Theatre Design & Technology - Winter 1979 - 20

General Design Goals
Transmission Channel
1. Digital, fully multiplexed transmission.
2. Compatibility between hardwire and IR modes.
3. Immunity to interruption of control signal.
4. Real-time control.
Dimmer Design
1. Compact, rugged and reliable.
2. Quiet and light weight.
3. Temperature compensated
4. Low cost

Figure 2

A first round list of the general design
goals in each of the two principle areas:
the transmission channel and the remote
dimmer.

where it is desirable to gang instruments together under the
same control. Unlike patching/twoforing, however, there is no
limit to the number of dimmers that can be set to respond to
the same control information. In the extreme case, all dimmers
could be set to respond to the same control information.
The result of the work summarized above was the set of
preliminary general design goals shown in Figure 2. Such
goals could not be completely determined in advance since
one of the reasons for this project was the determination of
these goals, based on the results of field trials. Thus, the
goals shown in Figure 2 represent a first round "best guess."

Prototype Construction
In order to build an actual system specific numbers were
needed in areas such as those listed in Figure 3. The first column of numbers indicates the system parameters which are
feasibly obtained with the available technology. The tech-

Light-Link Specifications
Parameters
Number of Channels
Dimmer Load Capacity
Hardwire Range
IR Range
Dimmer Setting
Accuracy
Maximum Dimmer
Temperature
Dimmer Size
Dimmer Weight

Figure 3

Feasible
to Build
1000
6 kW
10,000 ft
1 ,000 ft

Evaluation
Prototype
128
2 kW
10,000 ft
100 ft

±0.1%

±0.5%

200°F(93°C)
64 cu in
(4x4x4")
3.71bs

140 0 F(60°C)
166 cu in
(3x6.5x8.5")
71bs

Light-Link specifications indicating the
capabilities of a production system and
the design parameters of the evaluationprototype system.

nology has the capacity to cover the needs of present systems
as well as to meet the needs of any forseeable expansion.
The prototype system that evolved was built to the specifications shown in the second column of Figure 3. It consisted
of two principle components: a control console, and a collection of remote dimmers. A third very useful, but optional, component is the IR emitter.
A picture of the remote dimmer, C-clamped to a stanchion,
is shown in Figure 4. Before discussing this unit, it is important to point out that this dimmer was intended to serve primarily as an evaluation prototype, and secondarily as a production prototype. The need for a flexible open layout in which
additional features could easily be included took precedence
over the desire to make the unit as compact as possible.
Starting at the left end of the dimmer is the photodetector; it
is supported by a swivel mount. This enables the user to
quickly and easily perform a coarse allignment with the IR
emitter. The operator is aided in this process by the "data
valid" indicator that signifies when a link with the transmitter
has been established.
The choice of a directional photodetector, as opposed to
an ominidirectional one, was based primarily on economic
considerations. As the young open-air IR technology matures,
it will become economically feasible to incorporate an omnidirectional photodetector.
Two jacks are provided for the handling of data sent over
the hardwire means. Either jack can serve as the input to the
dimmer, receiving the data as passed on from a previous unit.
The other jack serves as output from the dimmer, sending the
data on to the next unit. Flexibility is quite high in such a system since at any point along the chain any number of dimmers
can either be inserted or removed without the necessity of
running additional cables from the control console.
Also shown in the photograph are: thumbwheel switches
that provide the flexible addressing feature discussed above;
power pigtails that connect the dimmer to line and load; and a
fuse for protection.
A block diagram of the remote dimmer is shown in Figure 5.
The received data, whether coming via hardware or IR, are received and amplified (not shown) before entering the decoding section. The control data, whose associated address
matches the address set on the flexible address switches, are
loaded into the temporary data buffer. Before these data are
allowed to update the long-term data buffer, they are subjected to several error tests. These tests are part of the means
by which the output of the remote dimmer is made immune to
interruptions of the control signal.
Following validation, the data are transferred to the longterm buffer and thence to the triac firing generator. As shown
in the lower left of Figure 5, a portion of the AC line is also
routed to the triac firing generator. (The use of a triac, instead
of 2 SCR's, is another consequence of this being an evaluation prototype.) The use of both of these signals enables the
remote dimmer to be operated on any phase relative to the
other remote dimmers as well as on any phase relative to the
control console.
In order to gain user feedback on the evaluation prototype,
a control console was required. Although the present data for-

Figure 4

Table of Contents for the Digital Edition of Theatre Design & Technology - Winter 1979