Hardware Hacking Projects for Geeks

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Chapter 8, How to Hack a Building-Size Display

Project Demo

After you’ve set up the system you’ve decided to build, carefully apply

power and load one of the Blinkenlights movies. You can see a desktop

system running in Figure 8-25.

And, as Figure 8-26 shows, having a whole building lit up with animated

movies can be very impressive.

Wireless Extensions

One of the most time-consuming tasks when setting up a large

building is running the wires to each lamp. With some addi-

tional hacking, it is possible to use low-cost wireless links to

eliminate the thousands of meters of wire.

A single transmitter attached to the control PC can broadcast

one frame of data 25 times per second. Each receiver has a

unique address and looks for its state (either on or off) for that

frame of data. At the end of the transmission, all of the receiv-

ers make their state change. Each receiver needs a strong signal

from the transmitter connected to the control PC. An easy way

to accomplish this is to place the control PC and transmitter in

front of the building windows at street level, therefore ensuring

line of sight to each receiver. An overview of this idea is shown

in Figure 8-27.

Figure 8-25: Desktop system LEDs

Figure 8-26: Large-scale system in action

Channel Coder/Decoder

A company called RadioTronix has

a nice application note on channel

coding (http://www.radiotronix.com/

an401.pdf). It outlines a simple and

effective method for encoding data

for a radio channel.

Building

Window light with receiver

Control PC with transmitter

Figure 8-27: Wireless system overview

Wireless Extensions

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164

Part II: Advanced Hacks, Tools, and Techniques

Each lamp (or set of lamps) must be outfitted with an addressable radio

receiver. Several companies sell low-cost radio receivers that can be used for

this purpose. Figure 8-28 shows a block diagram of a wireless lamp control-

ler, and Figure 8-29 shows a block diagram for a simple transmitter.

The radio link provided by the inexpensive receivers will require additional

software to allow for reliable data transmission and reception. Figure 8-30

shows the different operations that the data must undergo in order to be

reliably transmitted over a “raw” radio link. There are many other methods

available, but this one is simple and reliable.

In order to receive a radio channel coded packet, the inverse of the transmit

operations must be performed. This chain of functions is shown in Figure

8-31.

Error Control Coder

The radio channel will likely have

noise on it from time to time. To

prevent noise from hindering reliable

data reception, an error-correcting

code should be applied to all data.

You’ll find a good explanation of

error correcting codes at http://www.

eccpage.com/. The web page also

includes C source code. For this sim-

ple system, I recommend a BCH code.

Each frame should be organized into

a packet and a few command codes

defined.

Error coder

Data input (data

payload, address,

desired function)

Packetizer Channel coder Radio hardware

Figure 8-30: Wireless transmitter software

Radio hardware

Data output (data

payload, address,

desired function)

Channel decoder Depacketizer

Error code

decoder

Figure 8-31: Wireless receiver software

Micro-

controller

Low-cost

radio

receiver

Relay

Relay driver

transistor

120V socket

120V plug

Lamp

Figure 8-28: Wireless lamp receiver

RS-232

interface

From PC Serial port

Low-cost

radio

transmitter

Figure 8-29: Wireless transmitter

Wireless Extensions

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