So, as always I need to apologise for neglecting things on the site but I've just been so busy. Besides being busy at work I've also been busy in the workshop. One project that I'd like to share with you is my attempt to re-create an iconic British 8-bit computer; the Acorn System 1. 
The Wiki for this computer is here: https://en.wikipedia.org/wiki/Acorn_System_1

By todays standards, the Acorn System 1 was probably less powerful than a modern calculator with only a very limited amount of RAM (just over 1K on board) and a clock speed of around 1MHz, but for many people in the 1970's this would be an introduction to computers that would send them off on a journey that, hopefully, is still ongoing for them today. 

I wasn't rich enough to possess one of these beasts, but my brother was, and it has always held a certain fascination for me. I would see him slaving over the keypad for hours at end, with the result being a few flashing LEDs or some terrible sounding music coming from a tiny loudspeaker he'd connected to the thing. Even so, it was the most fascinating thing I'd ever seen and that wonder has stuck with me all these years. We will return to this in a minute.

A few months ago, I discovered JLCPCB. I used to create my own PCB's but they were limited. Typically single sided and whilst you can do some really clever things making your own boards, there are just some things that are too difficult. To be able to have double sided, high quality boards with a silk screen always seemed like a very expensive luxury. I'm going to create an article on getting PCB's made with JLCPCB. Long story short for now, I needed to order some PCB's and though since I'm paying for postage, is there anything else I want. It was at that point that the Acorn System 1 popped into my head and I decided to slip some PCB's that could, in theory, allow me to re-create the computer onto the order. This was a rush decision. I needed to order the original PCBs ASAP as I had a time critical job that was waiting for them, so I gave myself 24hrs to design a set of PCBs for the Acorn.

I had some hi-res pictures of the System 1 boards, and circuit diagrams, but the pictures were only of populated boards so you can't see all the tracks and their end points and I did find some discrepancies between the board pictures and the circuit diagrams. Also, the Acorn System 1 as was, uses some chips that are not exactly modern these days, and one (or two depending on the final configuration) are all but impossible to get hold of. I will cover exactly how, what and why I did to get the PCBs created in a separate article, but suffice to say for this blog entry, the PCB's arrived yesterday. 

JLCPCB have a minimum qty of 5 boards for each design, which on the one had seems a bit wasteful; especially since I don't even know if my design will work, BUT the boards are so darn cheap you don't really need to worry.

So, the above picture shows the boards and the partially assembled Keypad / display board. 

I wonder if this will work. 

The last 12 months have been hectic and my hobby has suffered as a result. However, things are "hopefully" starting to calm down now and I can return to my beloved pastime. 

I always book vacation time over Christmas and as it's a nice block of time, that's when I undertake my Christmas project. I did have something in mind for this year, but as usual, events overtook me.

Long story short, I made a new friend Dennis, and Dennis has a hearing problem. Whilst we were chatting he mentioned that for various reasons he didn't want to use a commercial hearing aid, so he'd tried to assemble one for himself using an off-the-shelf headphone amplifier but it wasn't really working. After inspection and a little test, the reason became obvious. It was designed for line-level audio input and not direct connection to a microphone. 

Now I could just have added a pre-amplifier, but even then the performance wasn't going to be great. Since size isn't too much of a problem, it occurred to me that some of the available space could be used for adding some audio filters. This opened a complete can of worms.

Depending on the listeners situation, the filter requirements are going to be different. Maybe you're just wanting to enhance the speech audio from someone you are talking too face to face. Or maybe you want to filter out some background noise. Maybe you are at the opera and want to enhance the vocal range but subdue the music from the orchestra and it soon became apparent that this is the sort of project you can make as simple or complex as you want. 

So, some type of adjustable filter would be required. A band-pass filter would probably be ideal. This would allow frequencies within a certain range to pass through, but attenuate frequencies that are either too high, or too low. To make it truly configurable, you need to be able to adjust the lower minimum frequency, and the upper maximum frequency thus leaving the pass frequency range or pass band in the middle.

Something with a preamplifier, band-pass filter, and headphone audio amplifier in a box with a rechargeable power source seems like it would do the job nicely. But how to design and test a suitable filter design.

I don't have any equipment that I can easily utilise to measure the performance or profile audio filters. My spectrum analyser has a sweep generator built it, so this is ideal... but it only works down to around 30 KHz which is wayyy too high for audio work. I need something that will go from near DC (0 Hz) to around 20 KHz.
I need an audio spectrum analyser; or Audio Analyser as they are called. They exist, and you can buy them, but when I saw the price I thought bugger that, I'll make my own. 

The process to profile or measure the response characteristics of a filter is pretty simple. Inject a signal of a known frequency and amplitude into the filter input, and measure the amplitude at the output. Repeat that for a selection of frequencies from the filters minimum to maximum frequency range, record the results, plot them on a piece of graph paper and hey-presto, you get the filter profile, and yes, I "could" do it that way. But it's very labour intensive and if you are want to tweak your filter design and re-profile it, it starts getting very time consuming. What's needed is a gizmo to do the hard work for you. 

More soon...

I like 19" card racks. I've made use of 19" racks in several projects including a custom computer and an extendable bench power supply unit. The only real problem with them is cost.
The basic 19" frame is reasonable enough, but the card front panels and some of the other bits are really expensive.

So, several months ago I purchased a 3D printer kit. Historically I've been really unlucky with CNC machines and they have always been more trouble than they are worth. However, I've been wanting a 3D printer for ages and I've been really looking forward to being able to print some custom parts including these PCB mounts for my 19" racks.

These plastic parts are used to mount the PCB to the card front panel, and I'm amazed at how much they cost; talk about daylight robbery. So, after a lot of fiddling and experimentation, I managed to print a reasonably accurate version of the above. It even worked and this then got me thinking if it would be possible to print the front card panels as well.

A couple of cards in my extendable bench Power Supply

So, after some hacking around in Microsoft 3D builder (which is a really primitive 3D building tool but does the job) I created a model that prints a front plate with integrated PCB card mount bracket.

The next step is to print the front with all the holes pre-made for the switches and LEDs etc. It will save a small fortune in buying metal front plates, and a lot of time in prototyping and building as there should be no more drilling and endless hours at the bench with a file trying to make pretty square cut-outs for displays. 

The digital thermometer clock has been a real success. Many people have contacted me to say they've built it successfully; most of those people had no issues in construction. However, the performance of the clock; mainly it's long term accuracy has been an issue. The DS1307 RTC chip was good (and cheap at the time), but it's not great in a clock like this and constantly seems to need adjusting.

A little while ago I stumbled on EBay suppliers selling DS3231 modules for around £1.50 each for a complete module so I purchased a couple to experiment with and see how easy they are to use.

Today I upgraded my clock to use that module. A little bit of fiddling with the original PCB is required but it can adapted relatively easily. A few changes were also required to the clocks original firmware.

Anyway it's been running on the bench for a couple of hours without any problem, so I'll publish upgrade details soon. I'm also going to re-publish an updated version of the original design.

Details on how to update the original design can be found here.

I've written a review about these buck voltage regulator modules that Ebay is awash with.
You can read it here.
I won't spoil the surprise by giving you the ending.

This was one of those projects that should have taken an afternoon and didn't, however it's now complete and working rather nicely.

I already had a box from a defunct project that had the 4x20 LCD display installed and power switch, mains transformer and a mains IEC socket on the rear so I decided to re-task it for this.

Inside is a PIC18F25K22 running this show, and the device can monitor up to three channels. Each channel has a 5-pin din connector and cable that connects to a Maxim DS18B20 temperature sensor.
You can set the alarm temperature independently on each channel, and the project starts beeping and flashing the LCD backlight if a maximum is exceeded. I also included a relay that can be used as a power interrupter for the project under test. This way I can leave a project on soak but if it starts to over heat, the power can be cut automatically.

The reason it took longer than anticipated was for some reason the PCB never etched correctly and I had some messing around to do. The software only took around an hour to write and debug. I love Proton BASIC.

...and the reason it's three channels and not more, well I happened to have three 5-pin din sockets in my junk box. If I'd had more... who knows.

If anybody is interested I'll make available the circuit diagram, PCB foil and PIC firmware but this really is an easy project to design and build.

So, now that's up and running, I can get back to the original task of designing a decent voltage boost converter.

So, over the last few evenings I've managed to finish the first useful board for my new extendable PSU; a 5v board that can supply around 3 amps. It has a trim control that allows the output to be adjusted from 5v to 6v, and two op-amps wired as voltage comparators that illuminate a couple of LEDs to indicate over or under voltage conditions (less than 5v or more than 6v).

Now all happy that I'd got my first board up and working I set about designing the next one that can provide a variable 0 to 30v at around 1 amp.
Happily playing with some ideas on my breadboard I suddenly noticed smoke coming from the circuit, a bang, and then a piece of the voltage regulator went flying past my head.

The problem was, besides a fault in my design, I hadn't noticed that the regulator was starting to fry.
I did have an amp meter in series with the board but I didn't see what it was reading.

What's needed I thought, is something that can alert me to when things are getting hot.

So, I've started on a new project. It will have the ability to monitor multiple temperature sensors and report audibly if one or more of them exceeds a pre-defined temperature. All this so I can eventually measure and record the discharge curve of a battery pack. See previous blog entries to find out more about that.

That's another breadboard with a half finished prototype put to one side, and the start of another project; multi-temperature sensor alarm project. I need to think of a more snazzy name than that. Whilst it would have just been simpler to pick up some cheap data logger off Ebay.... it wouldn't have been so much fun.

Oh.. and a word of warning. Electronics on the whole is a safe hobby. Baring the old soldering iron burn or stabbed finger when a screw driver slips, you should be pretty safe if you are sensible and carful. However, things do go wrong and most components will complain venomously if they are stressed beyond their design parameters; or just connected the wrong way around. I'm fortunate in that I wear spectacles and they offer a limited amount of eye protection, but you do need to be carful. When things start to go thermonuclear, it usually happens fast. 
If you're ever present when a tantalum capacitor explodes, you will wish you had a gas mask and fire extinguisher handy never mind a pair of spectacles.

I'm currently working on a +5v plug-in module for my new PSU (see previous blog post).
I had several requirements for this module including the ability to withstand a dead-short circuit and the final output voltage to be trimable to +/- 0.5v. Because of this I decided not to use the existing +5v rail that's already present, instead opting to down convert the existing +12v.

Good old linear regulators are almost indestructible if a few sensible precautions are taken, but they give off a fair amount of heat when dropping a large voltage, especially if you are wanting to pull a couple of amps.

After some research I opted for the LM2576-ADJ (also because for some reason I have a stack of them to hand).

The above circuit is straight from the datasheet and I'm using this almost as is. The only component required that I didn't have was the 150uH inductor.

Like most seasoned hobbyists I've learned to keep things that are useful and I've a drawer full of old inductors, ferrite rods and toroids that I've salvaged over the years but I decided to wind my own inductor on a ferrite toroid.

Now I've never really given this much thought before, but when buying inductors they list the inductance value, the amperage and sometimes the resistance. A wire inductor is after all just a long piece of wire, usually wound around a former of some kind so it's bound to have resistance and the more wire you have, the higher resistance. Because it's wire it has a maximum current carrying capacity and hence a maximum working current.

However, whilst experimenting with winding an inductor for this project I noticed something that in hindsight is obvious, but that I'd never really thought about before.

I wound two different inductors and after some experimentation managed to get them both pretty close to the inductance required; and I've opted to use a slightly higher value of around 220uH for this project.

The left one was wound on a much smaller toroid; you can't tell from the picture but the larger core is also around double the thickness of the smaller one.

Both inductors work in my test circuit but the larger one is more efficient.

With the input voltages the same for both tests, the regulator circuit draws 472 ma under test with the larger inductor, and 493 ma with the smaller one.

I'm not sure how many turns there are on the small inductor, probably around 70, but you can see there are only 14 on the large one and even taking into account the physical size of the core, much less wire was required and hence it has a lower resistance.

Oh, and if you're wondering how I wind my cores and check their inductance, I'll create a blog entry about that when I get a chance.

Yesterday evening I completed the first of the plug in modules that simply allows me switch the thing on and off, monitor the output voltages and fault conditions.

This is as basic a module as you can have, consisting of only LEDs and resistors. Later, I will create a better monitoring card that can actually monitor the output voltages. For now, I will rely on the PSUs own low-voltage signal.

These PC PSU's need a decent load on their outputs for them to function correctly. After some research and experimentation I found that 50 ohms across the +12v did the trick and the output voltages remain steady and seem well regulated. One article I found suggested loading the +5v line as well, but this PSU really didn't like that and refused to start.

Next step is to start designing some output cards. A simple card to start containing a switching regulator for +5v with several switched outputs and a couple of amps to begin with.

After this I really need a stable 0 to 10v fine adjustable output at around an amp. This will allow me to calibrate my data logger project.

The long bank holiday weekend arrived and I decided I would complete a battery powered project that's been sitting around for a while; I have a lot of half completed projects sitting around the workshop.
I needed to calibrate the software that monitors the internal rechargeable batteries and realised I didn't have any type of suitable data logger. So a nice quick little project idea was born, and anything to distract me from what I should be doing.

Whilst trying to calibrate my new little data logger I realised that my trusty bench PSU drifts around, a lot.
I built it around 10 years ago and it does the job, but it's a bit limited. I also think that the reservoir capacitors could do with being replaced. Also the always on fans are a bit noisy sometimes.
So, the data logger project got dumped on the growing pile of half completed projects and I set about designing a new power supply.

I've been thinking about doing this for a couple of years, and I've even been slowly acquiring the parts required.

The problem I've found with bench PSU's is that they never do everything I want. Some fixed 3.3v, 5v and 12v outputs that can supply a couple of amps each would be nice. Then I started experimenting with Nixie tubes and realised I needed something that can produce a couple of hundred volts at a few mA. Then I needed a 12v PSU that could supply around 10A to allow me to repair and test a CB radio for a friend. Then I wanted something that was suitable for battery charging. Now 1.8v electronics are starting to become popular and my bench PSU won't go down that low. And the list goes on.

I decided I needed a modular PSU, one that I could plug in personality modules designed for specific purposes and hey presto, the universal modular power supply was born.

It can supply around 24 amps at 3.3v, 30 amps at 5v and a whopping 50 amps at 12v and has a single quiet, variable speed fan. It can also supply -12v at 1 amp. Pretty impressive I hear you say.

Ok, so I cheated a bit and based the design on a 650w PC power supply.

However, I'm NOT going to do what many of these cowboys do and just mod a PC PSU.
It's dangerous and to be honest, a complete waste of time. I've seen many projects that start off by drilling holes in a standard PC PSU, fitting binding posts and then saying its a complete project, only for the PSU to fail (of old age or by your own actions) and you have to start again.

A bench PSU needs to be robust and capable of handling short circuits and other silly mistakes.
Current limiting is also an essential component when your PSU can deliver 50 odd amps in a heart beat.

The PSU I'm using is an unmodified (almost) stock PC PSU. The only modification is to cut off the low voltage connectors as I didn't have suitable male connectors to fit. As it happens, it's a neater job if you cut the cables to the desired length and use your own connectors and that's it. You don't even have to open it up or remove the lid.

My design is based on a 19" Vero rack, and with the PSU fitted there are 10 card slots available for custom modules. The great thing is that if I run out of slots, I can remove modules that aren't currently required or I could extend the unit to a second rack.

The above images show the completed frame.

You will notice from the first image there appears to be three metal spacers from the top of the frame to the PSU. These are just holding the PSU in place and stop it moving around. They are not fixed to the PSU housing.
You may also notice a couple "stop" in the bottom left of the picture that stops the PSU sliding forward and another one on the bottom right that stops the PSU sliding sideward. These stops hold the PSU really snugly and there's no movement at all. Also, if I need to replace the PSU for a different size one, I can move the stops around as required.

I did add an earth wire from the PSU (using one of the PSU's mounting holes) to bond it to the chassis and so scrapped the paint off the PSU to get a good connection.

The backplane consists or two identical (well almost as I changed the design slightly for the second card to correct a minor mistake I noticed during drilling), five slot panels. Each panel is fed with +12v, +5v and 0v connections.
You can just see the control signal connector on the right panel in the bottom right corner that connects to the PSUs control signals.

There is a nice metal cover that screws on the back of this rack to protect everything, and whilst you can't really see it from the pictures, the top and bottom panels are perforated so there is plenty of ventilation for the PSU and the plug in modules. The fact that the frame is metal will also mean it acts as a giant heat-sink.  

As it happens, I probably won't make use much of the PSU's native 3.3v and 5v rails as there is no short-circuit or current limiting protection on these lines, so most of what I need will be driven from the 12v rail. However, some of the plug in modules may require a low voltage for some logic so using the existing 3.3v and 5v rails will save me some effort.

After checking for short circuits between the rails, I connected the PSU plugs and plugged in. As expected nothing much happened. The +5VSB line is active but that's it.

The first module will actually be a monitoring and controller card. The PSU requires a dummy load on the 12v line before it will power up correctly.

I will create a proper project for this once I've got a little time.