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Technical Design   -   Installation Requirements

Technical Design

The original P2P proposal called for indoor installation, with a distance of only 25 metres (75') between the switch panel and the marquee. The technical design for this indoor installation was relatively simple, if repetitive: each switch would be wired in series with the corresponding light bulb, switching the 120 volt supply directly.

The curatorial committee of CAFKA.02, for which the piece was created, preferred to showcase the piece in Civic Square, in front of City Hall. This outdoor installation meant that P2P had to be completely redesigned, while maintaining the functional simplicity which makes the piece so compelling. The outdoor installation imposed two main constraints:

  1. Artistically, it was important that the piece use common household light switches. These switches are not waterproof, and the outdoor installation meant that performance may degrade if the switches rusted over the course of the exhibit. In addition, it was out of the question to have these switches switching 120 volts outdoors, out of concern for the safety of participants who may use the piece in the rain.

  2. The other main constraint was the distance between the switch panel and the marquee: 150 metres (450') of cable would be required to make one run from the switch panel to the marquee. This meant nearly 20 kilometers (12mi) of cable, in order to direct-wire switches to bulbs. Aside from the cost, it was simply impractical; another way had to be found.

The solution to the safety concerns was to confine high voltages to the marquee and the relay box, which the public did not have direct access to. This left the possibility that some switches might need to be replaced if it rained too much. We did have to shake out a couple of switches after a particularly long, hard day of rain.

In order to reduce the amount of cabling between the switch panel and the relay box, a 128-bit serial latch was designed from eight 16-bit latches (Fairchild 74F676). The switch outputs are all latched in parallel, and that data is shifted serially along a CAT5 cable to a decoder circuit in the relay box, 500 feet away. The decoder circuit is essentially the opposite: a 128-bit serial-input parallel-output register was constructed from sixteen 8-bit latching relay driver chips (Allegro Micro UCN5841/5842). Each of the outputs drives a relay on a modified RIB101 relay interface board (BSoft Software, Inc), which turns the corresponding lamp on or off.

A single microcontroller (MicroChip PIC16F84A) provides the timing, clocking the serial data line at 50kHz, meaning the whole display is refreshed roughly 400 times per second. RS422 drivers (Maxim MAX3043/3095) were used to transmit data and clock differentially along the CAT5 cable. Two unused wires in the CAT5 were used to bring power back from the relay box to the switch panel electronics, meaning the only thing leaving the switch panel is the CAT5 cable.

Suprisingly few people asked how it actually worked. Part of our goal was to make the interface design easily-understood and accessible to all. The workings are actually quite complicated, but it was done in such a way that people were not surprised at being able to control 125 bulbs from so far away. Or maybe they just didn't think about it. Either way, the interface design was a success, from our point of view, but we were still surprised that more people didn't ask how it worked.

Installation Requirements

The final P2P design is made up of four components: the Marquee, the Relay Circuits (which switch the power to the bulbs), the Switch Panel, and the Switch Panel Electronics (which reads the switch values and transmits to the Relay Circuits).

Marquee Relay Circuits Switch Panel Switch Panel Electronics

Several aspects of the installation are site-specific. Please see the images above for details of the local installation requirements described below. Images will open in a seperate window.

  • A frame at least 2x8 metres (6'x24')
    This is constructed locally mainly because it is relatively easy to build and very hard to transport. Once constructed, the frame is covered in black Textilene, a wind-permeable fabric, and airline cable is strung to which the bulbs will be affixed.
  • A support structure for the frame
    The complexity of this requirement depends on what can be used locally: it can be hung on the solid wall of a building, or on pillars or other supports as it was in these images. The bulbs are installed after the frame has been hung.

Relay Circuits
  • 60A @ 110VAC (ideally in 6 sockets)
    The relays are divided into five 25-relay control boards. Each relay operates one 50W light bulb. In addition to the bulb power, each relay control board requires ~1.5A @ 5VDC, and the decoder electronics which drives the relays requires approximately another 1A. The relays are rated up to 250VAC, but unfortunately the special bulbs we use in the piece are only rated to 130V.
  • A housing of some kind
    The type of housing required will depend if the relay circuits will be located outside or inside. This is site-specific and hard to transport, so needs to be sourced locally.

Switch Panel
  • location
    The switch panel is housed in a structure which is ~1 metre (36") high and has a footprint of approximately 70x310 centimetres (27"x10').
  • CAT5, 8-conductor cable
    A length of 8-conductor CAT5 cable sufficiently long to connect the switch panel to the relay circuits. Since we are not communicating at Ethernet data rates, repeaters are not required beyond the regular limit of 100 metres (300'). Previous installations have been successful with 500' of cable without a repeater.

Switch Panel Electronics
  • no local needs
    Power for the electronics arrives from the relay circuit box via two of the CAT5 wires.

Other Technical Information

Aside from installation requirements, the following technical information may be of interest.

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