Blog Posts

New digital thermometer clock firmware v8

31/1/2016

I was contacted a couple of days ago about the possibility of changing my digital themo clock to also show the date.
It sounded like a quick change and as the RTC chip does have the date available, it seem like a quick project for a rainy afternoon.

It actually took a lot longer than I thought as I had to also make provision for setting the date, including the year. Then I found I needed to restructure everything and as I made changes, more and more of the code needed to be updated. It's almost been rewritten now.
The new firmware is available on the project page for those that want to try it.

I took the opportunity to make a couple of other changes.
I've removed the need for the crystal on the PIC; the one for the RTC is still needed of course. It wasn't required as the PIC's internal timing is good enough and I needed an additional output pin to connect to my debugger. You can leave the crystal in place if you want and it shouldn't cause any issues. However, you now have two free I/O pins to play with if you need them.

The interrupt called display routine has been completely re-written and is now tiny compared to the original. It causes a lot less flicker and the display runs brighter as a result.

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Problems accessing this site

9/1/2016

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At some point, Weebly, the cowboys who host this site, did something that broke all the sites internal links. I've repaired these as best I can but if you do happen to spot anything that's broken, please drop me a line.

Sorry for the inconvenience.

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Replacement florescent tubes - fading over time

4/5/2015

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I have a moveable bench light that has two 15w florescent tubes that I've had for several years. A few days ago one of the tubes started flickering and then refused to ignite. They are 18 inches long and I thought I would have a hard time finding them but actually they seemed quite common.

I've had "issues" with buying cheap unbranded long tubes from high street outlets so was a little nervous. I found a place on-line http://www.lampshoponline.com/ and was really surprised at how cheap they were. Because I've had problems before, I bought four branded tubes (£2.48 each) and a couple of replacement starters, just in case.

The tubes I bought were Philips T8 Triphosphor - Colour 865 Daylight which were the same specification as the existing ones - though the original ones seem to be unbranded.

After replacing the one faulty tube and switching on it was like looking into the sun and I couldn't believe how bright the new tube was compared to the other one. I'd never really thought about if tubes fade over time and just assumed they eventually burnt out. Anyway, I replaced the other old tube and I'm very impressed with the amount of light being produced. So I'm guessing the original tubes have been fading over time and I never really noticed.

So, don't know if this was something odd with the old tubes, or if they really should be replaced periodically but I've put a note in my diary to check them next year. Since I bought four and they are probably from the same batch, next year I will replace just one tube with one of my spares and compare the light output.

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Breadboard wiring aid tool

1/5/2015

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I received an Email from some kids at Stanford University in the USA about their Kickstarter project for a tool to aid with cutting wires and component leads to the right length when breadboarding.

I's a really nice, simple idea.
You can watch a short video presentation here.

https://www.kickstarter.com/projects/1094241997/ohmkara-breadboarding-multitool-electronics-protot

Good luck guys.

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Bat detector

25/4/2015

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I've added breadboard construction details for my bat detector mentioned some time ago. Breadboard documentation software is getting better all the time and after a recent search I found a pretty good one to play with, but it's far from perfect. It's a pig for example to create new parts and the existing library is pretty good but it's by no means complete.

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Stepper Motors

17/3/2015

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I've been neglecting the site and I apologise for that. Work has been really busy, I got suckered into building a website for somebody (actually suckered isn't correct as I volunteered though I didn't know what I was letting myself in for), and I've been working on some custom gizmo orders for people.

That said I've still had time to play in the workshop and I've got some projects to write-up that I think people may be interested in.

This last week I've been consumed with stepper motors. Well to be specific, trying to find some that are fairly small, powerful and cheap which is proving a challenge.

Ebay is awash right now with 5v steppers that are geared - model number is 28BYJ-48 rated at 5v.
I ordered several but I probably should have grabbed just the one first to experiment with.
Now they are nice steppers for the money. However they have a built in gear box that reduces the speed and means their maximum output speed is very limited. The gear box does mean they have a fair amount of torque though.

28BYJ-48 Available in 5v or 12v models

They would be great for clocks, robotics and even drive motors as long as you don't want your creation to whizz around the place to fast, but for my application much to slow.

However, they weren't a complete waste. Each motor was supplied with a little driver board. Now I don't use stepper motors often, and I've always opted for dedicated driver chips for them. However these driver boards are simply based around a ULN2003 Darlington driver chip. Each IC's contain seven Darlington drivers, rated at around 500ma and up to 50v. They even have the suppression diodes built in and are perfect for driving low current small steppers, motors, bulbs and relays.
It's one of those "why didn't I think of that" type of situations. You simply drive the IC from your CPU, and with four output lines you can drive the stepper in either direction and run in different stepping modes, all for peanuts. These IC's (or one of the many alternate variants) are usually easy to get hold of, and in the case of the ULN2003, you get three spare driver channels for other things as a bonus. I think there is an 8-channel variant which would be useful for a robot with two steppers.

So, whilst it turned unto a useful exercise after all it doesn't solve my current problem.

One solution that I've tried previously is using a modified radio modellers servo motor.
You open the servo and make a change (usually simple) to allow the shaft continuous 360 degree rotation.
However, whilst usually having masses of torque, servo's are rather large, expensive and quite noisy.
There are some micro servos available but they are tricky to modify for full 360 degree rotation.

So, back to searching Ebay.

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Refurbishment of old PSU

4/1/2015

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I've been sitting on a piece of equipment for a little while and my Christmas project was to get the thing working again, and then make some modifications. It's around 20 years old and it's had a hard life. When it was first plugged in to the mains, nothing happened. After some initial checks of fuses it was time to remove the lid and have a look.

A poke around the PSU showed mains going in, but nothing coming out, There was also some liquid residue on the inside of the case.

Anybody who's played with old equipment will probably have experience of PSU problems - specifically with the capacitors. Over time they break down as they dry out and that can cause all sorts of problems, from low or wandering output voltages, to spectacular bangs and flashes followed by clouds of the most foul smelling smoke.

I pulled the PSU module and inspected the PSB underside. There were signs of corrosion on the board but a quick inspection of the rest of the equipment showed no such damage. This was unlikely to be water damage and so is a pretty good sign that one or more of the electrolytic capacitors have leaked.
The copper side corrosion was also under a bank of three electrolytic capacitors so I pulled them to have a look.

You can see the mess under C12. I've never figured out how the capacitor fluid manages to get through the PCB to attack the underside solder joints and tracks... but it often does. Corroded solder joints on the underside are a good indication that the component has leaked something.

A quick inventory showed there were six electrolytic capacitors on the board. Four of them were 1000uf at 35v. One was a small 100uf at 16v and a big 200uf beast rated at 450v.

This isn't my first time repairing vintage power supplies so I replaced all six. Only the big 450v capacitor could be classed as unusual and I do try and keep a few of these types in the spares box. I also used 105 degree low-impedance types for the 1000uf ones.

However, the other capacitors to consider replacing are the EMI suppression / filter capacitors. These capacitors are usually wired across the Live - Neutral and when they fail often go short circuit leaving a trail of stinky smoke and bits of capacitor housing and fluff all over the place.
Even if these capacitors are still intact, it really is just a matter of time before they fail. They may have already failed open-circuit. They only cost a few pence and take seconds to replace so you may as well do it now whilst you've got the equipment dismantled.

But this failure mode raises an important point.

You MUST replace mains side suppression / filter capacitors with at least the same voltage and X/Y class. The actual capacitance value is usually less important.
The voltage rating should be obvious, but the X/Y class can cause confusion, and getting this wrong can cause a future equipment failure to be fatal.

The circuit above shows a typical basic mains filter / EMI suppression circuit though they can be much more complex of course. It's also common especially in older equipment to find very simple arrangements consisting of nothing more that a single C-X.

Looking at the above circuit, C-X is wired directly across the Live and Neutral, and the C-Y capacitors go from each line to earth.

If C-X was to fail by going open circuit, then there would be no real harm done (other than a loss of effectiveness of the circuit). If it failed by going short circuit a fuse or breaker somewhere back along the power line should blow or trip. Annoying but in the end, no serious harm done.

I've had equipment where the chassis fuse keeps blowing because the X capacitor has failed short-circuit.

If one of the C-Y capacitors were to fail open circuit again, again no real harm done. However, if they were to fail by going short-circuit a situation could arise where the chassis is left at mains voltage.

So typically class Y capacitors are designed to fail by going open circuit.
Of course, an obvious question is why don't they just use Y class capacitors in both applications. It would certainly save a lot of mess and acrid smoke being released.
I suspect the main reason is cost. Y class capacitors tend to be more expensive. A secondary consideration is probably that if a capacitor has failed the suppression characteristics of the circuit have been compromised.

Of course, nothings ever simple and there are sub groups for each of the two capacitor classes and it's important to match or exceed the specification of the faulty one.

Subgroup                 Peak Service Voltage
X1                           > 2,500v and <= 4,000v
X2                           <= 2,500v
X3                           < 1,200v

Peak Service Voltage is not the components rated working voltage. The working voltage of these capacitors seems to start at around 275V AC so should be more than suitable in domestic mains equipment. X2 capacitors are very common in UK mains equipment.

Subgroup                 Rated Voltage
Y1                           <= 500v
Y2                           Between 150v and 300v
Y3                           <= 250v
Y4                           <= 150v

These capacitors tend to be supplied in rectangular plastic packages that can be soldered flush with the PCB. Once soldered in place, it would be almost impossible to push over without causing permanent and obvious damage to the package unlike regular ceramic disc capacitors.
They also tend to be self-healing and can often recover from a voltage spike, again unlike cheaper ceramic disc capacitors.

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If it doesn't work, always check the simple things first

14/12/2014

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Checking the obvious when something doesn't work is of course, well, obvious.

A friend recently asked to me look at a piece of kit that had been giving her trouble. It was intermittent and a swift whack or fiddling with the connections would usually bring it back to life but it had gotten to the point where it was unusable so she had sent it to repair When it arrived back it was completely dead and not wanting to fork out even more money or be without the device for an extended period of time, asked me to have a look.

It's a mains powered device and since UK plugs have a cartridge fuse inside, this is always the first thing to check. Unfortunately the fuse was fine.

I opened the device, couldn't see anything obviously loose or damaged so started checking the continuity of the mains flex from the plug to the device. A-ha. Both the live and neutral were intermittent, so it was probably a break in the cable some place.
I cut the plug off, disconnected the flex from the equipment end, and replaced it. When I came to remove the plug from the old flex to attach to the new one, I found that both the live and neutral screws were really loose. What should have been a five minute repair took around an hour.

Always, always check the simple things first.

On the plus side, the equipment looks nicer with a new piece of black flex rather than the grubby original white one.

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The Small Custom Computer (SCC)... rises again... MK4

13/9/2014

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I've decided to resurrect my Small Custom Computer (SCC) project again. This will be the fourth time I've kicked this project off... hence it's the MK4. I've got a better handle on what I want from this project and the direction it should take. Hopefully, I'll actually finish this one.

Will it be practical... Not overly.

Will it be blisteringly fast and able to solve many of the problems facing mankind today... Nope

Will it be challenging... Yes

Will it just be useful... As an academic exercise in how not to design your own CPU instruction set, almost certainly.

On the plus side you can build it yourself as a test-bed for experimentation and who knows, it may be a stepping stone for "you" helping to solve many of the problems facing mankind. So, if in the future it does help you in this endeavour, I'd like a mention please :)

In the mean time, you can follow my progress, or lack of here.

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Cheap and dangerous

11/9/2014

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I purchased an external 3.5” disk drive enclosure from Ebay the other day which has just arrived. Really cheap and you supply and fit your own drive, but it’s just for moving stuff around so I didn’t want to spend much.
When it arrived my attention was immediately directed to the 3-pin UK mains plug and power lead supplied.
British Standard regulations specify that the earth pin (top pin) should be all metal if the earth is actually needed for the equipment, or can be plastic if the equipment isn’t earthed.

The 50/50 metal/plastic combinations are NOT allowed (left picture) for the Earth pin.

And anyway, as a generic cable it’s supposed to be a standard IEC earthed power cord so should be solid metal.

Also, notice the British Standards mark in the top right corner of the right hand picture... British Standards approved my foot.

This then got me thinking about the cable and connector. The IEC connector has 10A 250V stamped on it but the plug was fitted with a 13A fuse.

In the UK, mains plugs have a cartridge fuse fitted and these fuses are generally available in 3A, 5A and 13A ratings, yet the majority of these IEC cables are rated at 10A. I suspect it's because of the Europeans, however, the lead manufactures know this fact and so use cable that can handle at least 13A... don't they????

When I looked at the actual cable it has 3CX0.75mm2 stamped on it so off I went to the cable manufactures web site.

The below table is an excerpt from one of their data tables.
3Cx0.5mm2           16/0.20           6A
3Cx0.75mm2         24/0.20           9A
3Cx1.0mm2           32/0.20         14A

The manufacturer states that the cable in my lead is capable of carrying 9A. I’m not really happy at that as its well below the 13A rating of the fuse and I would have been happier to see 3C1.0mm2 being used with its 14A capability instead.

However, if the plug can be fake - there's no way that was ever BS approved, could the cable also be fake?

The manufacturer data states that the 3Cx0.75mm2 cable should be made from 24 strands of 0.20mm wire.
I checked the actual cable specified by cutting it in half and it was obvious this wasn’t going to take anywhere near 9A. The actual conductors were made from 30 strands of 0.10 mm wire, and a quick check shows that in fact, this cable is only physically rated for around 1.5A.

The above image shows the fake cable on the left, and a true piece of 32/0.20 rated at 14A on the right.

Now I don’t know about other people, but I tend to grab the power cable that’s closest to hand and “assume” that they are all fit for purpose, but this cable is a disaster waiting to happen.

Be warned !!!

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New digital thermometer clock firmware v8

Problems accessing this site

Replacement florescent tubes - fading over time

Breadboard wiring aid tool

Bat detector

Stepper Motors

Refurbishment of old PSU

If it doesn't work, always check the simple things first

The Small Custom Computer (SCC)... rises again... MK4

Cheap and dangerous

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