Monday 16 October 2023


 If you need to message me, join up to the Dozcord Discord server.

There's an invite link

Tuesday 28 March 2023

Making an attic antenna for LoRa APRS & TinyGS

Recently I changed my job. 

Sadly this led to the loss of use of the wideband Discone antenna mounted for test purposes on the side of the old building, which I'd used for my TinyGS and LORA APRS igate. 

I have got an iGate at my home, however I'm disappointed by it's woeful performance on the co-linear antenna I fitted in November last year, it requires further investigation when the weather improves!

The plan is to mount a simple quarter wave ground-plane antenna in the attic. 

First things first, an antenna is made up around an SMA connector, and given a tune-up using the Nano VNA. You can get rough dimensions from this great on-line calculator. 

Next, I need to come up with a method to mount the antenna. 

The plan is to mount the antenna on the top of a piece of 20mm electrical conduit, as a simple mast. 

A slotted cylinder and locking ring is 3D printed. It's a bit on the thick side, as the first one I made was a bit thin, and split...

The slotted cylinder fits over the antenna radials, and secures it to the 20mm PVC conduit. As the cylinder is now so thick, it's a really snug fit, and the locking ring is not required. 

The 3D model for the cylinder can be found here :

As I want to feed two receivers (one APRS, one TinyGS) I've fitted an LNA and a splitter (both procured from AliExpress). The LNA will overcome the losses in the splitter (especially as I ordered a 4 output splitter by mistake!). You can just see in this photo, that I coiled the coax around the PVC conduit. This type of thing is usually used as a choke balun. It's not really needed here, but was just a convenient way to tidy up the coax! 


I also connected a third receiver to the output, just as an experiment. This feeds a Raspberry Pi running ionosphere, which as an APRS decoder. This one's tuned to 144.8 MHz, and yes, this antenna isn't going to be much use at that frequency, but let's see how it fares...
Ionosphere can be found at
It supports linux, mac os, windows, freebsd, and, of course,  our Raspberry Pi. There's a handy script on the page for installing the software on a pi automatically. 

There's some config to set, but it's quite straightforward.

The Raspberry Pi and SDR are zip tied to the mast...
A supply distribution is made up, consisting of a redundant USB hub & a buck converter. There's a clip-on ferrite between the buck converter and the hub, in an attempt to keep any noise out. This will be fed from a 12v plug in supply once mounted. 

Off to the attic! 
A collegue of mine tells me there's a russian phrase for this kind of construction ... "Проводной с соплями" - "Wired with snot" Yep, it's not tidy!

The Base of the antenna can just been seen, sticking out of a redundant water tank. 

Thankfully, my wifi reaches the attic, and all devices log on to their respective servers...

The two APRS iGates appear on the map ...

and TinyGS is working very well.... This picture was taken after a few days, you can see the increase in the numbers of packets received...
... including some well out of footprint! (Tropospheric ducting, perhaps?) 

Sadly, neither my VHF or UHF APRS stations have captured anything yet, as there's not much traffic here... I'll update you on those as it goes ...

Friday 17 March 2023

PiCow - the Raspberry Pi Computer Originated World

There I was, minding my own business, when the lovely owner of commented on my PiPatGen thread.....

Dammit! I was supposed to be tidying the workshop (in all fairness, some tidying occurred!)

A PiCow ... why not ?!?? (if you don't know what C.O.W. is ... look here.)

So I did a spot of research into video playback, and found a brilliant video playback program called hello_video designed for continuously looping video for use on advertising hoardings and art installations.


Some footage was conjured up, and exported from my video editing software (Kdenlive) as an .mp4

hello_video needs a .h264 fileformat to work, so I used ffmpeg to create a raw stream. 

ffmpeg -i input_file.avi -vcodec copy -an -bsf:v h264_mp4toannexb output_file.h264


I created a COW directory and placed the .h264 file in the directory using scp from my main linux PC.

Issuing the command

hello_video.bin --loop  ~/COW/cow.h264

created a wonderful display! It looped seamlessly!

The command was written into the .bashrc file, so it would start up on boot.

The shutdown scripts were added, in a manner identical to the PiPatGen.

I wanted something a little better to shut down the Pi, rather than having to push a toggle button. 

A small timer was conjured up
5V comes into our circuit, and is used to power up a 4049 inverter IC. There's also a feed to a relay coil, and it's normally open contact. SW1 is a normal DPDT ON-ON toggle. On one switch, the normally closed switch is connected to pins 6 & 12 of the raspberry pi's IO's. The NO contacts on the other pole, are connected to +5V via D1 (actually, D1 is a throwback to an earlier iteration, you can just couple the switch straight to +5V!). This is the power switch. When it's operated, pins 6 & 12 are disconnected, and C1 is rapidly charged up to 5V, which switches on U1A, taking the output of U1A low, this in turn feeds U1B, inverting the signal again, and turns on Q1, which energises the relay and supplies power to the raspberry pi.

When the power switch is turned off, pins 6 & 12 are connected, which initiates the pi shutdown script. C1 is still charged up, and starts to slowly discharge through R2. When it gets to about 50% of it's voltage level, the logic does it's thing, and switches off Q1, removing power from the pi. 

R1 is made adjustable, as the time to shutdown a pi varies on which version of the pi you have and the speed of the SD card. When building the device, manually shut down the pi by shorting pins 6 & 12, and set a timer to see how long the pi takes to completely shut down (you can tell as the SD card access LED stops flashing), then set R1 to allow the relay to switch off in a little longer than the Pi takes to shut down, to allow for a bit of a safety margin. My Pi Zero W takes about 9 seconds to shut down, so I set my timer to keep power connected for ~15 seconds. I used a 10-turn pot to make setting a little easier. (Incidentally, the Pi model B takes about 15 seconds, with the same image)

The 4049 is there just as a buffer. I could have used a FET to switch the relay, as that has a very high impedance gate, and eliminated the logic all together, but I didn't!

If your Pi takes longer than the maximum time, make the value of C1 bigger. 

The circuit was built up on a piece of perf-board.
This hardware also works great on PiPatGen, which would eliminate the shutdown pushbutton.

I did try the image on my PiPatGen's hardware, and even on my resource-shy Pi B, the output was smooth. 

Onto making another case!

The PiPatGen's front panel was modified, and despite my Pi Zero having no audio output, I left the audio jack in place, as I can feed some audio into the front panel if required. 

Clearly no shutdown button is now required. 

Another PlayStation modulator was procured from eBay, another case from eBay seller seemoreitems, and a simplified 3D printed front panel was made.

If you're not using an earlier Pi with a video output socket, you'll need to add a couple of header pins, and connect to that... this is the location of the port on my Pi Zero W. The port is marked up "TV" on this Pi Zero W. Check your Pi is capable of a video output before buying, as the newest models seem to have lost this (certainly the 400) 
The image file for the project can be downloaded from 

Here's the obligatory YouTube video ...

Monday 27 February 2023

PiPatGen - The Raspberry Pi Television Pattern Generator.

Recently I came by an old Model B Raspberry Pi, whilst once the cutting edge of single board computing, is now showing it's age a bit.

So what projects can we create today with it?

How about a pattern generator for our vintage analogue TV sets?

It's got suitable composite video output (I *think* even the current Pi's (although sadly not the 400) have this (if you can find one)), so this project *may* work fine on something newer.

Here's the plan:

Have a set of test patterns available, that we can select, say Test Card F, Test Card C, a PM5544 type, maybe the "Native American" pattern, a grey scale, colour bars, SMPTE, cross hatch for setting up convergence... all running straight at boot, selected by switches on the front panel, and a button to safely shut it down.

The latest version of Raspberry Pi OS lite is downloaded and written to an SD card, we don't want (or need) the additional hassle of running a resource hungry desktop GUI.

The system is booted, with a keyboard and monitor attached.

Using raspi-config, the system was set up with SSH enabled, auto-login at boot, composite output enabled etc etc ... 

I installed the FBI (Frame buffer imageviewer), and wrote a bit of python to act conditionally on the position of a 10-way rotary switch in an attempt to display 10 different pictures. 

Whilst FBI does a wonderful job of displaying the images in the console, no way could I persuade it to act on the position of my switch, unless I issued an ESC or CTRL-C between changes on the keyboard. Even using the signals library and attempting to send a killall would work. Once FBI is running, it grabs hold of the keyboard interface and refuses to allow any changes until ESC or CTRL-C is issued on the physical keyboard. Googling the problem shows I'm not alone in my quest.

So, what else can I do? I tried a package called FIM (FBI improved), which performed no differently. 

I could get FBI to scroll through all the images one at a time, by pressing J to step back and K to step forward in the image sequence. This would work, except I didn't want a keyboard attached. I needed a method of generating a "J" and a "K" and getting it into the USB interface as if it were a keyboard. I did contemplate using an Arduino as an HDI (human device interface, that's a keyboard (or mouse) to you and I!) , there's code that I could implement to do this freely available.
Instead I asked on our street's whatsapp group to see if anyone had an old USB keyboard that they were going to get rid of, and someone did!

Excellent. A Dell keyboard duly showed up at the front door, an ideal candidate. The keyboard was quickly disassembled and the keyboard driver PCB extracted... 

Keyboards are generally wired in a matrix, each key connecting an X and a Y (or rows and columns if you like) , and each switch closes one of the X and one of the Y, giving a unique coordinate for each switch, a bit like this...
In this fictitious example above, we can see that J (SW15) puts a short between X7 and Y2, and K (SW16) puts a short between X8 and Y2. All we need to do if discover what connections are made for J and K on our scrap keyboard, and make connections to the encoder board to a couple of push buttons, to make a keyboard with just J & K on it... simple...

The keyboard matrix consists of three layers of plastic film, the top one contains all our "Y" traces, there's an insulating layer with no traces (just some holes that allow the keys to press through and complete the circuit) and a third bottom layer that contains the "X" traces. 
Identifying the two J & K keys, and their contacts is easy enough, and the traces are discovered by testing with a multimeter for continuity. 

Once these are discovered, it's just a matter if wiring up out two push buttons to the appropriate contacts on the USB encoder PCB. My PCB had a layer of black conductive material deposited on the contacts, to facilitate a reliable connection with the plastic traces on the original matrix. This was removed with a fibreglass abrasive pencil, to reveal the copper traces underneath, so they would take solder. 

Now I had a method of skipping forward and backwards through patterns as required.

Next a shutdown button was implemented using a python script in an identical manner to the hospital radio streaming  codec and monitor. Thankfully FBI didn't interfere with the operation of this. The shutdown button connects between pins 6 & 12 on the Pi's GPIO header.

Now, what about some audio. I created a stereo 44.1KHz wav file in audacity and placed some music on one channel (left), and a continuous 1KHz tone on the other (right), this is set to play using the aplay function on boot. I tried using an MP3 player, but it's glitchy with this resource-shy pi.

Next, everything is set to run on boot, and it all works nicely.

As most of my TVs don't have a video input, I'll need to create a UHF output. This is achieved using a redundant PlayStation modulator, purchased for little money on eBay. The modulator case is opened up to save space, and the small RF metal enclosure mounted inside a clamshell case, along with the Pi. Note the european modulator has a setting for I & G, this is for 6 MHz (I) or 5.5 MHz (G) sound carrier settings. Sadly the PI won't output SECAM. American users wishing NTSC output may wish to use a VHF modulator. 

Note on PlayStation modulator wiring:-

White is video in
Yellow is +5V
Red is audio in
Black is Ground. 

The audio is switched between left and right channels of the Pi's 3.5mm audio jack to select tone or music with a simple SPDT toggle switch. 

A front panel was 3D Printed with the holes in it to accommodate all the required functions. The RF is leaving on the rear of the case, via the coax already attached to the modulator. Power is supplied from a USB lead, via a switch, to the Pi and the modulator. Audio from the Pi is switched between L & R channels, and sent to the modulator, and a front panel RCA socket. Composite video is also made available to the front panel.

A front panel legend is created in inkscape, and printed out and laminated. This is then stuck to the front panel. 

I've made an image of the OS etc, which you're free to download and modify here:

Note the username is doz , and the password is DTW.

Unzip this with 7Zip or similar. Don't use the windows native unzipper. Write the image to a card using Raspberry Pi Imager or Belena Etcher etc ..

CTRL-C or escape will quit FBI (although the music will still be playing in the background!) ...

It may be wise to change the default password (DTW) at this stage, if you're connecting your Pi to the outside world. 

sudo raspi-config 

whilst you're in there , press 1 , and expand the filesystem to make use of all the space on your card.

sudo reboot

If you need to change the overscan settings or change to NTSC, you'll need to edit the boot config file

sudo nano /boot/config.txt

and edit the overscan lines. Note this is in pixels. 


and then reboot

sudo reboot

For NTSC change:




There's 3 images and a 1K/ Music file loaded, you can change these to suit. Images are found in ~/doz/TestPatterns/ and images should be PNG or JPG format. These are cycled through in alphabetical order. To display correctly, images should be 720*576 for PAL, or 720*480 for NTSC.

The music file can be found in ~/doz/Music/ and must be called TCM.wav 

You can use WinSCP on windows to change these via the network, or copy them straight to the card if you run a linux distro (or use the SCP command in a terminal)

Tuesday 8 November 2022

Aliexpress LoRa Amplifier AB-IOT-433

I've been experimenting with APRS using the T-Beam LoRa hardware, as described in Andreas Spiess HB9BLA Wireless' YouTube video here. (Andreas has two YouTube channels, both are very much worth subscribing to, here and here.)

Now the standard T-beam hardware outputs around 100mW, and I'm getting a range of around 9 miles, although one packet managed to make a 12 mile journey one morning!

Perhaps a little more output power would help.

AliExpress advertises a small RX/TX amplifier. A quick search for "Lora amplifier" pulls up a few, and I bought this one.

It arrives from the seller very promptly. 

It advertises a 2.3W output power at 5V supply, so you will need to have a suitable license to operate this. I have a ham radio license, so I'm good on 433 MHz.

I gave it a quick test on the bench, and it makes 2W happily at 5V, so a real improvement, however the second harmonic is not very well suppressed... 
We're going to need a simple filter. 

Using the tracking oscillator of my Spectrum analyser, I cut an open circuit length of coax to make a suitable stub filter, and once it's cut to the correct length, It's connected via an SMA T connector to the output. 
The output is measured again...

Much improved :) 

I've curled up the stub, which does not effect it's performance, and connected the amp up. It's probably best not to run the amplifier without an antenna or load connected. 
Lashed up in the car for testing... 
And the range now?

Almost no change! I'm not sure what I was expecting. UHF at these frequencies is very line-of-sight, and the range in this case is ultimately defined by terrain. A testament to the robustness of the LoRa protocol at just 100mW, and an interesting, but ultimately pointless experiment! 

Thanks to Jayne, M0JNE, for her help and thoughts on this project.

Wednesday 12 October 2022

Building the XUM1541 USB to IEEE Serial interface

A while ago I repaired a 1541 Disc drive (here), and despite fixing the VIC-20 (here) I hadn't tested the drive. 

I had, however, seen a project called XUM1541 (details are here) which gives us a USB to IEEE serial.... 

So simple! 

An Arduino Pro Micro is pressed into service, but V8 of the firmware resisted many attempts to flash onto my Pro Micro... the webpage described hitting the Reset button just before running the avrdude command line.... but the Pro Micro doesn't have a reset button. I used a pair of tweezers to short out the RST line to GND on the Pro Micro, but still couldn't get it to work ...

After a lot of messing about, what you need to do is plug in your Pro Micro (into a windows machine), and open device manager. Expand on "Ports and LPT" , and you should see your board appear as "Arduino leonardo" .... this is where it goes wrong...  Now short out RST to GND Twice in quick succession. Your board should now appear as "Arduino Leonardo Bootloader" ... note THAT com port number and insert it into the command line. 

Now, short out RST and GND twice again and hit return on the command line within 8 seconds..... bingo! 

All the software and drivers can be found in this handy download

Once everything is installed, it all works superbly... 

A simple box is printed out.. 

Software works great!

Now , I really need a real C64 to try it all out!

Thursday 6 October 2022

Lamp Limiter / Dim Bulb Tester

One piece of test gear I've never had is a lamp limiter or dim bulb tester. 

They're ideal for s-l-o-w-l-y awakening our vintage electronics up from years of slumber, without too much drama. 

I've been helping my neighbour, Steve, build a pub in his back garden, and while we were testing out some of the electrics, he donated a box of three incandescent 60w lamps to the cause. Ideal. You can't use LED lamps, and halogen lamps are far from ideal.

Here's the schematic.

It's a simple enough circuit. The live side of the mains is supplied via two 60w lamps (in series), each lamp can be switched out as required. This will limit the current and give us a (very) visible sign if there's any issue! 

I purchased a cheap energy monitor from eBay. With both lamps switched out it'll give similar functionality to the Hopi or Killawatt testers. 

I got it from eBay from eBay seller topwoodfly, and it arrived promptly. It has it's own current transformer, as you can see in the picture. 

It's mounted in a box, along with the lamp holders and switches. 

I've put a change over switch on the input to the energy monitor, so we can measure the output voltage. I expect if the output voltage falls too low the energy monitor will stop functioning. I'm not expecting any miracles of accuracy here!

All wired up in a jolly shade of yellow, just to make fault finding on the unit as difficult as possible.

The neutral wire from the mains is passed straight from the input to the output, via the centre of the
current transformer. 

Here's the finished box, running off the bench variac at about 138V

There's a short circuit across the output, just for demonstration purposes!

I must say I'm actually quite impressed by the accuracy of the energy monitor!

Quite a useful addition to the workshop :)

Friday 5 August 2022

The C2N'terface

So, we're here, sorting out our micros, and I've hit a problem... 

I'd love to build a Raspberry Pi based 1541 emulator, but Pi's are currently unavailable due to the dreaded chip shortage, and I won't pay "scalper" prices, so that's out. What's needed is to connect my age-old trusty MP3 player to our VIC-20 (or C-64/128 , PET ... anything that wants a C2N plugged in!)

Now the PET, VIC and C64 don't really have an audio in, like a ZX81 or Spectrum. They expect a processed square wave pulse train. 

A bit of an internet search, followed by a good look at the schematic of a C2N, and a plan is hatched. 

Saving is easy ... what's needed is a simple voltage divider, a shade of filtration, and a capacitor to block any potential DC upsetting the record function of my MP3 player.

Playback not so easy ... we need to peak the signal with an op amp, and square it off with some buffers. 

Here's the original C2N diagram:

The signal comes from the record/play head at a very low level, and amplified by the first 3 op-amps, IC1-2, IC1-1, IC2-1. The signal is centered around 2.5V, as there's a voltage divider formed by R18&19. The last op-amp, IC2-2 starts to clip the waveform, as it's run out of headroom. IC3-1 & IC3-2 are inverters (even if they aren't shown as such, look at the waveforms) and square off our signal to +5V square wave.

Recording is the reverse, except the inverters directly drive the record/play head. 

There's motor drive and "sense" which detects the play key being depressed. 

So here's our MP3 player interface... 

Audio comes in at J2, is DC blocked (just in case), and passes through a level control pot RV1. It's given a bunch of amplification by our op-amp, and the output is already pretty much a square wave at this point, but it's followed by either 2 or 3 inverters. The absolute phase of the interface is defined by the jumper, JP1. There's some talk on the internet that this is important. I'm not convinced, but I'm already into an inverter IC, so it may as well be a hex one! The resultant square waves are sent to the computer. A crude level indicator is provided by U1D driving an LED. This should be flickering a bit during loading. A motor drive indicator LED is also provided, driven by another spare inverter! 

Saving is easy, a voltage divider is formed by R8 and R1, and a bit of filtering by C4, the output should be in the region of 640mV pk to pk. This is fed to the line level input of my mp3 recorder via a DC blocking capacitor, C1.

The sense line is held low, so the computer will always think the play key is down. 

A board is duly designed, and sent off for manufacture. 

... and arrives a few weeks later and is populated. There's a snafu on the original layout, which involved a couple of bodge wires. This has been sorted in the latest revision!

Both saving and loading to both a Tascam wav recorder, and to my aging Acer MP3 player work well... there's a tip to setting the volume level to achieve reliable loading. The LED should be *just* glowing when the carrier is present. Once it's set it can be left alone.

Using the audiotap programme for Windows (here) it's easy to convert downloaded .prg or .tap files to a .wav file for playback. MP3 also worked flawlessly at 256 kbps. I didn't try less. 
A few rounds of Llamasoft's Gridrunner were enjoyed :)