerations that occur
when the beam is commanded to jump from one side of the
projection area to the other, the scanner amplifier will
momentarily require high peak currents from the power supply.
Typically these peak currents could reach up to 4 amps. If the
wires that connect the power supply to the scanner amplifier
are 1 ohm, this will momentarily raise the voltage on the
“Power Ground” connection to 4 volts! (This is referred to as
“Ground Bounce”.) If this “Power Ground” signal is then
‘daisy-chain’ connected to another scanner amplifier, or to
other components inside the projector, then those other
projector components will also momentarily experience a
voltage of 4 volts, thus corrupting the signal integrity of
the other components.
single-point grounding scheme is used, the “Ground Bounce”
stays localized to the particular projector component that
generated it (in this case, the X scanner amplifier), thus
preventing the “ground bounce” from corrupting other projector
signals. And since the scanner amplifier has differential
inputs, the amplifier itself will be able to subtract out the
“ground bounce”, thus maintaining its own signal integrity.
illustrates why a single-point grounding scheme is the best
way to connect projector components. Without this scheme,
images often appear distorted, as shown in Figure 3:
A scan fail
monitor may also be connected to the scanner amplifiers as
shown on the diagram. This is required for audience-scanning
projectors, and might also be advisable for graphics
projectors as well. Most scan fail monitors connect to the
same power supply that operates the scanner amplifiers. In any
event, the ground connection of the scan fail monitor must
make a “home run” connection to the “Central Grounding Point”
as shown on the diagram.
POWER CONNECTIONS FOR THE LASERS
to showing the connections for the scanner power supply,
Figure 1 also shows the connections for the lasers. However,
unlike the connections for the scanner amplifiers, the
connections shown for the lasers are intended to be more
conceptual than literal. The reason is because, although
scanner amplifiers are highly standardized and generally all
require the same kind of power supplies and connections, the
lasers themselves may have integrated AC power supplies and
laser diode drivers, or may each operate on a separate power
supply and driver.
shows a single power supply operating three solid-state laser
diode drivers, but there may be only a single laser, or in
fact the laser may be an ion laser with completely separate
power supply. Nevertheless, the diagram shows conceptually
what must be accomplished for best results. If the laser power
supply is small enough to fit within the projector, then the
“Power Ground” from the power supply, as well as the “Power
Ground” from the laser diode driver should each be routed to
the “Central Grounding Point”. If an ion laser were used, then
a PCAOM would be used to modulate the beam. In this case, you
would connect the Power Supply of the PCAOM driver, as well as
the PCAOM driver each to the “Central Grounding Point” using a
“home-run” type connection.
PROJECTOR INTERLOCK (required by the
Figure 1 also
shows a relay placed in series with the laser power supplies.
This relay is intended to facilitate the “interlock” feature
of the ILDA Standard Projector. When connected as shown, the
laser diode drivers (and optionally, the shutter driver) will
only receive power when the interlock loop is closed. The
interlock facilitates an additional layer of safety for laser
projectors, and is required by the ISP standard.
facilitates an additional layer of safety for laser
projectors, and is required by the ISP standard. The interlock
signals form a “loop” that starts at the laser projector, goes
through all of the cabling that connects the projector to the
signal source (often a computer), and goes right back to the
laser projector. If this loop is broken somehow, for example
by someone tripping over the signal cable, the projector
should not output light. It is also possible for users to
place a “Red Mushroom Switch” in series with the ILDA cable
and connected to the interlock lines. This provides an easy
mechanism for laser operators to prevent laser output when
needed. And lastly, since the connector on an ILDA Standard
Projector is a DB-25, which is the same connector used by
desktop computers for SCSI, Printer, and Serial connections,
it is possible that someone might mistakenly plug the laser
projector into one of these connectors. The SCSI, Printer, and
Serial DB-25 connectors all output some kind of signals, and
thus, might cause light to be emitted from the laser
projector. The interlock portion of the ISP standard is
intended to help prevent light from coming out of the
projector if the projector is mistakenly connected to a
non-laser signal source.
is shown in the diagram as a relay, which applies or removes
power from reaching the laser diode drivers, the interlock
system may be implemented in other ways, including as an
additional shutter within the projector.
is shown in the diagram as a relay, which applies or removes
power from reaching the laser diode drivers, the interlock
system may be implemented in other ways, including as an
additional shutter within the projector.
specification allows for voltages up to 25 volts, and currents
up to around 160 milliamps to exist on the DB-25 pins. Thus,
the projector interlock must be facilitated in such a way that
these values are not exceeded. However, I recommend that you
try to implement an interlock that uses far less voltage and
current – for example 5 volts and 5 milliamps. This could be
done using an electronic relay instead of an
electro-mechanical relay. The interlock signals may also be
implemented using other methods that might provide an
increased level of safety – for example, by outputting a small
sine-wave signal on pin 4, and comparing it to the voltage
received on pin 17.
It is also a
good idea to put an LED or some other indicator somewhere
within the interlock system so that the user can see when the
interlock is enabled or disabled. In the diagram, we show an
LED connected as the Laser Emission Indicator.
We will now
discuss the connections that are related to the ILDA DB-25
signals, starting with a discussion of the scanner amplifiers.
DB-25 connector primarily contains signals that control the
motion of the beam (i.e. X-Y scanning), and the color and
brightness of the beam (i.e. R, G, B beam power). There are
other signals on the DB-25, such as the projector interlock
mentioned above, a shutter signal, and some multi-propose
“user” signals, but this article only discusses motion and
color related signals, as well as the shutter and projector
and color-related signals are arranged as differential pairs.
For the purpose of the laser projector, the word
“differential” means that the laser projector must derive the
actual signal level by taking the difference between two
signals (i.e. by subtracting). For example, ILDA DB-25 pin 1
contains the X+ signal, and pin 14 contains the X- signal.
Often times, when pin 1 is going from 0V to +5V, pin 14 will
be going from 0V to –5V. The actual signal level is found by
subtracting; thus +5V minus –5V = +10V. This means that the
voltage level that the X scanner amp should sense is +10V.
Note that I
used the term “often times” above. The reality is that there
is no strict requirement for the X- signal to be “equal but
opposite” when compared to the X+ signal. As far as the
projector is concerned, the same X position could be commanded
if the X+ signal goes to +10V and the X- signal stays at 0V,
because +10V minus 0V = +10V. Likewise, the same result could
be generated if the X+ signal goes to +20V and the X- signal
goes to +10V, because +20V minus +10V = +10V.
This is an
important concept, because the ISP standard absolutely
requires projectors to derive all motion and color signals by
taking the difference between two signals. No motion or color
signal should be assumed to be referenced to ground, and –
within the projector – no motion or color signal should be
connected to ground.
With that in
mind, you will notice that we connect ILDA DB25 pin 1 to the
X+ input on the X scanner amplifier, and we connect pin 14 to
the X- input.
that is not shown in the diagram, but could be handy to have
is an “Invert switch” on the projector. This switch should be
placed in the path of the X+ and X- signals, so that you could
easily invert the projected image if needed.
The ILDA ISP
standard states that the beam should move to the right, when
there is a positive differential signal on the X input. This
provides correct orientation for a front-projection scenario.
For rear-projection, the polarity of the X axis will need to
be inverted, and the “Invert switch” allows this to be
accomplished very easy. What’s more, an “Invert switch” often
comes in handy when doing dual projector audience scanning
For the Y
axis, we connect pin 2 to the Y+ input of the Y scanner
amplifier, and we connect pin 15 to the Y- input.
although the scanner amplifiers themselves may have a “Signal
Ground” input (labeled SG in the diagram), this is NOT
connected!! The reason this is not connected is because if it
were, this would destroy the single-point grounding scheme
that was established and discussed above in the Power
Connections section of this article. Basically, since the
scanner amplifiers have differential inputs, it is only those
differential inputs that are used for ILDA DB-25 signals. The
only ground connection is made from the “Power Ground” to the
“Central Grounding Point” already mentioned above.
scan-fail interlock were used, it would be connected to the X
POSITION and Y POSITION signal from the scanner amplifier.
But, there is something tricky to watch out for. You should
consult the manufacturer of the scan-fail interlock to
determine whether or not the scan-fail interlock itself has a
differential position input. If the scan-fail monitor does not
have a differential position input, you should NOT connect the
“Signal ground” or “Position ground” from the scanner
amplifier to the scan-fail monitor. Doing so would, again,
destroy the single-point grounding scheme. The scan-fail
monitor already has a ground connection to the “Central
Grounding Point” established on the Power Connections diagram.
You should only connect the scan-fail monitor to the scanner
amplifier’s ground connection if the scan-fail monitor itself
has differential inputs. Pangolin’s PASS safety system does
have a differential position input, but most others do not.
monitor would also be connected to a shutter or to the laser
diode drivers, but the method of connection depends on the
exact scan-fail monitor being used. Consult the manufacturer
of the scan-fail monitor for details.
signals are connected in exactly the same way as the X and Y
signals were connected, using differential signaling. ILDA
DB-25 pin 5 is connected to the “positive modulation input” on
the red laser diode driver, while pin 18 is connected to the
“negative modulation input”. ILDA DB-25 pin 6 is connected to
the “positive modulation input” on the green laser diode
driver, while pin 19 is connected to the “negative modulation
input”. ILDA DB-25 pin 7 is connected to the “positive
modulation input” on the blue laser diode driver, while pin 20
is connected to the “negative modulation input”.
discussion about color signals above, as well as the diagram
makes the assumption that the laser diode driver has
differential inputs to begin with. As two examples, laser
diode drivers made by Laserwave and by Viasho do have
differential inputs, but, as two other examples, laser diode
drivers made by CNI and Melles Griot do not. Also, it is
possible that the laser projector would have an ion laser and
PCAOM used for color modulation. In this case, the same
connection scheme and discussion applies. PCAOM drivers made
by NEOS do have differential inputs, but PCAOM drivers made by
AA do not. (In general scanner amplifiers to have a
differential input, however some very low cost scanner
amplifiers may not.)
Since the ISP
standard absolutely requires all motion and color signals to
be implemented as differential pairs within the projector,
this means that if you have scanner amplifiers or laser diode
drivers that do not have differential inputs, you will need to
implement the differential receiver as a separate circuit.
One easy way
to do this is with a difference amplifier, as shown in Figure
5, below. A single op-amp along with four resistors can be
used to receive the differential signal from the X, Y, R, G,
or B signal, and generate a “single ended” signal which is
then connected to the scanner amplifier or laser diode driver.
Note that this circuit is drawn and implemented in such a way
that it has two inputs and also two outputs. One of the
outputs is the “single-ended” signal that drives the
component, but the other output is a “ground reference”. This
needs to be connected to the “Signal ground” input terminal of
the scanner amplifier or laser diode driver. The connection is
made this way in lieu of connecting this to any other ground,
so that “Ground Bounce” can be detected and rejected by this
circuit. Also note that this diagram does not show the pin
numbers of the op-amp, and also omits the op-amp power supply
connections for clarity. The op-amp must receive power from a
power supply that is capable of feeding it a minimum of +5V
Figure 5, Difference Amplifier
TTL VERSUS ANALOG COLOR MODULATION
standard requires the color signals to respond in an analog
fashion, such that 0V does not produce any light from the
projector, 2.5V produces around half the nominal laser power,
and 5V produces the full laser power. The ISP standard also
assumes that if the laser projector is disconnected from the
signal source, there should be NO light coming out of the
laser projector (because there would be no difference between
the differential color signals).
recently seen several projectors that used laser diode drivers
that used TTL modulation inputs rather than analog modulation
inputs. TTL modulation basically means that the laser can be
fully on, or fully off. This does not conform to the ISP
what’s worse is that the TTL modulation inputs “float high”,
which means that when the projector is disconnected from the
signal source, it produces a full-power, non-moving beam. On a
recent trip, I saw this happen on two separate occasions, by
projectors made by two separate companies. And in both cases,
the non-moving full-power beam landed on dark fabric, which
actually caught fire!!
One thing to
keep in mind: under very bad circumstances, laser projectors
may present hazards. Projectors must be designed to be safe,
and one of the safety aspects is to prevent light from coming
out of them when the projector interlock is opened, and when
they are not connected. TTL modulation should really be
avoided, but if it is used, at the very least, a differential
receiver should be used which would force the TTL lasers to
“float low” instead of “float high”.
SHUTTER AND DB-25 COMMON SIGNALS
DB-25 pin 13 provides a signal to control a shutter, but its
presence is somewhat optional, and may depend on local or
federal laws where the laser projector is to be used. For
example, for projectors that use solid-state lasers, it might
be argued that between the fast and complete extinction
offered by the laser diode driver itself, coupled with the
fact that the projector interlock actually removes the power
from the laser diode drivers, a shutter is therefore not
needed. But for ion lasers, even those modulated by a PCAOM, a
shutter would still be desired. In general, it can be said
that the shutter offers an additional layer of safety for the
laser projector, and thus it is a desirable thing to have.
motion control signals, and color control signals, the shutter
signal is not considered to be an “analog” signal. It is TTL
in nature, and thus, the shutter is either fully opened, or
fully closed. The shutter is fully opened when pin 13 is
roughly 5V when compared to pin 25. Also, the shutter signal
is not a true differential signal, since noise immunity is not
really needed, due to the fact that it is TTL in nature.
However, for the purpose of projector connections, it should
be considered to be a differential signal whose counterpart is
ILDA DB25 pin
25 is considered to be the “Common” signal of the ILDA
connector. However, this “Common” signal is not necessarily a
“Ground” signal, since, under many circumstances, this signal
is not connected to “Ground” within the projector.
shutter signal is TTL in nature, and referenced to DB-25 pin
25, one good way to receive this signal is using an optical
isolator. The optical isolator will receive the TTL-level
signal between pin 13 and pin 25 and allow isolation of these
signals from the rest of the projector components.
projectors, the ISP specification states that the DB-25
connector should be a male connector. However, I recommend
putting two connectors on the projector – one male, and one
female. This maximizes ease-of-use because it allows the use
of any type of DB-25 cable between the projector and signal
source, and also allows easy daisy-chaining of multiple
projectors when needed.
BEWARE OF PHANTOM GROUND
Above we have discussed the benefits of using a
single-point grounding scheme. One thing to be aware of is
that even when the wire-connections are made carefully, it is
possible that the components themselves might provide phantom
connections to ground, thus destroying the single-point
grounding scheme and creating more of a spider-web-shaped
For example, this would happen if the metal housing of a
DPSS laser is connected to the laser diode driver’s ground
connection. A phantom ground connection would be made by
screwing the metal DPSS laser housing down to the metal
base-plate in the laser projector. Therefore, when assembling
the laser projector, you will need to use an ohm-meter, to
identify which components have metal parts connected to the
electrical ground, and which might come in contact with the
base-plate, or other conductive projector parts. Once such
parts have been identified, they should be isolated from the
I always recommend using nylon spacers under any laser, to
provide a physical and electrical separation between the laser
and the base-plate. The metal case of scanners, and thus, the
X-Y mount of the scanners are another notorious place to
generate a phantom ground connection, and this should be
isolated where possible.
OTHER PROJECTOR PARTS THAT MAY
to the fundamental components described above which are
certainly a part of most laser projectors, additional
components might also be needed or desired. For example, the
United States and certain other countries require additional
safety features for laser projectors, including “laser
emission indicator”, “key-switch”, “cover interlock”,
“external interlock”, “time delay” and a “manual reset”.
emission indicator was described above. The other elements
are described below.
key-switch is required for laser projectors because,
under the wrong circumstances, laser projectors could be
hazardous, and should be used only by trained laser
operators. To prevent untrained personnel from operating the
laser, a key-switch is used. The key should only be given to
the trained laser operator.
projector covers could be easily opened or removed by a user
or operator, the laser projector is required to have a
cover interlock, which will prevent the lasers from
operating when the cover is opened. This cover interlock is
ideally implemented as two spring-loaded switches connected
in series with the general projector interlock loop
described above. Two switches are used instead of one, to
provide redundancy in this safety feature.
governments may require an additional connector to be placed
on the projector called an external interlock connector.
Electrically, this connector only needs to have two pins,
and this is connected in series with the general projector
interlock loop. One example of how this connector might be
used is when the laser projector is only supposed to project
a certain effect (such as a cone shape), at only a certain
time (such as, when a performer is in just the right place
that the cone will surround them). The external interlock
can be connected to a “pressure pad” that is only active
when the performer is standing in place.
delay is used so that, when the projector is turned on,
it won’t start emitting laser light right away. This is in
place so that the laser operator can turn on the laser
projector, and then advise others who might be in the room
that the laser is coming on. The time delay allows the laser
operator and others to take their place to avoid accidental
exposure to the laser. Of course, this assumes that exposure
would be harmful in the first place, which isn’t always the
case. Nevertheless, the US and other countries require a
time delay to be a part of the projector. The time delay is
normally set for 20 to 30 seconds.
reset is required so that if power is interrupted from
the laser projector, and then power is restored at some
point in the future, the laser projector will not
automatically start outputting light until some manual
intervention takes place (such as pressing a button, or
cycling the position of the key-switch). The manual reset
provides an additional level of safety.
Consider this scenario: a laser projector is operating
in a nightclub, but at some point, the power to the entire
night club goes out. Then everybody leaves because there is
no power. Some time later the power comes back on. If the
laser also comes back on and emits a non-moving beam, and if
this non-moving beam is parked onto dark fabric, it could
start a fire. Because of factors such as this, the manual
reset is required by certain countries.
key-switch, cover interlock and external interlock are all
easily understood and implemented, but the time delay and
manual reset provide some difficulty since off-the-shelf
components such as laser diode drivers do not have a time
delay or manual reset feature. Often times these two features
are implemented in the form of an electronic circuit made by
the projector manufacturer.
As a point of
Professional Audience Safety System (PASS) incorporates
differential receivers for both the color signals and position
signals, and also implements the time delay and manual reset
features, thus, this multi-purpose projector control board can
help to solve some of the most difficult problems in
constructing a projector.
able to create stunning high-visibility graphic displays, as
well as breathtaking audience scanning displays. Lasers serve
a niche that can’t be accomplished by any other light form.
Getting the connections right inside a laser projector will
ensure maximum quality of the projected image, as well as a
safe display for all.