The Filter Works!

I don't usually blow my own trumpet. I usually use somebody else's But I can't keep this to myself.

The low pass filter works and works well. Of course it helps to have an LC meter to measure the coils. A bit tricky for the ones less than a microhenry but it does give a close approximation.

It also helps to be able to "sweep" the filter when finished. This I did with an HP spectrum analyser and tracking generator. In the 'old days' [last century] I would plot the response on a XY plotter. To send it out to others would need photo copying. Now I use the phone's camera. 

Of course I forgot the impedance is 200 Ohms not 50 Ohms. So the first test showed up the mistake. After making two resistor matching sections I got the desired result.

It should look a lot better but the loss of the matching sections modifies the overall loss. However it shows at least 40dB suppression for the second and third harmonics. It will do the job.

The 'test' construction isn't very 'pretty'. But that also works. The coils are wound on the plastic outside of a ball point pen. It doesn't matter that it is not perfectly round. Nor does the gauge of the wire matter very much. So long as it is 'close' to the specified gauge.

The spreadsheet will be available on my web site later. It allows for any characteristic impedance. Or any cut-off frequency with the ratios of frequency to the cut off frequency. A sample is shown below:-




What was I thinking?

First of all, I needed to design a low-pass filter for the Raspberry Pi. To 'clean up' the output. It is a square wave after all, with only odd harmonics. A simple task for a Cauer [elliptic] filter.

So I sat down and opened my filter book. Decided on a design I had used many times before. This one 'notches' out harmonics providing very good suppression.

Having developed a spreadsheet to calculate the values, I took a careful look at the components. You generally have two choices, one uses capacitors the other uses coils. The lowest cost version is where you make the coils from scavenged wire. The capacitors can be recovered from old radio equipment. And no they don't have to be Silver-Mica types. Ceramic will be fine.

[I will make the spreadsheet available at a later stage. This will allow the constructor to design the cut-off frequency and impedance to his requirement. Not mine.]

Then it hit me!

It is possible to use ferrites for the coils. But they are neither 'cheap' or readily available here in SA. So I started the secondary phase of this project...

Coil Making in the 21st Century [in South Africa]

  • Getting hold of the wire. [do not steal!]
  • Transformer Disassembly.
  • ATX Power Supply Toroids.
  • Having got your wire, how do you wind it?
  • Measuring your coil.

During this 'process', I came across a lot of reference works. Some are available on the Internet. A lot I had photocopied in the 70's. Now scanned they provide valuable references. Quite a few are IEEE papers. If you want them nowadays, you will have to pay $ for the privilege.


What I wanted to do with some pictures...

Having discovered that the GoPro "free" software only works on 64bit PC's with more than 4GB of memory. It also doesn't have a Linux version... I went looking into the Open Source software. Turns out academia has the solution. So you can process the GoPro pictures under Linux as well as communicate with the camera as well. Just as well. The GoPro is a Linux appliance!

I learnt about making an 'animated GIF' in GIMP. That was fun. But the resultant pictures were only 256 colours and played back on FireFox with shading levels. So I tried making an animated PNG file. That had the quality but was in danger of taking over the hard disk. Certainly wasn't small enough to email anyone.

I discovered a built-in utility inside ImageMagick. Which is there in Lubuntu [Ubuntu]. The utility was "convert" and "mogrify". Both will convert single graphic files to another type. Multiple files will need the mogrify utility.

I then wanted to convert 100+ files to a lower resolution. This should reduce the size of the animated PNG. Or so I thought. Googling produced this link:-


Convert 'issues'
But the example given doesn't work...

convert: invalid argument for option `-resize': 800×600 @ error/convert.c/ConvertImageCommand/2553.

BUT if you use 'X' for 800X600 :-
          convert frame001.png -resize 800X600 example.png
[no issues]

[Oh WOW!]
convert frame*.png -resize 800X600 example*.png
[also works!]

So I tried telling the 'how to geek' about this. But the comments/feedback is closed...

closed Oct 11, '15

This topic was automatically closed 10 days after the last reply. New replies are no longer allowed.

I then used 'apngasm' to put the 'frame's together. This works well. But the file is still 80+ MB in size...


The plumber has left the building!

So the plumber has gone away with a smile on his face. Sten came and in-between other jobs, fixed my leaky pipe. The pipe had been leaking in the wall for some time...maybe years?

Well the wall and plaster and floor are now drying out.

Me, I am just thawing out. The temperature has climbed to 18C in my study. I wanted to do several things today, but events and people have diverted me again!

So before I forget, let me type this in...

USB bootable stick creation in Windwoes

Insert a USB flash drive into your computer:-

Start a Command Prompt as an administrator (right-click on the entry and select Run as administrator) and type 'diskpart' and then press ENTER.

Next, type the following lines a line at a time, pressing ENTER at the end of each line after the command has completed:

DISKPART> list disk
DISKPART> select disk X
[Note: X is the number for your USB flash drive you obtained from the list command from the previous step. ]
DISKPART> create partition primary
DISKPART> select partition 1
DISKPART> active
DISKPART> assign

Next you need to make your USB flash drive bootable.
Start another Command Prompt as an administrator and navigate to the boot folder of the Windows 8 ISO image then type the following:
bootsect /nt60 X: [Enter]
[Note: Where X is the drive letter for your USB flash drive]

From a Command Prompt type the following:
XCOPY X:\*.* Y:\ /E /J /H
[Note: Where Y is the drive letter for your USB flash drive and X is the drive letter of your mounted Windows 8 .ISO file]

Amateur Radio in the 21st Century [1]

As I write this, we are still without power after an hour. What made me write this? Well I thought of this subject this morning. All day it has been at the back of my mind. Something for the younger and newcomers to the hobby of “Amateur Radio”.

In the old days, that is before 2000, we amateurs “hacked” our hardware. Whenever someone threw away some thing that we could use, we 'acquired' it. Adapting wartime radios, assembling kits or developing new hardware, we 'hacked' it.

Last century as a constructor you could drive into Johannesburg and visit a component store. Here on the West Rand, you could drive to the top of CR Swart and visit “Vidi-Kits”. These stores have mostly disappeared or service our small electronics companies. Our market here in SA is minute in comparison to a major city in Europe or USA. It is not big enough to support the small number of constructors or electronic enthusiasts. All the components are imported. Attracting import duty. With the exchange rate not in our favour, the value of a component is way beyond a youngster with pocket money or a pensioner.

Some time ago, I suggested to some members that the only way 'we' will acquire components in future, will be by taking electronic hardware apart. After all some Africans are already gaining notoriety on Youtube constructing wood-burning-cutting laser devices using DVD writers! Whilst others [Germans] are tutoring hackers in CNC machines using parts scrounged from CD/DVD drives and mice [the computer kind!].

Component Mining

A lot of hardware survives a catastrophic failure or electrical surge. Ironically the fuse in the ATX power supply used in most modern desktop PC's, never blows! Most of the transistors are re-usable in virtually any application. They were supposed/reckoned to last 150 years in the 1960's. Most will lie unused in a landfill. Certain IC's will also provide a long and useful life, if you can determine the function of the IC from the obscure number. Fortunately for you these days we have very good search engines on the Internet. A small IC inside a CD-ROM drive turned out to be a 300 milliWatt stereo audio amplifier.

A lot of the cases or housings can be re-used. The metal [thin!] box the ATX power supply uses can house a variety of constructed projects. The power supply used in old [out of date] networking product can power a lot of TTL and CMOS circuitry. Usually these are non-switch-mode power supplies. Therefore they are 'RF clean'.

Old television sets, radios and computers, can provide suitable parts for use in a multitude of projects. Sometimes the construction or assembly can be obstinate in getting at the parts. Expertise is quickly gained after a few mishaps. At the very least this will improve your soldering! De-soldering joints is not an art but a skill. After the first dozen components it becomes second nature to apply sufficient heat and some new solder, to release that 1% resistor from the PCB.

More to come ...


Amateur Radio in the 21st Century.

Hi All

I have decided to write articles about the hobby. Based in South Africa and relevant to "hackers" of the veteran sort and newcomers to the hobby.

It has become obvious over the last 30 years that the hobby has a shrinking number of "young blood". That is not an ageist term and does not refer to the youth of today. But the "newbies" to the hobby.

The 'hobby' has grown in appeal to certain interested people. Especially with experimentation and communication. No longer confined to H.F. or slow speed analogue modulation. Our hobby has grown in various parts of the world and shrunk in others.

My personal activity has always been hardware 'hacking'. With writing software coming a close second. With the local electronic market at the end of an expensive exchange rate pipeline, it has become almost impossible for low income enthusiasts to build anything. I hope to change that.

With this in mind, let me know what you think.

John Brock ZS6WL


Raspberry Pi - session n

First of all, when did you last see an organ grinder?

Today's Q and A session revealed a few things. Dave made copious notes before setting off early for the theatre. But for those who couldn't make it I made a few as well...

Software Mentions

I mentioned it and then couldn't get to the examples on the internet. The club house is a victim of a 'slow internet' connection.

GNU Plot home page  
Gnuplot Primer 
Plotting sensor data using GnuPlot 
interfacing early 90’s HQ digital oscilloscope from LINUX 
FIR Filters by Windowing - using gnuplot 

GIMP - Graphics Manipulation Package 
4 Themes That Will Make GIMP 2.8 Look and Work Like Photoshop 

LinPsk - PSK31 for Linux  [download from Sourceforge]
A Slice of Raspberry Pi

WSPR on the Pi.
/WsprryPi forked from threeme3/WsprryPi
Raspberry Pi QRP TX Shield for WSPR on 20 Meters 

Also mentioned

A Cross Platform and Open Source Electronics Design Automation Suite
Thanks Nic!

Audacity - Audacity® is free, open source, cross-platform software for recording and editing sounds.

Audacity does work on my 'old' Pi. But would work much better on a later model. JB


'Issues' in ugrading Lubuntu

When I made my main PC a dual boot Windwoes XP and Lubuntu, little did I realise that my hard disk was going to run out of inodes...

On researching this 'issue' [we don't call them problems any more], I found that lubuntu or ubuntu keeps updating the kernel. Whilst not removing the old ones. Auto remove [in apt-get] does not flush the old ones. So gradually I had used up all my inodes. This became apparent when I tried to install some new software. It also ran out of disk space.

So when I had a message telling me I had an 'issue' with an update/upgrade with disk space, I checked using 'df'. Nope someone wasn't telling the whole truth. Df said I had a few Gigabytes to play with. So what was wrong?

[Use df -i to show inodes]
More research said that I should remove the old kernels, thus freeing up the inodes. Well I did and we went back to the happy situation. For a while. Maybe two months...

Some months ago I had removed [with extreme prejudice] Windwoes XP. I no longer touched/rebooted into XP. I did not need the XP taking up space on my boot drive. So I made it an ext4 drive and used it for downloading storage. In previous years as drives got bigger and bigger, it was necessary to re-install Windwoes to a newer larger drive. This could take days of transferring settings ensuring all the little things like country got set correctly. In one case I forgot to set the country to South Africa, all the Office settings of course used US spelling! This needed a re-install.

Now for the good part. I backed up my home directory [folder in Windwoes] and my spare part [downloads]. Commence install of lubuntu 15.10...on a whole disk not a partitioned one of 30GB each.

A couple of hours later, I restored my backups to the wide open space on the same disk. When I run df -i now I get loads of inodes. No 'issues' and hair pulling.

I should tell you that the disk drive is a Samsung solid state 60GB disk. Which has performed for several years superbly. It really sped up Windwoes, booting in 4 to 5 seconds. Unfortunately the speed was beaten by an identical PC running Ubuntu!

Now I am happy to boot into lubuntu in a matter of about 3 seconds. But I cannot take a sip of coffee between switching on and logging in!