The prototyper consists of a small piece of standard breadboard attached to one half of the front panel of a desktop-style project box. A large header at the bottom of the board connects the breadboard to the input and output devices on the other side of the panel.

The switches are dual-mode toggle switches that have a momentary action in the “up” position and latch in the “down” position. When activated, the switches pull their outputs from +5V (via a 10k pull-up) to GND.

The three potentiometers vary their output from 0v (fully anti-clockwise) to 5v (fully clockwise).

The LED indicators have two modes. They can (individually) be configured to either be “off” for a logic-high and “on-red” for a logic-low, or “on-green” for a logic-high and “off” for a logic-low.

Power is derived from either the internal battery or an external PSU (7-35v). The internal voltage regulator supplies 5v at up to 1A, and is connected to the red/black posts on the breadboard.

The device is pretty simple – it is basically a timer that can be configured to sound an alert after either 5, 10, 15, or 20 minutes. The alert can either be audible (buzzer) plus visual (flashing LEDs), or just visual. I find it useful to clip the timer to a pad of paper with a “to-do” list written on it, so that I know what task the alert is supposed to be reminding me of!

In a rare period of intense focus I managed to complete this project over a weekend – only a few hours in total, but spread over Saturday (hardware design and construction), and Sunday (microcontroller code). The hardware consists of a PICmicro microcontroller, voltage regulator, switches, a buzzer from an old PC, plus a few other bits I had lying about.

The WebX10 system utilises (surprise, surprise) X10 mains-wiring control signals. If you don’t know what X10 is then Wikipedia has a good article here.

The WebX10 system was quick(ish) to build thanks to several ready-made modules:

• Siteplayer Embedded Webserver
• Basic Stamp 2 (BS2-IC) Microcontroller
• XM10U TTL-X10 Interface

The Siteplayer and BS2-IC, along with a handful of passive components are mounted on a piece of stripboard. A bracket from an old network card was attached so the device could be mounted in my Linux server. Power is provided by the standby power line from the server’s ATX PSU (i.e. the WebX10 receives power whether the server is either on or off).

This project is taking far longer than I originally thought. Still, I’ve started so I’ll finish (eventually!). To be fair to myself my sum total of electronics experience before starting the project was building an electronic doorbell at school – at that didn’t even work.

Laser Tag

Be it Paintball, Airsoft, Laser-tag or Nerf, running around an arena playing guns with a group of friends is absolutely fantastic fun no matter what age you are (be honest!). The advantages of the Laser-tag type game are three-fold:

• It cuts down on cheating (you can’t argue with a computer)
• Zero ammo cost
• Being shot-at doesn’t hurt

Decent equipment can difficult to get hold of, so I decided to design and build my own.

Guns

Surprisingly, the actual laser on a Laser-tag gun is only cosmetic – coded focused infrared beams are used to register hits, etc. This is good news for the home constructor as modulatable lasers can cost in excess of £100, whereas infrared diodes cost a few pence.

Kudos must go to Dave Bodger, whose website provided invaluable info.

Packs

The packs will house infrared sensors and the batteries to power the pack and gun. The electronics in the pack are quite simple, but mounting it within a wearable harness is another matter. To do.

Target Units

A target unit will be kept in each team’s “base” and will keep a count of each time a member of the opposing team
manages to shoot it. Still at the “bare circuit board” stage.

Emitter Units

The function of these units is to periodically emit infrared codes to give players extra ammo, more lives, etc. (they are
configurable). So far, 4 have been built.

The MD5 implementation in sCrypto was coded by Paul Johnston, and the code for the TEA was written by Donald Bindner. Thomas Dixon wrote the Java TEA implementation used in fCrypto. All other code (glue, GUI, etc.) was written by me.

sCrypto and fCrypto are distrbuted as Free/Libre Open-Source Software (FLOSS) – Share and enjoy: crytpo.zip

sCrypto

SCrypto is a simple encryption program implemented in Javascript/XHTML/CSS. Consisting of a single 29K html file (8K when compressed!), it can be easily stored on a USB ‘thumb drive’, and has been tested with Internet Explorer on Windows and with Firefox on both Windows and Linux.

To encrypt a message simply type the text in the message area, enter a passphrase, and click ‘encrypt’. The message plain-text will be converted to unreadable cypher-text. Copy and paste the cypher-text into an email, word-processing document, or whatever.

To decrypt the cypher-text paste it into the message area, enter the same passphrase used to encrypt it and click ‘decrypt’.

fCrypto

Whereas sCrypto works on copy & pasted text, fCrypto works on files. It is a simple command-line program written in Java, and therefore requires a Java Runtime Environment (JRE) to be present on your machine.

fCrypto is used thus:

For example, if you wanted to encrypt a file called photo.jpg with the key
snowman, then you’d use the following:

This would create a new, encrypted file: photo.jpg.fcrypto

To decrypt the new file the following is used:

The would re-create the original file: photo.jpg

Of course it would be more useful if fCrypto could encrypt a directory into a single secure file, but I really don’t have the time to add the functionality. As a work-around, I archive collections of files and then encrypt the zip.

Warning!

• This software is provided “as is” – I use this program myself without any trouble, but if it exposes and/or corrupts your data due to a bug I’ve missed then you’re on your own!
• fCrypto is a very simple program and lacks some of the safeguards of more sophisticated software. Be aware that fCrypto will not stop to ask you if it is about to create a file with the same name as an existing one – it will just write over the existing one (deleting it in the process).
• Keep your keys safe! If you forget the key(s) you encypted your files with you will not be able to read them.
• Some countries have restrictions on the use of strong encryption – you might want to check to see if you’re violating any local laws by downloading and using this software.

Our design criteria included the following:

• Insulated for year-round use
• A usable veranda (many commercial summerhouses feature impractically narrow verandas)
• The entire front of the structure must open out
• Large enough to take a two-seat fold-out sofa-bed

The images on this page pretty much show how the structure was built, and also show some of the special touches that we added during construction. Of particular note is the window that can be easily removed and replaced with a bar-top for those long summer evenings. Other features include: built-in lighting, mains electrical sockets, 100BaseTX network, and a stereo system.

Flock

Back in the late eighties, when I was studying for my degree, I recall being in a spectacularly dull maths lecture. As was usual, my attention turned to what I could see through the window – a flock of what must have been several thousand birds flying over Plymouth harbour. In the distance I couldn’t make out individual birds, but the large waves and other dynamic structures produced by the collective movements in the flock looked amazing.
Around that time I would make regular visits to the library to search out material on artificial intelligence. Among the usual stuff about computer vision, chess playing, natural language processing, etc, I came across a recording of a documentary which featured a piece on a program called “Boids” by Craig Reynolds. It made a deep impression on me.
Craig Reynolds wrote “Boids” while working as a computer animator at the Symbolics Computer Company. The idea behind the program is to simulate the flocking behaviour of birds (or schooling behaviour of fish) by making each individual bird follow a few simple rules:

• a tendency to fly towards the centre of the flock
• an ability to match the velocity of birds in the rest of the flock
• an ability to avoid getting too close to nearby birds

My “Flock” applet is a variation on this approach. The main difference is that in my program the “Boids” only move in two dimensions. Reynolds’ original was in full 3D.

Genetic Algorithm

Melanie Mitchell’s book “An Introduction to Genetic Algorithms” provides an excellent primer to the field of genetic algorithms (GA’s). Section 1.6 of the book describes a very simple example, and my applet is an implementation and visualisation of an almost identical GA.

Fractal Tree

Recursion based algorithm to render a simple tree image

The physics simulation in modern games is much more sophisticated – check out Crysis (PC) and LittleBigPlanet (Playstation 3).

Download sprdmp.zip for a (much simpler) program written in Blitz3D (the PC version of Blitz2.1) that demonstrates how the equations are used (source code included – GNU GPL open source licence).

Overview

Two particles P₁ & P₂ moving with velocites V₁ & V₂ respectively are connected by a spring with natural length n and stiffness k, and a damper with resistence r.

Let us say that the particles’ position and velocity vectors are represented in the Cartesian coordinate system thus:

Now let us define:

Spring force

The force due to the spring is equal to the difference between the expanded/contracted spring’s length and its natural length multiplied by the spring’s stiffness divided by spring’s natural length:

Damper force

The force due to the damper is equal to the rate of expansion of the damper multiplied by the damper’s resistence.

The distance between P₁ and P₂ as a function of time t is:

Which expands to:

To get the rate of exansion we differentiate with respect to t:

We evaluate this at t=0:

Which simplifies to:

Finally, multiply by the resistence to get the force:

With that, here’s a video I made about how to fold a crisp-packet into a triangle:

Many thanks to my cousin, Susan, for passing on this particular piece of  “pub lore” many years ago.

Journey’s end.

My flight back to London Heathrow wasn’t until the afternoon, so I had time to savour my last morning as a backpacker. Curiously, as I packed my rucksack one last time I can only remember thinking: “At least I wont have to listen to other people’s snoring anymore!”. In the time following my return I really missed that sound!

LAX