Relay Bounce

October 9, 2012 on 9:57 am | In Electronics | No Comments

I was using an oscilloscope to do a little troubleshooting on a project that involved having a microcontroller flip a relay and I stumbled across something interesting.  I was graphing the voltage across the relay contacts (there was a good reason for this !) and I spotted this:-

When the relay snaps shut the voltage is supposed to suddenly drop to 0 as the contacts short.  As you can see that’s not exactly what happened.  Instead it bounces between open and shut 4 times before finally settling on closed.  This all happens over 16ms so you might never know about it if you weren’t looking at it on an oscilloscope.  I figure what is happening here is that the moving relay contact is quite literally bouncing off the fixed one.  Note how the closed period (0v) of each successive bounce becomes longer until it finally settles.

Arduino-based Audio Cable Tester

August 6, 2012 on 12:30 am | In Arduino, Electronics | No Comments

I love the Arduino platform – it makes it so quick and easy to get things done.

The local music school had a need to test a bunch of audio cables (1/4″ and XLR) in a hurry in preparation for the annual students’ concert.  Yes, it can be done with a multimeter but if you have lots of them to do this becomes very tedious very quickly.  Sounds like a job for an Arduino

It does XLRs also…

The work is all done by an Atmel Atmega mircocontroller

All of the development work is done in the Arduino environment.  Once it was all working I just transferred the microcontroller onto a piece of perfboard (so that I could reuse the Arduino board for my next project).  The connectors on the far-left of the board (connected to pins 1,2,3,7 and 8 ) allow the Arduino board to be used as a programmer (see That link explains how to configure the Arduino to use the built-in 8MHz RC clock in the Atmega so no external crystal is required (neither accuracy nor flat-out CPU speed are remotely important in this application).

I also stuck a normal ICSP connector on it so that it can be programmed using a normal programmer (bottom-left corner).  This is handy:  once the code was complete I was able to blow the Arduino boot-loader away so that the sketch starts immediately rather than after a pause.  The downside is that if I want to use an Arduino to program it again I need to reinstall the boot-loader (for which that ICSP header will be very useful !)

The source-code is posted here.  The comments in the code list the required connections.  I haven’t bothered to draw a circuit-diagram but if anyone wants one just leave a note in the comments below and I will post it here.




Laptop Transport

August 5, 2012 on 11:39 pm | In Bicycles | No Comments

I needed a better way to carry my laptop on my folding bike.  I used a backpack for a long time which works fine except on hot days (sweaty-back syndrome !).  I tried a handlebar mount (the excellent but expensive  Klickfix system) which also works well but it adversely affects the stability of a bike which is already a little on the twitchy side.  The bike has a carrier but it is so low down that my heel clips anything that overhangs its perimeter.  There had to be a better way.

My simple solution is to make an attachment for the carrier which will allow the laptop case to be mounted vertically without moving around.

The band around the laptop back is a strip of 40mm x 2mm aluminium which is attached to carrier

It works so well I did the same thing for the folding bike’s big brother…

I worry a little about the laptop having the guts shaken out of it (its a very nice laptop !) so I use a folded-up and partially inflated tube to give it a little “suspension”:

I hope someone finds this useful.  If you have thought of a better way to solve the same problem I’d love to hear from you.


The Sound of Music

April 25, 2012 on 5:12 pm | In Guitar | No Comments

Oscilloscopes and guitars are a wonderful combination :-)

This is what an open-E played on the 6th string of my Stratocaster knock-off looks like:-

The two vertical dashed lines measure that time between two consecutive peaks (12.19ms) and from this we can work out that the fundamental frequency is 82Hz (as it should be…E on the 6th string is supposed to be 82.4Hz).

The scope can also do spectrum analysis (showing the harmonic content).  Here’s the same open-6th string:-

Unsurprisingly, you see a strong peak at 82.4Hz (the fundamental) with 1st harmonic (164Hz, one octave above), 2nd (247Hz, which is B on the 2nd string, nicely illustrating the relationship to the 5th note in a chord) and 3rd (329Hz, which is the E on the 1st string, two octaves above the fundamental) all strongly present.  Although there isn’t much of it, 4th harmonic occurs at 412Hz (you can just barely see it).  This is G# played on the 1st string which illustrates the role of the major 3rd note in a chord.  I think its interesting to see how the notes of a major chord all occur as harmonics of the fundamental.

Another interesting thing to look at is how tuning using harmonics works.  This is the 6th string with me barely touching the string just above the 12th fret to damp the first harmonic:-

Two interesting things:  the strongest peak is at 164Hz (one octave above that open E, just as you would expect).  The fundamental frequency of that string is almost completely gone (because my finger prevents the string from moving at its centre point).  Also, there is very little other harmonic content.  Sure enough, if I graph the signal it is much closer to being a pure sine wave at 164Hz:-

If you play that with another (nearly) pure sine wave which is only slightly different (i.e. fractionally out of tune) they will interfere with each other, creating that “fading in and out” effect that we listen for when tuning using harmonics.  They will fade in and out at a rate equal to the frequency difference (which is why you can only hear it when you are very close…tuned to within a few Hz)

These graphs illustrate this pretty well.  Here are harmonics on two strings (6th string 5th fret and 5th string 7th fret) which are slightly out of tune:-

The “waviness” in the signal shows the interference between the two (at ~3Hz, measured with the vertical dashed lines) which I can hear as the characteristic fading in and out.  When I correct the tuning and repeat the test…

…the “waviness” is gone.  This video illustrates what is going on:-


Finally, it is interesting to see the harmonic content of the same note played on two different strings.  Here’s the open 5th:-

And here’s the same A played on the 5th fret of the 6th string:-

Note the much stronger fundamental (110Hz) compared to the 1st harmonic.

Saorview on the cheap

April 25, 2012 on 4:31 pm | In TV & Satellite | No Comments

Irish readers who aren’t living under a rock should know by now that the analogue terrestrial TV service will be switched off in October of this year (i.e. 2012).  If you haven’t make some arrangements to receive digital TV by then you will be enjoying uninterrupted views of cosmic noise, left over from the big bang (otherwise known as “snow” or “static”).

I came across plans for a homebrew antenna at  I just had to tried it and it worked beyond my wildest expectations.  Here it is, mounted in my attic, facing roughly in the direction of the Kippure transmitter:-

Signal strength…100%, bit-error rate…0%:-

The only part I had to buy was the 300Ω <-> 75Ω balun (part number FD78K from Maplin, about €3.50)


I have been running with this for over a year now and it works perfectly.  I have made a few more of them for family and friends with equal success, even in locations where analogue RTE service was poor.

If you really really can’t pick up the Saorview terrestrial service, there is also the Saorsat satellite service.  You will need a Ka-band LNB (which is different to the normal type of LNB you would use for picking up Sky/Freesat etc. from Astra 2) and a dish aligned on 9oE.  I tried it with an ordinary cheap-and-cheerful satellite receiver and a dinky 40cm dish mounted in a bicycle-repair stand…

Saorsat dish

…and it works a treat.  You don’t get TV3 or 3e at the moment but hopefully that will change.  I haven’t tried it, but I understand that it is possible to use a slightly bigger dish and mount a normal LNB off-centre and you can pick up both Saorsat and Sky/Freesat etc. using a single dish. (where I bought the Ka-band LNB among other goodies) sell a mounting bracket sized for exactly this purpose (the positioning of the LNB is critical for this trick to work).  Its a great site with a great range of well-priced products.  They also make their own excellent tutorial videos.  Highly recommended.


Diary of a Hard Disk Upgrade

January 6, 2009 on 4:43 pm | In Windows | No Comments

I recently found myself needing to upgrade the hard disks in a few PCs to bigger and and faster ones. The thought of having to reinstall Windows from scratch an all of them made my heart sink into the pit of my stomach so I set about seeing what would be involved in “cloning” the contents of the existing disks onto the new replacements. I could have just paid for Acronis Migrate Easy (which seems to be generally well-regarded), but where’s the fun in that ?! In the end, I successfully migrated a number of PCs using free software. This article describes how.

The usual disclaimers apply…this is what worked for me – if you try it and it goes wrong and you haven’t taken the normal sensible precautions before embarking on an endeavour like this then please don’t come crying to me.

Step 1 – Partition the new hard disk

I found it easiest to do this from Windows. I connected the new hard disk to the PC and created the partitions using the normal Disk Management tool. You will save yourself some trouble if you “match up” the partition numbers on the old and new disks (i.e. if your Windows partition is on partition #1 on the old disk, make sure it is on partition #1 on the new disk also). On some of the PCs there was a Dell diagnostics partition present (so the Windows partition was actually the second partition). More about this below.

Step 2 – Delete (or rename) the HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices key in the registry

Doing this now will save you a world of pain later on. The problem is that Windows maintains a mapping of volume IDs to drive letters under this registry key

MountedDevices registry key

Here, my new drive was mounted as drive N: when I partitioned it. If I do nothing now, Windows will remount it as drive N: (instead of C:) when I later boot from it, which won’t end well. If I just zap this registry key, Windows will recreate it when it next boots (although it will have forgotten about what drive letters you liked for your USB keys etc.) I guess you could just delete the entries for C: and N:…I didn’t try this.

Step 3 – Reboot into Linux

I variously used Fedora Core 8 and Knoppix 5.1.1…both contained the required NTFS tools and both worked perfectly. At this stage, I have both the old and the new drives attached to the PC. I did this with various combinations to attachment from direct PATA to USB-to-PATA or USB-to-SATA converters and they all worked perfectly.

Step 4 – Dell Diagnostics Partition

One of the candidates was a Dell PC and I wanted to preserve the Dell Diagnostics partition, partly because it is a good thing to have and partly – as I mentioned above – to keep my partition numbering consistent between the old and the new drives (which saves a bunch of trouble). I did this by using the gparted utility to create a new partition on the new disk, similar in size to the original (but rounded up to an even cylinder boundary) with a system ID of ‘de’ (copied from the original). Then I simply did a “dump” sector-by-sector copy of the diagnostics partition from the old to the new:-

# dd if=/dev/sda1 of=/dev/sdb1 bs=32768

This worked a treat. I could then recreate my NTFS partition (again using gparted) as the second partition on the drive. It may seem that this rather wastes the effort of partitioning the disk in Step 1 above, but by doing it this way the standard Microsoft boot code is installed in the MBR. Of course, there are lots of other ways the same thing could be achieved, such as copying the MBR from the original disk to the new disk using the command

# dd if=/dev/sda of=/dev/sdb bs=512 count=1

…and then using gparted to resize the partitions accordingly. Either way, be very careful. If you manage to zap the partition table on your original disk at this stage you are going to have a seriously bad day !

The fun isn’t over yet…see below !

Step 5 – Clone the NTFS parition

The next step is to clone the contents of the old NTFS partition into the new one. This is done using the aptly-named ntflsclone command:-

# ntfsclone -O /dev/sdb2 /dev/sda2

In this form, the command clones /dev/sda2 to /dev/sdb2. Once again, be careful. This command doesn’t ask for any confirmation and if you get your partitions mixed up (e.g. specify them the wrong way around) it will hurt.

Step 6 – Resize the partition

At this point, all of the data is copied from the old disk to the new disk, but the filesystem on the new disk still “thinks” it is the same size as it was on the old disk. If you just reboot at this point you will find – to your dismay – that the new filesystem is no bigger than the old one, even though the partition containing the filesystem is bigger.

To fix this, we run ntfsresize on the new partition

# ntfsresize /dev/sdb2

Step 7 – Fix the number of “hidden sectors” in the NTFS Boot Sector

If the new NTFS partition doesn’t start on the same sector number as the old one, you will need to adjust the number of “hidden sectors” recorded at offset 0x1C in the NTFS boot sector.

TODO: Describe using “hexedit” to complete this job

I did it a different way (kind of by accident). I wound up booting from the Windows Vista installation DVD and selecting “Repair”. This located and repaired the incorrect boot sector for me automatically.

Step 8 – Insert the new disk and reboot

All going well, it will boot up exactly as before – except possibly faster – and you will miraculously have lots of available space

Step 9 – Defragment

If your original disk was heavily fragmented (which it probably was if you were running close to capacity for a good while), then the copy is still heavily fragmented. Now would be an excellent time to defragment the new partition.

Other Alternatives

In the course of my research I came across a promising tool called XXCLONE. This works a different way, being file-oriented rather than sector/block-oriented. Apparently, it copies individual files from the old to the new. As an experiment, I cloned one of the PCs using this tool rather than using the procedure described above. Initially all seemed well…I could boot from it and everything appeared to work OK. However, a few days later I was trying to install Microsoft.NET Framework 3.5 and found that it simply would not install. I can’t be certain that the problem was with the method used to clone the disk, but when I went back and recloned using the Linux tools I could then install the same .NET Framework without any problems. If the problem was caused by XXCLONE it is a pity…it is very elegant and simple to use. I have sent some feedback to the developers…hopefully it will be helpful to them.


I can’t claim to have worked all of this out from scratch by myself. The most useful online guide I found was Michael Dominok’s at

Finding IP addresses hidden in “ifconfig -a”

January 6, 2009 on 4:31 pm | In Apple, Linux | No Comments

Wow…is it really over a year since my last blog post ?

Anyway, here’s something which I hope will be useful.

I find it awkward to pick out my Mac’s IP addresses at a glance from the output of “ifconfig -a”.  Here’s a command-line that makes it very easy…

[eamonn@stig ~]$ ifconfig -a | perl -ne '$interface=$1 if (/^([a-z]+[0-9]+:)/); print "$interface $1\n" if (/inet (\d+\.\d+\.\d+\.\d+)/);'

Now even the most commited command-line disciple is likely to baulk at typing all that, so I have an alias set up in ~/.bash_login.  The quotes are a little fiddly to get right, so here’s the alias…

alias ipa='ifconfig -a | perl -ne '\''$interface=$1 if (/^([a-z]+[0-9]+:)/); print "$interface $1\n" if (/inet (\d+\.\d+\.\d+\.\d+)/);'\'
A slight variation of the same thing works on Linux also (with adjustments to compensate for the differences  in output format)…
alias ipa='ifconfig -a | perl -ne '\''$interface=$1 if (/^([a-z]+[0-9]+) /); print "$interface $1\n" if (/inet addr:(\d+\.\d+\.\d+\.\d+)/);'\'

I hope its useful…

The eircom Master Socket

December 28, 2007 on 2:16 am | In Telecomms | No Comments

Have you ever wondered what lurks inside the master telephone socket that eircom provide?  Surprisingly little, as it turns out.  Here is a photo of the insides of one of them:-

Photograph of the inside of an eircom master socket

(click on the photo above for a bigger version) 

The circuit diagram of the master socket is:-

Circuit diagram of the eircom master socket

There are two sets of three connectors.  The connectors are labelled L1, L2 and R.  Each connector is directly connected to its counterpart on the other side via a breakable link on the circuit board.  The breakable links are tracks on the circuit board which run between pairs of oblong holes (visible in the photo above on the far right-hand side).  I think the idea is that two lines could be delivered via the same socket by breaking these links (although I have never seen or heard of this being done).  The (two-wire) eircom line is connected to L1 and L2.  L1 from the left-hand side connects directly to pin 6 on the RJ11 socket and via the breakable link to pin 4 in the RJ11 socket.  Similarly, L2 from the left-hand side connects directly to pin 1 on the RJ11 socket and via another breakable link to ping 3 in the RJ11 socket.  Since pretty much all modern telephones only connect to pins 3 and 4 of the RJ11 socket, this is really all you need.  For historical reasons, there is a 1.8uF capacitor connecting L1 on the left-hand side to the R pin on the left-hand side.  This is connected to pin 2 directly (and pin 5 via another breakable link) on the RJ11 socket.  This provides a separate ringing signal for (old) phones that need it.

Apart from the capacitor, the only other component on the circuit board is a 470K resistor.  Between R and L2.  This (in series with the capacitor) provides a load which eircom can use to test the line remotely when there is nothing else connected (even at a few tens of Hertz, the impedance of the resistor will dominate that of the capacitor).

 I had expected to find some sort of surge arrestor or something like that in there but there isn’t one (the equivalent BT master socket has a surge arrestor, but is otherwise identical internally – although obviously the socket is physically different).

 Practically speaking, you can ignore the R pin completely…I think pretty much all modern telephones will derive their own ringing signal from the line and don’t rely on it being delivered separately from the master socket.  So the only significant connections in the master socket are these:-

Essential connections in the eircom master socket

The idea is that the eircom line will connect to L1 and L2 on the left-hand side (I think these are labelled S1 and S2 in newer sockets) and you will connect your own internal wiring to L1 and L2 on the right-hand side.  Then, in the event of a fault, it is easy to isolate your internal wiring from the bit eircom are responsible for (thus avoiding a costly call-out charge if you haul out and eircom engineer to a fault which turns out to be with your internal wiring !). 

There are stories told of old modems which require you to snip everything except pings 3 and 4.  I have only come across it twice in my long and distinguished career fiddling with such things.  I can’t claim to have a clear understanding of exactly why this is (sometimes) necessary…I have been offered several conflicting explanations.  I think it is to do with some (cheap and nasty) modems shorting some pins together internally.  If anyone would care to volunteer an explanation I will update this article accordingly.

The source of all network and security problems finally identified!!!

December 20, 2007 on 1:01 pm | In Networking, Security | No Comments

[This post is a slight departure from my stated policy of trying not to increase the average level of inane wittering on the Internet any further by keeping my opinions to myself in this blog, but this particular insight is just too penetrating not to share it with the world. Like all of my opinions, it is - of course - entirely correct ;-) ]

I have had an epiphany. I now know what causes pretty much all network problems: GUIs. At first blush, this may sound like a slightly sweeping statement but I have come to believe in it very deeply. Stick with me here and I will explain why.

Take today for example: Myself and one of my esteemed colleagues squandered an hour of our lives dealing with a guy in a secondary school where we support the Internet router. His Internet access was broken. I’ll spare you the long and painful details of the hour…suffice it to say that by the end of it we determined that there was a server sitting between his 140 snotty, insolent teenagers and our router. It took a startlingly large fraction of that hour to glean from this barely-adequate specimen of humanity that this server even existed. We also figured out that – somehow – the server was at the core of the problem. After a conversation reminiscent of having teeth pulled, our hero volunteered that – infact – there had been a change to the server that morning: he had uninstalled Microsoft ISA server off it !! Somehow – and it completely eludes me how anyone can be quite this gormless – it never entered his head that perhaps uninstalling the proxy/firewall software off the server separating the hormonal masses from the Internet router might be somehow related to his current predicament (140 horny teenagers separated from their porn supply and becoming increasingly antsy about it).

So, how do I extrapolate from this to my theory about GUIs being the root of all evil ? Well, if that server had been a Linux server, there is no way on earth that this guy would have taken it upon himself to touch it. The slightly arcane (yes…I admit it) Linux command-line has a way of scaring off people like this who really need most of their brain power just so they remember to breathe regularly and are taking huge risks by trying to apply their limited stock of intelligence to anything else. In short, command-lines have a way of making things appear a little harder to do than they actually are and therefore act as a built-in safety-net, preventing “special” people from trying to do things they are simply not equipped to do. GUIs, have exactly the opposite effect: they allow the dimmest of knuckle-dragging troglodytes to poke and prod at things they don’t really understand until eventually they manage to break it.

I formulated a more limited form of this theory some years ago when I formed the opinion that Checkpoint Firewall-1 was the source of all security problems on the Internet. When I first started working in the field of data security I could never really understand how hackers seemed to be able to waltz past the best of access lists and firewalls as if they weren’t there. How could it be that the hackers were all so clever and the developers of firewalls were all apparently dribbling idiots ? Then, one day, I was on-site with a large multinational customer watching the guys in there trying to get an application working through a Checkpoint firewall. So, they fired up Checkpoint’s very lovely GUI and they added the rule they thought should do the trick. It didn’t, so the relaxed the rule a little further. It still didn’t, so they relaxed it a little further again, and so the cycle continued through several iterations until eventually the application did work and the “firewall” was reduced to the functional equivalent of a piece of wire. At that moment I understood for the first time that security holes were rarely caused by weaknesses in firewalls and far more often caused by mental deficiencies in those charged with configuring them. I also understood that the GUI was at fault: Checkpoint’s (lovely) GUI makes it very easy to set up rules without the bothersome inconvenience of having to have the remotest understanding of what the hell you are doing. If they had a PIX rather than a Checkpoint (these were the halcyon days before PIX Device Manager, when all was right with the world), this would not have happened. The only thing I didn’t grasp at the time was exactly how generally-applicable the GUI theory was.

Adventures With Windows Vista and the svchost CPU Hog

July 12, 2007 on 12:57 am | In Windows | No Comments

System tray icon showing 50% CPU utilisationImagine my dismay when my new, fast-everything-dual-core-with-tons-of-RAM PC suddenly started running at around 50% CPU utilisation more-or-less constantly, even when it wasn’t supposed to be doing anything. You just don’t have these problems with Linux ;-) . Anyway so began the detective-work.

Most of the CPU utilisation appears to be Kernel-space:-

Graphs showing CPU utilisation

The obvious place to start is to find out which process(es) are doing the damage:-

Pretty consistently the culprit seems to be svchost with a process ID of 1468…it is running steadily at around 50%.

So what is it doing ? Microsoft’s Knowledgebase article 314056 says:-

The Svchost.exe file is located in the %SystemRoot%\System32 folder. At startup, Svchost.exe checks the services part of the registry to construct a list of services that it must load. Multiple instances of Svchost.exe can run at the same time. Each Svchost.exe session can contain a grouping of services. Therefore, separate services can run, depending on how and where Svchost.exe is started. This grouping of services permits better control and easier debugging.

Easier debugging my ass. Anyway, it goes on to give some information about how to find out what this particular instance of svchost is doing. The command:-

tasklist /svc /fi "pid eq 1468"

shows exactly which services are running under this particular process ID:-

tasklist /svc output

So it only remains to figure out which of these services has gone awry. A little bit of guesswork comes in here, matching the abbreviated names from the output above to services. This isn’t too difficult. All I had to do was stop the services listed one-by-one until the CPU utilisation dropped to zero. At it turned out, the culprit was the “TapiSrv”, the Telephony service. This wouldn’t stop when I asked politely (which was a bad sign all by itself) and when it was the only one remaining the CPU utilisation was still at 50%. When I forcibly killed the svchost process from Task Manager the CPU utilisation finally dropped to 0%.

Hindsight is wonderful. It turned out that the reason the Telephony service wouldn’t stop was because it depended on the Fax service and it was the Fax service that was really stuck. Once I had figured that out it dawned on me that the problem began around the time I was (unsuccessfully) trying to send a Fax to my bank to berate them for their stupidity (but that’s a whole other blog entry). I have worked around the problem for the moment by disabling the Fax service and all is well again (by Windows standards, at any rate). I haven’t yet got around to working out what is wrong with the Fax service (the urgency has passed…I found another way to beat up the bank !)

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