A Burning Man installation © 1999 Tim Black
Imagine the Man surrounded with a wheel of lights, flashing, dancing and rushing patterns of time. Alien sequences whirling around, somehow related to the time, but not simple clock wheels. Patterns created on site by visitors to the Pattern Buffer Lounge. More complex, more interesting, as the days count down…
In keeping with the "wheel of time" theme, we are building a circle of light 636 feet in diameter. Surrounding the man and buried in the playa, patterns will move around the circle with great speed. The patterns will change with the time of night, making a kind of clock surrounding the Man. (See The Site Plan.)
The circumference is made of 2000 LEDs built in 200 strings of 10 each. Each string will have LEDs spaced 1 foot (or more) apart on a wiring harness. Each group of 10 LEDs is attached to a controller board. These boards have microchips that drive the LEDs by receiving pattern data from a central control chip. Each LED is sealed into a clear plastic lens. Each lens is mounted on a 3 Lb concrete casting to form a lightpod.
A thin steel cable 106 yards long will be used to scribe a circle around the location of the Man. A truck or tractor with a single steel plow blade will follow this line to open a trench in the playa. The 200 strings will be placed in the trench and volunteers will fill in the trench while placing the LED lens at ground level. (i.e. 20 people doing 100 feet each) Battery power supplies and data repeaters will be in shrines at the four quarter points. Power feed lines will be run to one of the shrines.
The loop circuit is made of 200 strands, each with 10 LEDs and a microcontroller on a small circuit board. A pair of 12 Ga. power wires feed the loop. The wires carry a 6 volt power supply with data on a third wire. The data line is daisy chained, so each board acts as a data repeater.
Each strand will have a PIC16C622-OTP micro controller chip. There is a pulse drive circuit that hits the LEDs with 70 Ma for 100Ms then drops to 25 Ma. This makes a very nice snap to the leading edge of the light and produces good visibility at lower duty cycles. Each driver has a 10,000 uF Cap for energy storage.
Driving all 10 LEDs will be 250Ma per segment. Transistor drivers will be used to isolate the pulse currents from the PIC outputs. The PIC chip draws only 2Ma, so the total is 252Ma. Adding a 20% safety margin (50Ma) brings the Max current for each strand to 302Ma. With 200 strands the maximum current comes to 60.4 Amps at 6 volts. (362.4 peak watts)
There will be eight shrines holding 220 Ah Lead-acid batteries. Each shrine will feed one eighth of the circle from both ends. Each quarter needs a peak current of 15.1 amps, each eighth needs 7.55. Each shrine will feed 3.77 Amps Max into each end of the eighth circle. These loads are peak values with a low duty cycle.. All shrines will have a 6-volt battery, and power limiter circuits. Each strand will have filtering and energy storage provided by a 10,00uf Cap to reduce the ripple in the load. The ring will run all week on a single charge.
Here are some interesting numbers for the playa ring:
With 1760 amp hours of battery we can run 25% patterns for 176 hours. At 12 hours a night that comes to 14.6 days. This is enough margin to make me happy.
A laptop computer will load the patterns into the control system. A program will be written to create the patterns from a GUI or from a pattern board. The control system will clock the pattern into the loop of strands. Each strand microcontroller will have two functions. One function will store a set of trigger events based on the system frame counter. (an RLL encoded pattern for the lights on each strand) The other mode looks like a 2000 bit shift register. Patterns entered into the loop will be displayed in real time (2000 cps) based on the loop count. Each strand can be instructed to display the shifted pattern with variations, such as invert, backwards, reflect, add, or subtract. The RLL mode will allow complex patterns to be preloaded and run, while the shift register mode will allow real time interaction with players.
Each PIC will store 30 sub-patterns for the 10 lights attached to it. Each sub-pattern has a 11 bit trip count and a 10 bit lamp pattern. There is a sync command that sets each chips version of the frame counter to a fixed offset. When the frame counter matches a stored sub-pattern trip value, the matching light pattern is displayed on the LEDs. There is an end count option that can reset the frame counter early to force high speed repeats of sub-pattern sequences.
Each strand is in a data loop, with each PIC receiving and sending 24 bit packets simultaneously. The packet rate is 2000 per second. I have hand tuned assembly code that can make a 4Mhz PIC do this full duplex I/O at over 100K baud burst speed. This still leaves about 50% of the chip cycles for doing the rest of the work (that should be enough). With this data rate I can update the entire ring 10 times a sec, load all 6000 RLL pattern events in 3 secs, or feed shift patterns at 2000 cps.
Because of the daisy chain interconnection of the PICs there is a one frame + one bit delay in the communication around the ring. The frame delay can be avoided be sending the data packets in an address reverse sequence. (address 200 sent first, then 199 ...) this causes each PIC to pass the packet to it's neighbor without taking any action on the packet. When all 200 packets have been sent round the ring, all the addresses match on the same frame. There is still a fixed offset of one bit time per board. This appears as a total of 8 frames of error, one frame every 25 boards. The 11 bits of frame count value allow a max count of 2047, so there is plenty of room to compensate for this error.
The strands will be build from a single 2" x 4" circuit board and hand assembled with the 10 LEDs into a wiring harness. The LEDs will then be fixed into clear plastic lens mold and potted with liquid plastic resin. All connections will be waterproof.