Category Archives: General

An LTSpice screencast using screentoaster.com

I was trying to remember how I did screencasts before, but instead discovered the interesting online service at screentoaster.com. It doesn’t require that you download any software: you can quickly record what occurs on the screen and upload it either to youtube.com or their own server, and then embed in a blog post. Here is my first attempt: demonstrating a simple astable multivibrator circuit that I cribbed from Hans Summers’ low powered QRSS beacon project. Using LTSpice, you can easily reproduce the schematic and then find out how it varies the voltage and current.


This is really just a test, I hope to have some more interesting circuit examples in the not too distant future.

Wolves And Rabbits

I really like programmpraxis.com. Today, they had an interesting little task, not so much for the program itself, but for what it reminded me of. But first, the task: basically to simulate a simple set of differential equations which model a predator prey relationship. It’s not that hard really, this just implements the model using a Runge-Kutta integrator.

Wolves And Rabbits « Programming Praxis.

#!/usr/bin/env python

# http://programmingpraxis.com/2009/12/01/wolves-and-rabbits/

def population(r, w, rg, wg, rd, wd):
    def dr(x, y):
        return rg * x - rd * x * y
    def dw(x, y):
        return wg * x * y - wd * x 
    while True:
        yield r, w
        rh = r + dr(r, w) / 2.
        wh = w + dw(w, r) / 2.
        r = r + dr(rh, wh)
        w = w + dw(wh, rh)


g = population(40, 15, 0.1, 0.005, 0.01, 0.1)

for x in range(201):
        r, w = g.next()
        print r, w 

If you plot out the data, you get curves looking like this:

wolves-rabbits

This reminded me of an article by A. K. Dewdeny from Scientific American back in December of 1984, entitled Shark and fish wage an ecological war on the toroidal planet Wa-Tor. Wator was an implementation of a shark/fish predator prey model, but displays a similar set of interleaved periodic population changes. Unlike this program though, it works by directly simulating the world by creating individual sharks and fish, and modeling their interactions. It was a fun, simple project which didn’t require any real understanding of mathematical methods.

New Google Voice Number

Courtesy of Zeid, I finally got an invite to use Google Voice. I’ll write more about this when I figure more about it, but at a minimum, it gives you a real phone number (amazingly, for free) which anyone can call and leave voice mail. You can configure it to ring a different phone (say, your cell phone) or send it directly to voicemail. You can access your voice mail in the conventional way, or by using a computer online. It even automatically transcribes the message into text! Very cool, and thanks Zeid for hooking me up.

Here’s an example of what you can do. This is a little flash widget. If you click on it, you can type your own name and a number that you can be reached at. Google will then call you back and hook me to my voicemail (or, if I configured it differently, ring my cell). Give it a try if you like. I’m just starting this, and it seems that you can’t select the “Conceal my number” checkbox if you want this to work. It also won’t work from international numbers.

Addendum: Here is an example of what Google Voice can do with a voicemail message.

LM3909 Oscillator

I was wandering the net at random, following various links, and ended up researching a rather interesting integrated circuit that I’ve used before: an LM3909 LED flasher chip. It’s an interesting chip which can flash a regular LED with a 2v voltage drop using only a single 1.5v battery. I used to have this breadboarded up on my bookshelf ages ago, and it would flash for months using a single AA cell, and probably for years using a D cell. The problem is that this chip is kind of hard to find these days, and when you can find them, you end up paying $5 each. What’s cool is that someone actually worked out the circuit from the databook and drew up a completely discrete version.

Discrete LM3909 Oscillator

From the Brainwagon Archive: Silly Arduino Project #1: A Trivial Beacon

One year ago tody, I tinkered together a very simple program and keying circuit to use my Arduino to send out a simple Morse Code beacon message.

brainwagon » Blog Archive » Silly Arduino Project #1: A Trivial Beacon

It seems like complete overkill to me to use a $30 microcontroller to do this task though. A couple of different groups have created PICs and the like which can be used to do the same, with a total cost of well under $10 (the chips are probably around $2 or so a piece). I keep thinking that what I should do is code up a simple version of this kind of application in ATMEL AVR assembly code and make it available. I can envision a simple web based application that would enable you to enter a list of beacon messages you want, and then hit a button and download the necessary firmware, all assembled and ready to go. Just burn and go…

I need more hours in the day.

Addendum: K1EL makes a $6 Keyer chip. Expanded Spectrum Systems makes a nifty “Freakin’ Beacon” controller that includes the ability to sync with a GPS for timing.

Using the 74HC240 in radio applications…

There are a number of interesting radios based upon using things like hex inverters or octal drivers as both oscillator and amplifier. In digging around, I found the most common chip used in this application is the 74HC240. Here’s a link to a page which contains many interesting links:

Testes com 74HC240

Another link to a similar design which points at a QST article which you can access if you are a registered ARRL member.

Picture 1

PROPNOMICON: From the Mountains of Madness

Shifting gears a little from my normal subject matter, I’ll confess to a certain literary guilty pleasure: I like the writings of H. P. Lovecraft. I’m not saying they are the highest literature: I just find them to be an interesting style, creating a mythos that I find a compelling backdrop for a collection of creepy stories. I also like things like fossils and the like (I keep an Elrathi kingii on my desk). So, I find the following craft project strangely compelling: to create Lovecraft-themed props.

Gads. Next, I’ll be making steampunk stuff…

PROPNOMICON: From the Mountains of Madness

cpu_from_scratch [bradthx.net projects]

Every few months, I read about some one who embarks and completes a homebrew CPU project. A few months have gone by, and here’s another. Someday, I really, really, really am going to have to do this myself.

Recently I decided to take on a very ambitious project to design my own microprocessor from scratch. A couple of things inspired me on this project, the first being a book I finished, Digital Apollo by David A. Mindell. This book details the design and engineering behind the AGC (Apollo guidance computer) and I was amazed to learn how well designed both the hardware and software were for it's time. The fact that it was designed completely with 3-input nor gates made it even more amazing. The second reason is the fact that several other people have done the exact same thing, built entire computers from basic TTL logic. A few blog posts have been floating around the internet detailing other peoples CPU designs, looking at these I knew it was something I could easily do myself and would be a very fun project.

via cpu_from_scratch [bradthx.net projects].

Bokode @ Home – Recreating MIT’s bokode tech with everyday materials

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