Remote Phosphors

I became aware of remote phosphors recently through a video on laser headlights by mikeselectricstuff where he shows a piece of remote phosphor in passing at about 5 minutes in. I’d never seen this material before and it got me interested in having a play around with it.

For a bit of background, there’s not really any such thing as a white LED. LEDs are essentially monochromatic as a result of the quantum mechanics which underpins their operation, which makes producing white light with an LED alone essentially impossible. One way around this is to use a red, green and blue LED in combination but by far the most common way to produce white light from LEDs is to use a blue LED and a yellow phosphor: the phosphor absorbs some of the blue light and re-emits is as various longer wavelengths, which appear as yellow. The combination of blue and yellow appears as white. The phosphor is typically embedded within the package of the LED.

Remote phosphors don’t substantially change this, except that the phosphor is separated out from the LED. One potential advantage of this is that the remote phosphor acts as a diffuser, giving the light a softer glow and more even light distribution. Another is that they essentially remove the influence of the emitter on the quality of the light produced: if a remote phosphor is used, the phosphor gives the same CRI regardless of what blue LED is used behind it, allowing more flexibility with component substitutions. I bought a few Intematix Chromalit parts from digikey to have a play around with. The effect is really bizarre, as you can see below.

My first target to make use of this technology was some LED downlighters which came with my flat. These are pretty awful, producing flickery light.

I thought these were GU10 or MR16 but actually when I took them apart I found that the bulbs are some strange proprietary type called G40. Despite being apparently purpose-built for LED bulbs, the fitting seems to suffer all of the failings of LED retrofits: the compact form factor places hot LEDs next to a hot driver circuit, reducing the reliability of both, and the lack of space leads to insufficient decoupling and large amounts of flicker. This seems like a stupid design decision when the ceiling space gives ample opportunity to separate out the driver.

I ordered some 5W blue LEDs and a constant-current driver module off aliexpress and fitted these in the original fitting. The phosphor I use here was CL-927-LR-PC: 90CRI at 2700K so it should produce a very good quality warm white!

I’m pretty happy with the outcome, at least for a first attempt. The quality of the light seems good, although what isn’t obvious from the photos (given the camera’s auto-exposure) is that it’s not quite as bright as I’d hoped. I used a 600mA LED driver with a 2x series LED package, for about 3.6W. The light it replaced was 5W. I’ve ordered a larger 3x series LED package and a driver to suit it, so in the future I’m going to have a go at upgrading the LED. I think I might also need to have another look at the thermal design. The transfer from the LED package to the fitting case doesn’t seem great. I’m hoping I can find some way to scrape off the paint and improve contact.

Having done this, I’m inspired to have a go at using this material for some other projects: it’s also available in strips, which I think I might use for some under-counter lights in my kitchen.


Weird Extension Lead

Quick blog post… Go!

I found this weird extension lead dumped at the side of the road this evening.

It looks like a normal 4-way extension lead but for the bit in the middle with the “GreenPlanet” sticker and a curious “SMALL/BIG” switch on the side. There seems to be no information on the exact model anywhere on the internet, but the model number is “TL0893INFOB”. I wasn’t sure what this was about at first but I know there’s a lot of snake oil sold around magic “energy saving plugs” so I knew the possibilities were endless and it was certainly going to be something stupid. From the “master” and “slave” labels I figured out eventually what its purpose must be: to turn off the phantom load on the “slave” sockets when the “master” is switched off. When I got home I googled it and my suspicions were confirmed. The reviews I found for this thing (or similar devices under the same brand) were almost universally negative, with complaints including:

  • Doesn’t switch off devices when the master is switched off
  • Randomly switches off “slave” devices while the master device is on
  • A loud hum while it’s working

I’ll admit: I didn’t fully test it before I took it apart. I meant to but I tested it with a laptop charger and it turns out my laptop isn’t great at telling you when it’s plugged in so I got entirely false negative results and assumed there was an internal short blowing the fuse. I did notice though that plugging my laptop into the master socket didn’t do anything (no indicator light, no sound of a relay closing), which is probably why it got thrown away.

Taking it apart, it’s exactly what you’d expect: a board with some crude load detection circuitry and a relay.

I had absolutely no interest in reviving the magic power saving circuitry so I just ripped it out. It didn’t look interesting enough to fully reverse engineer but it looked like a cement resistor (which I ripped off while removing it) measured the current to the master socket, the circuit would detect this above a threshold and switch on the relay to enable current to the “slave” sockets. I’m not sure what the SMALL/BIG switch does but I wouldn’t be surprised if it’s a threshold selection. The power supply seems to involve a capacitive dropper. The IC in the middle is an LM324 quad op amp and there’s a 2N5551 which I suspect is activating the 24V relay. I wonder if the mains frequency ripple across the relay coil is the cause of the noise which some reviewers complained about.

With a couple of terminal blocks, the circuit is bypassed and it’s just a normal extension lead. If I had the desire to, this could have been a fun project for making a “smart plug” with the convenient place to install the circuitry.

I’m always looking for an extension lead so this was a handy find, and interesting to take apart too. It was always a stupid idea because it’s overcomplicated. Instead of something like this, it’s more important for new equipment to be designed with low standby power, eliminating the need for this kind of trickery. The drawbacks are too many: essentially there’s no reliable way for it to tell if equipment is “on” or “off” and the inclusion of the “SMALL/BIG” switch makes it user unfriendly by giving you another hurdle to cross before you’re able to use it. I’m not surprised it ended up dumped at the side of the road.


A Quick Power Bank Repair

Everything breaks at the most inconvenient time – that’s a fact of life. So it’s hardly surprising that I discovered my power bank was broken on a several-hour long train/bus journey. Once I got home, I decided I’d fix it.

The opened power bank

Opening it up, I could see the circuit was divided into two boards: one with the battery and the charging and protection circuitry, and one with the boost converter. I didn’t do too much reverse-engineering of either board but I did discover that the boost converter board had a logic input from the charging/protection board, which enabled or disabled power to all of the circuitry.

The problem seemed to be that nothing would make the protection board drive this high. It has a button to switch it on and, even with the battery fully charged, it would just blink its LEDs to tell you the battery was flat when you pressed this button. Connecting the boost converter board to a bench power supply at 4V and pulling the enable input high, the converter was working fine. So the fault seemed to be with the battery protection.

Rather than bypassing it completely and sacrificing all protection on the battery, I figured an adequate protection for the battery would be an undervoltage cutout. It should cut out at around 3V. It should have some hysteresis too: once the load is removed the battery voltage will recover a bit. I don’t want this to cause oscillation. Once it trips, it shouldn’t reset until the battery’s recharged a fair bit. For this, I’d set a rising threshold of 3.6V.

To achieve this I’d need a low-power comparator and a reference. I thought my chances of finding something lying around were pretty slim – and I didn’t think this project was worth buying parts for. Power banks are easy to come by and if I couldn’t find the stuff to fix this right now, it would have just gone in the bin. So it’s lucky that, looking through a drawer of “Misc ICs”, I found a MAX931:

MAX931 datasheet snippet

An ultra low-power comparator with internal reference was exactly what I needed! And the supply voltage range conveniently covers the 3V-4.2V range over which I need this to work. The circuit I used is a very simple comparator circuit with the inverting input fed from the internal reference. The “HYST” pin controls an internal hysteresis. I decided not to use that – the maximum hysteresis it can add is around 10% but I want closer to 20%. Instead, I achieve the hysteresis by adding a feedback resistor.

For the falling threshold of 3V and the rising threshold of 3.6V, I want a nominal 3.3V threshold (halfway between the two) with a 600mV hysteresis. R1 as 1M and R2 as 560k means the non-inverting input would match the 1.182V reference at the inverting input with an input voltage of 3.29V – pretty much spot on the nominal threshold I was looking for. Divider current will only be about 3.7V/1560kohm=2.4uA. Compare this with the typical input leakage of 10pA with a maximum of 5nA over temperature: this divider current is enough that input leakage can be ignored.

The low threshold is the most critical one, so I calculated the hysteresis based on this. We want to trip at 3V. At 3V, the Thévenin equivalent of R1 and R2 would be a 3*560/1560=1.077V source, with an impedance of 560||1000=359kohm. To bring this up to the 1.182V reference and make this trip, we need to deliver a current into it of (1.182-1.077)/359k=292nA. For output currents <1mA or so, the output will more or less be at the rail:

Datasheet snippet

So the feedback resistor needs to be set to deliver that 292nA with 3V-1.182V across it. That means for R3 we’d like (3-1.182)/292n=6.23Mohm. 6.8Mohm was the closest value to this that I had so that’s what went in. The current draw of this circuit will be around 5uA, about equally split between the potential divider and the comparator quiescent current. The battery is supposedly 5Ah – if that’s the case this circuit will take around a million hours or 114 years to fully discharge the battery.

I threw the circuit together on breadboard:

Circuit with LED to indicate state

I cranked the voltage up and down to check the trip points and as expected, the output would switch on at about 3.6V and off at about 3.0V.

This is far from my finest soldering job but I airwired all the components together:

Circuit before adding a 47nF decoupling capacitor

I’m never going to need to charge four devices at once, so I removed a couple of USB ports to make space for this circuit:

I’m not sure that anyone actually needs to charge four devices at once from a powerbank. I suspect the number of USB ports is partly a historical thing: I remember a time when a lot of USB devices were very picky about what a charger did with the data lines. You can see in the above picture that the ports are labelled with different current ratings and have different resistor networks between D+ and D-. All of my devices charge fine from all of these ports so I just removed the two ports labelled with the lowest currents and wired the circuit in.

This was a real mess and I’d accepted that this is a total bodge job – hence the comparator wrapped up in insulation tape.

I found that the boost regulator, when idle, would draw around 24mA. This would run the (supposedly) 5Ah battery flat in a little over a week. For this reason I bodged a switch in series with the enable output, so that the power bank could be switched off when not in use.

The end result is a mess. But it works. And it has a switchy switch! It goes click! Waste not, want not – hopefully it will last me a little while longer now. Until it breaks at the most inconvenient time – but I guess that’s just a fact of life.


A Dynamic Teleswitch Update – Successful Switch!

I’ve finally managed to switch electricity suppliers! It’s been a long journey, and I described some of the background in my previous post. In this post, I’m giving an update on my journey, I’ve written a guide on how to follow in my footsteps, and I’ve added a few more practical thoughts related to these meters.

My Journey

As I mentioned in my previous post, suppliers other than scottish power are only required to offer me single rate tariffs: meaning I’d be unable to take advantage of cheap night time electricity for my space and water heating. Even in light of this, my scottishpower tariff is so expensive that this still works out cheaper than staying with them. The company I ended up switching to was Octopus: simply because they were the cheapest “big name” supplier for a single-rate tariff with my ~5300kWh/year usage. This turned out to be a good choice, as they did an amazing job of supporting me with my switch.

I initiated the switch using MSE’s cheap energy club and then emailed them immediately to make sure they had all the relevant information about my meter (both of the MPANs) and check that they’ll only be charging a single standing charge. They initially told me they’d be doubling my standing charge on account of my dual MPANs, but I cited the “Energy Market Investigation (Restricted Meters) Order 2016” and I never heard anything about the notion of doubling my standing charge again.

Once I got talking to Lucas at Octopus, I was surprised that he was able to offer me a 2-rate tariff. I was delighted at this since it means I can save a lot more money than I was ever expecting! The switch took a little bit longer than it should have done because of problems with the MPANs not being switched at the same time and scottishpower objecting because of this, but I think lessons were learned and this is all ironed out now.

Unlike on scottishpower, my octopus account shows my two MPANs as if they’re separate meters:

My octopus dashboard

One MPAN works exactly as if it were an economy 7 meter, taking two meter readings (my day and night rate readings). The other MPAN also shows as being on the dual-rate tariff but I only submit one reading for it: my control reading, which gets charged on the night rate. I’m actually getting billed separate standing charges for each MPAN, but I was credited with a year’s worth of standing charges upfront so one standing charge is effectively zero. I haven’t been with them long enough yet to have received a bill, but when I submit meter readings my account balance gets reduced by the correct amount, so it seems like their billing is working fine! Since it’s the middle of summer, I haven’t used any control rate electricity though, so I can’t test that yet.

I’m really pleased with the outcome of this – I was never expecting to be able to escape scottishpower so this is a huge relief. I hope this information helps other people to make a choice with their restricted meters. Octopus have gone above and beyond their duty: all that Ofgem require of them is to offer me a single-rate tariff. I doubt it’s been worth their time and expenditure dealing with me, yet they’ve put me on a fair, dual-rate tariff which they had no obligation to do. The overhead of managing my meter probably outweighs any profit they’ll make from selling me electricity, but I think this is the fair thing to do: for suppliers to take the hit on the overhead associated with legacy infrastructure rather than this being passed to unwitting customers. I hope Ofgem make changes to reflect this in regulation and improve choice for us in the longer term.

Guide to Switching

This guide applies to anyone on exactly the same kind of meter as me: Scottish Power Comfort Plus White Meter. Please feel free to try it with any other restricted metering infrastructure and let me know how you get on! I know of someone else trying this with an SSE THTC meter now too.

  1. Bring your account up to date and in credit with Scottish Power. They may object to your switch if your account is in debt so submit a meter reading and, if you can, make a card payment to put your account comfortably in credit. This may be easier said than done in these times, but reducing the debt may still help if you’re able. Bear in mind that you might not get any credit balance refunded for up to 11 weeks from when you start your switch, so budget for being without the money for that time.
  2. Decide whether you want a single-rate or dual-rate tariff. If more than half of your electricity is on your night or control rate, a dual rate is probably preferable. Check your scottish power bill and you should have a summary of your annual usage. You can see mine at the end of this section, for example. My night+control rate summed together are 1892+2124=4016 – way higher than my day rate of 1348. So in my case, a dual rate tariff was definitely the right choice
  3. Decide which supplier you want to choose. Long story short, I’d recommend choosing Octopus. Admittedly, I do stand to gain if you use my referral code further down, but even if this weren’t the case, I still think they’re probably the easiest supplier to deal with if you have a restricted meter. Their rates are very competitive at the moment too. If you’ve decided you want a dual-rate tariff, as far as I’m aware the only option at the moment is Octopus. It might be worth comparing other suppliers’ economy 7 tariffs and asking them if they’ll supply your meter and with what charges, but be prepared for disappointment! If you’ve chosen a single rate tariff, you’re theoretically able to use any supplier, except for some smaller suppliers. You might want to compare suppliers using a tool like cheap energy club. Make sure to choose “show only big-name suppliers”, because this will exclude the ones who are too small to be required to take on your meter. Theoretically, your switch should be seamless but it’s likely you’ll have to contact them to confirm that you do indeed want their single rate tariff, and to make sure that they transfer both of your MPANs without charging the standing charge twice. Because of all of this, you might prefer to choose Octopus even if you want a single-rate tariff.
  4. If you’ve decided you want to switch to Octopus, it’s best to email them since a restricted meter switch will require some manual intervention on their end. Hopefully it should all be seamless for you though! Email and ask for Lucas – he dealt with all the quirks of my meter and he’s probably now Octopus’ expert on restricted meters! They’ll need your name and address to start the switch. If you know someone currently on octopus who can refer you, you’ll both get £50!
  5. Submit your initial meter readings with your new supplier as soon as they ask you for them, to maximise your savings
Annual usage example

Other Things to Consider

A few other things you might want to consider when selecting a supplier or tariff, or after you’ve switched:

  • If you’re asking for a dual-rate tariff, you’re relying on the willingness of the new supplier to support this. As far as I’m aware, no supplier is obliged to do so. If you don’t stand to gain much from a dual-rate tariff, you might find it easier just to just choose a single rate.
  • Consider how you might change your energy usage. If you’re on a Comfort Plus White meter tariff as I was previously, chances are you use most of your electricity at night since the rates strongly incentivise this. But maybe if you were on a single rate tariff you’d be able to reduce your consumption, and slightly offset the loss of a cheap night rate? For example, maybe you often find your storage heaters are wasting a lot of energy heating your home when you don’t need it? If so, perhaps you could stop using your storage heaters and switch on a plug-in heater as you need it instead. Or maybe you’re heating a full tank of hot water overnight then finding that you don’t need it all? Maybe stop running your immersion heater overnight and just hit “boost” when you need some water. While this is all generally bad advice if you’re able to get 2-rate electricity, you might decide that the hassle of securing a 2-rate tariff outweighs the savings. Before I found I could get a 2-rate tariff, I was planning to do all of these things and decommission my storage heaters!
  • Something I haven’t looked in to yet is whether the process of a meter change is easier after switching. I wouldn’t rely on this but once you’ve switched, it’s probably a good idea to discuss with your new supplier about the possibility of changing your meter to a more standard one. If you can do this, it will probably make switching easier next time. There’s a risk of this being difficult if you’re renting but it may be possible – especially if the replacement is also a smart meter, as I don’t think it’s possible for a landlord to object to a tenant getting a “dumb” meter being replaced with a smart meter.


All of this restricted metering nonsense has consumed a lot more of my time and energy than I’d expected. I think I’m about to wrap this project, but before I do I’m going to contact an organisation or two to try and “pass the baton” in terms of looking out for restricted metering customers. I hope what I’ve discovered helps someone else follow in my footsteps and raises more awareness of the struggles associated with these meters. If anyone else tries this approach, I’d be interested to hear how you get on!


Dynamic Teleswitch Meters

For most people in the UK, switching electricity and gas suppliers is an easy process – and you can save yourself a lot of money by comparing your options regularly and considering switching. I’d recommend Money Saving Expert’s Cheap Energy Club as a very easy-to use tool for doing this.

However, for a few of us, the options are much more limited. When I moved into my current flat in Edinburgh, I had to deal with a Dynamic Teleswitch Meter, a form of “Restricted Metering Infrastructure” – as Ofgem refer to it. Essentially, the meter has two circuits with three associated rates: one circuit behaves in a similar way to economy-7, being active all the time but switching between a day rate and a cheaper night rate at fixed times of day. The other circuit is switched on and off at the behest of a radio teleswitch signal, and only supplies my storage heaters. The switching times of this aren’t guaranteed – it can be anywhere within a certain window, but must be at least a certain number of hours per night.

My “Comfort Plus White Meter”, plus right hand fuse box for storage heaters and left hand fuse box for everything else

I suspect this form of metering was introduced before the electricity market was changed such that it was possible to change suppliers and so this wasn’t a concern. Technologically, the benefits are apparent: the grid have some flexibility over when they provide power for storage heaters, and this can be used for load balancing. The consumer is rewarded by a slightly lower rate for heat units vs night time electricity. For example, my current tariff is:

RateApprox usage/yrApprox cost/yr
Standing charge22.57p365 days£82.38

The “approx usage” I’ve given is based on my usage over the last 12 months. I’m not sure if this is high or low. Scottish power tell me I’m a high user, and MSE would have estimated my usage at about 1/2 of what it is. But also I live alone in a 1-bed flat which visitors complain is cold in winter, and I think I’m quite frugal with electricity – running washing overnight when I can and avoiding heating water on daytime electricity. So who knows. Anyway, this table illustrates the overarching problem with these meters: the tariffs available are uncompetitive. When I’ve tried to switch before, scottish power have told me that there are no alternative tariffs available from them. Other suppliers have said they’re unable to take on my supply, or are only able to offer me a single rate: to which I said no thanks, because running storage heaters on single rate electricity would be stupid, surely? To see just how bad a deal I’m getting, let’s imagine I had an economy 7 meter and all of my heating usage was on night rate instead, as is typical for storage heater installations. This is Vari-Fair by Bulb — the best deal I found from MSE’s comparison:

Unit rateEstimated usage/yrEstimated cost/yr
Standing20.558p/day365 days£75.04

So I’m paying around £200 extra per year because of this meter, compared to if I had economy 7. In light of this, I decided it would be worth trying to get the meter replaced, even if I had to incur some of the cost myself. I contacted Scottish Power and asked them whether I could change my meter (with my landlord’s permission) to an economy 7 one. I was repeatedly told that they’d be happy to change my meter, but only if I removed my storage heaters – they insisted that storage heaters could not be used with economy 7. Clearly this is an outright lie – the very point of economy 7 tariffs is to work with storage heaters. I asked them what other options I had for heating my home. I expected them to insist that I needed to use panel heaters, but instead they made the baffling claim that no form of electric heating could be used with an economy 7 meter, and so I’d need to remove electric heaters entirely. Eventually, when I agreed that they could disconnect my storage heaters (with the intention of getting an electrician to reconnect them with a timer afterwards), they told me they’d phone me within a few weeks to arrange replacing my meter. This was over a year ago and I’ve never heard back.

I was pretty angry about this, so I did some digging to find out more about these meters. I found a couple of reports commissioned by Ofgem: The state of the market for customers with dynamically teleswitched meters and Understanding the consumer experience of Dynamically Teleswitched (DTS) meters and tarrifs. These reports come across as quite damning of DTS meters, significantly reflecting my experience. In particular they find:

  • DTS customers often don’t understand how their meter works
  • Customers often didn’t choose to have their meter installed
  • Customers who had tried to switch tariffs had often given up in frustration
  • Tariffs are uncompetitive

As well as this, they found that DTS users are typically elderly, less affluent and less educated. This is worrying from a fuel poverty perspective – in that those who are more likely to struggle with fuel costs are also more likely to be taken advantage of through these tariffs. I’d be willing to bet that a lot of them are renting too, so need for landlord’s permission is another barrier to escaping these meters. The latter report is dated 2014, and I thought it seemed strange that six years later I was still suffering the same frustrating experience, so I contacted Ofgem to ask what they’ve been doing for DTS customers since then. They pointed me to a piece of legislation: Energy Market Investigation (Restricted Meters) Order 2016. This requires that electricity suppliers (with the exception of small suppliers serving fewer than 50k households) offer single-rate tariffs to customers with restricted meters, at no extra cost. This is far from perfect, as a single-rate supply is unlikely to be cost effective for running storage heaters and water heaters, but it’s a start. I was curious, so looked in to just how bad it would be to run my home on single rate electricity. Searching for single rate tariffs with an annual usage of 5320kWh, I found that the best deal was available was Symbio Energy’s “Low, Fair and Green 2020 Variable v7”:

Unit rateEstimated usage/yrEstimated cost/yr
Single rate11.055p5320kWh£588.13
Standing charge24.5p/day365 days£89.43

Amazingly, it seems like it would still be cheaper to accept a single rate rather than stick with Scottish Power! And that’s even with about 75% of my electricity usage at night, which is well above the national average. If my daytime electricity usage was higher, my current tariff would be even less competitive. However, Symbio are a small supplier and might not be required to take on my restricted meter (if they have fewer than 50k customers). Even still, there are plenty of single-rate tariffs available from big-name suppliers which would cost <£800 a year, so still a small saving compared with my current situation. And like all of the other tariffs I’ve compared here, some of them have no exit charges.

I’m now trying to switch to a single rate tariff. I’m not expecting it to be easy but I’ll see how it goes. When I’ve tried to switch before I’ve run into problems with dual MPANs — so this time I’ve made extra sure that I’ve made the new supplier aware of that. I hope that the situation improves for consumers with DTS meters. Especially with smart meters being rolled out, this could be a good opportunity to allow such customers more flexibility. While I’ m not particularly familiar with SMETS2 meters, my understanding is that they have multiple switched outputs and so could be programmed remotely to switch between DTS, single rate or economy 7 tariffs. I hope that Ofgem will consider taking advantage of this and pushing further legislation to allow us this flexibility and protect us from exploitation. Because what is happening now feels like exploitation of a captive market by Scottish Power, and a cynical attempt too hold on to these customers. The “state of the market” report by Ofgem sugests that 551k homes have DTS meters UK-wide, including 91k in the south of Scotland. We’re constantly reminded of the need to compare energy suppliers in adverts by price comparison sites. But half a million homes don’t have this option. Hopefully there are brighter (and warmer!) days ahead.


Kenwood Chef A701a Electronic Speed Controller Retrofit

Following on from my previous post, I gained another addition to my mixer family: a kenwood chef A701a.

Latest addition to my collection on the right

I got this as spares/repairs and when I opened it up, one thing was obviously wrong: it seemed to have swallowed half a bag of flour:

It was everywhere

Once I got it cleaned up, it turned out the fault was just a wire loose. The solder joint had snapped, I suppose. With this soldered back on, it lived again!

Broken wire – the neutral wire from the motor

Running it at a low speed, I could smell the residual flour burning off the 450 ohm wirewound resistor. This got pretty hot in low speed operation because at lower speeds it alternates between having this resistor in series with the motor, and shorting it out. From my research previously, this resistor seems to be the most common failure in these mixers. Aside from that, it’s wasting a tonne of power and making the motor hotter than it needs to be, so it worried me a little from a motor reliability perspective too. I realise it had worked fine for 40+ years and the chances are it would continue to be fine, but I thought it would be a fun project to try and retrofit the A901 TRIAC motor controller to this model.

The other reason I wanted to make this modification was that the design required the physical switch on the motor controller board to switch the full motor current. There’s some suppression circuitry which I suspect is partly to reduce the stresses on that switch, but it still gave me the fear – you could even see it sparking as the motor ran! The TRIAC controlled version, on the other hand, only carries the TRIAC gate current through the switch so the stress on the switch is more or less zero.

This snippet from the service manual shows the wiring of the mixer:

The input chokes don’t exist in my model. Also nor do any of the supression capacitors, except the 0.1uF across the resistor (value not labelled but it’s 450 ohm)

I can only assume the chopper circuit is placed in between the field and rotor windings to take advantage of the interference filtering offered by the windings of the motor – since it’s series wound it makes no odds electrically where it’s placed. The same is done on the A901. I haven’t found a service manual for this but I’ve reverse engineered the following schematic:

The switch in series with the gate is the one controlled by the centrifugal governor. It gets pushed open when the motor hits its target speed.

Since the A701a and the A901 are very similar and the motors seem to be more or less the same, I figured I’d just reproduce the A901 circuit verbatim and see if it works. I found this old solder terminal board on it lying around, and I thought it seemed ideal for this circuit. I already had the components as spares for my A901s. The TRIAC in the original circuit is unknown and has some non-standard marking, so I took a punt on a BT137-800E. In this simple circuit I suspect all triacs with a sufficient voltage and current rating are much of a muchness, and the 800V, 8A rating of this seems like it should be plenty.

I disconnected the motor controller circuit on the mixer and placed this new circuit in instead. It works! Here I’m just using a crocodile lead to simulate the switch actuated by the governor so there’s no actual speed control.

I captured this interesting video too, where you can see how the governor works:

With the basic circuit working, I set about figuring out how to fit it. First, I ripped out the old motor controller components, just leaving the board with the switch:

Next, the TRIAC needed mounting. It didn’t seem to get noticeably hot at all, but since the mixer has plenty of space inside it, I thought I may as well give it some heat sinking just to be on the safe side. I used one of the holes in the board to mount it onto a random bit of aluminium angle I found lying around, with a thermal pad to act as a gap filler:

I found this random bit of metal which conveniently fitted in the pillar which supported the capacitor, and retained an M3 screw:

With the circuit bolted down, annoyingly the TRIAC leads didn’t reach to any of the solder terminals:

This may have been a blessing in disguise though since I’m not sure I’d have been comfortable with the solder joints being exposed to the potential stresses caused by vibration in the mount. Anyway, I wired up the TRIAC with short lengths of wire instead. I’ve also wired in the switch.

And now putting it back together and soldering the various motor/mains input/switch wires:

And it works! Another happy mixer! It gets a lot less hot at low speeds now

I’m pretty happy with this result! If I end up getting another one, I’m tempted to do a bit more of a polished job of it by designing a PCB to go on the old board, and possibly make it easier to upgrade more of them. It seems like A701s appear with broken motor controllers from time to time, and the wirewound resistor is expensive to find a replacement for. Replacing just a PCB could be a good alternative…


Kenwood Chef A901 repair

I was in the market for a kitchen mixer a while ago, so when I saw a kenwood chef A901 in a charity shop, I jumped on the opportunity to own a piece of vintage kit like this.

As I walked down the road carrying it home, a woman stopped me to say that she recognises the mixer because it’s the first mixer she bought when she moved in with her husband in the ’60s! I’m not sure it’s that old so she might have been thinking of a slightly different model (maybe the A701) but it still goes to show how well the design has stood the test of time.

It served me well for a little while, before it suddenly started smoking and sounding weird while I was using it. Without the motivation to debug it at the time, I put it back in my cupboard as a project for another day. I feared it would be a failure in the motor insulation so the mixer might be a write-off.

Anyway, I revisited it recently and found that I was a fool to put it off for so long! The fault was a very common one and I fixed mine with little difficulty. Having put the effort into figuring out this fault, I thought maybe it would be fun to fix up some more. So I grabbed another one listed as “spares and repairs” off ebay, and sure enough, it had the same fault. Here’s a few highlights from my fixing it:

First, the top cover needs to come off. I found on my way in that a few parts were quite dirty/greasy. A little bit of soapy water cleaned these up nicely though.

Once the top comes off you can see the motor and the gearbox.

Next we need to get into the other end of the motor housing to slide the motor out. This turned out to be very frustrating. One of the screws was seized pretty badly. My screwdriver kept camming out and the screw wouldn’t turn. Even with penetrating oil, I couldn’t get it out. I was preparing myself for the possibility that I might have to drill it out but out of desparation, I tried a long shot: heating it with a hot air gun. It turned out this loosened it just enough to get it open!

With the motor out of the mixer, you can see that the problem is exactly what you’d expect: a failed capacitor.

In fact, I even found the other half of the capacitor rattling around in the housing!

The failed capacitor was a 150nF X-rated but there was also a 47nF capacitor which I figured I ought to replace too. I had some 47nF and 100nF X-rated capacitors in my parts box so I paralleled up both to replace the 150nF.

While I had the motor out, I figured I’d also check the brushes since these are the first thing to go on the motor. With about 14mm left on each side, these still seem perfectly adequate and don’t need replacement.

Almost ready to put it back together! Before I put the motor assembly back in the housing though, I decided I didn’t fancy putting that screw back in which I nearly wore the head off of trying to get it out: it might never come out again if the mixer needs maintenance in the future! I had a look around the workshop for any suitable replacement screws but I couldn’t find any with the same thread. It looked like some kind of self-tapper. The original screw was slightly smaller than M4 though, so I tapped out the holes to M4 and fitted it back together with M4 bolts instead.

Here’s the motor assembly back in the case with the new capacitors:

I tested it and it worked! But at the lowest speed it was a bit too slow, and it pulsed a lot. The reason is because of the ingenious (if, by modern standards, quite primitive) motor speed control. The motor axle has a kind of leaf spring arrangement on it, whose edges get flung out by centrifugal force (in their reference frame, before any pedants correct me) as the motor spins. This causes the top of the spring to push up on a switch on the control board, and stop the triac firing. So it spins up to a certain speed, then starts skipping mains cycles. It’s essentially an electrical version of a Watt governor or a hit-and-miss engine. Amazing! The effect is very pronounced at low speeds and with no load, since the motor’s inertia makes the control loop quite underdamped but adding a load makes it less noticeable. The speed is set just by adjusting the position of the control board relative to the rotor. To increase the minimum speed a little, I just loosened the spring loaded screws a little until the minimum speed seemed about right, without excessive pulsing.

Here’s the hinge mechanism put back together with nice socket head M4 machine screws, rather than the nasty phillips head self-tappers it had originally:

With it all put back together, there was one final step. The rubber feet on the bottom were completely worn down. It turns out you can buy replacements online, and the gap between the bottom of the mixer and the work surface may contribute to motor cooling. A corkscrew comes in handy for pulling out the old feet:

And it’s all happy again! That was quite a satisfying saturday project, now to find someone in the market for one of these… And maybe get some more and do the whole thing again!


Fake flash

Being a complete fanny, I managed to drop my phone and break the display assembly again.


I was in the pub the next day with a friend recounting my tale of woe, and as he was showing off his shiny new phone to me, it suddenly occurred to me that we used to have the same phone. I ended up buying his old phone off him for parts, and repaired my phone there and then.


Annoyingly, in doing so I managed to snap my SD card!

I bought a new 64GB SD card on ebay and when it arrived I decided to do a check of its integrity:

$ dd if=/dev/urandom bs=1M count=64000|pv|sudo tee /dev/mmcblk0|sha1sum -

Already things were seeming suspicious: It claimed to be a class 10 card (10MB/s) but it initially wrote (at least for the first few GB when I was paying attention) at about 2-2.5MB/s.
When I came back later, it was ~45GB in and had sped up to about 15MB/s.

I had planned to read it back and check that the data were the same:

$ sudo pv /dev/mmcblk0 | sha1sum -

But it read at about 10MB/s (rather than the advertised 50MB/s) so I lost interest and tried a different approach, writing to random locations and reading back. 54MiB in was fine, but 54722MiB in returned garbage.

I threw together a quick script to check random 1MiB blocks throughout the disk:

dd if=/dev/urandom bs=1M count=1 of=somerandom
for i in $(seq 1 1000) 
	thisblock=$(shuf -i 0-63999 -n 1)
	sudo dd if=somerandom of=/dev/mmcblk0 bs=1M seek=$thisblock
	sudo sync
	sudo dd if=/dev/mmcblk0 skip=$thisblock bs=1M count=1 | diff - somerandom
	if [ $? == 1 ]
		echo "bad"
		echo $thisblock >> badblocks
		echo "good"
		echo $thisblock >> goodblocks

(I don’t know if the sync is necessary – I wasn’t sure if maybe there’d be some transparent caching in play whereby the OS would assume what it’s just written is still there.)

This gave me two lists, one of the known good blocks and one of the known bad blocks. About 3/4 of the blocks were bad, so it was looking like this 64GB was a 16GB in disguise:

 $ wc -l goodblocks badblocks 
 240 goodblocks
 760 badblocks
1000 total

I threw together a piece of octave too, to make a visualisation:

for i=goodblocks'
for i=badblocks'
colourmap=[0 0 0; 0 1 0; 1 0 0];

The result:
Each pixel represents a 1MiB block of the SD card, top left being the start, working right then down, with the bottom right being 64GB (more accurately 64kMiB, or 64000MiB). Black pixels are unknown (the script took a random sample of 1000 blocks out of 64k), green are good (read back the same) and red are bad. This looks pretty conclusively like it’s a 16GB card in disguise.

The bad section seems to be the same 512 byte pattern repeated over and over for a bit, then the repeated pattern changes sometimes for no discernible reason. I haven’t invested much energy into figuring out why yet.


Puzzlehunt 2015: Dossier


Since 2012, CUCaTS have been running a puzzlehunt annually during Cambridge’s May Week. Usually lasting 24 hours, teams of up to three compete to solve as many puzzles as possible in the allotted time. The puzzles tend to cover a wide variety of fields, including steganography, number theory, encryption, signal processing, geometry and even linguistics. Some of them are pen-and-paper problems, some require URL hacking and the like, while some require physically going to a place to look for clues. I’ve taken part twice and helped organise it three times. And either side of the admin interface, it’s always been an amazing thing to be involved in.

The puzzle

This post is a writeup for one of the puzzles I set in 2015. This one’s called dossier and you can find it on the puzzlehunt 2015 page.

Uncharacteristically, the puzzle description isn’t that cryptic: you’re told that you need to decipher the two in-depth ciphertexts.

In other words, there are two plaintexts, let’s call them p_0 and p_1. These are both encrypted with the same key, say k for example. This means we have two ciphertexts c_0=p_0 \oplus k and c_1=p_1 \oplus k


Bitwise XORing the two ciphertexts together gives us the two plaintexts XORed together:

c_0 \oplus c_1 = ( p_0 \oplus k ) \oplus ( p_1 \oplus k ) = p_0 \oplus p_1

So if we know part of one of the plaintexts, we can work out the corresponding part of the key and, helpfully, the corresponding part of the other plaintext.

The body of the puzzle gives a few “crossword-style” clues, along with corresponding locations in the texts at which these words occur. Take the first clue, “0x014 Maniac Hat Time (13)” – this is an anagram of “mathematician”, which occurs starting at the 14th byte of the plaintext.

At this point, enter a little program I wrote to help with solving this: xorsolver. This is just a convenience thing really, and partly just a little exercise in tkinter for me. It does nothing which can’t be done with a few lines of c and a text editor. It’s also a terrible piece of code, with copy and pasted code lines all over the place.

Essentially it allows you to make guesses at parts of the plaintext and immediately see the result the other plaintext.

To start with, it initialises with a guess of the first plaintext as a long run of null bytes. The other plaintext is correspondingly nonsensical.


As soon as we enter our first piece of guessed plaintext (specifying a memory location and the text at the bottom), a piece of the other plaintext reveals itself:


” am writing t”. In fact we could guess that the word “mathematician” is surrounded by spaces: guessing ” mathematician ” at 0x13 yields: “I am writing th”. Now we can make a guess at the context for this. Let’s try ” I am writing this” at position 0x12.


” a mathematician fr”. Guessing a bit further ahead: ” a mathematician from ” gives ” I am writing this le”. Another bit of leapfrogging ahead, and we end up with “, I am writing this letter ” and ” a mathematician from Meche”. Guessing placenames beginning with “Meche” or a word to follow “letter” might be a bit tedious, so let’s leave this section for now and pick up another clue.


The clue “0x071 Not just a guess (6)” corresponds to “theory”. Guessing this at position 0x71 gives ” week.” in the other text. A decent guess at what comes before this might be ” last”. So we guess ” last week.” at 0x6C. This gives “ring theory”. A bit more leapfrogging and the furthest we can easily get with this section is ” in the fields of string theory and knot theory. He ” and “er I bought from you last week. Unfortunately, the a”.

Picking up some more clues, and doing a bit more leapfrogging, we might end up here with our two texts:

����������������� a mathematician from Meche����������������������������������������#548 in the fields of string theory and knot theory. He %,sm$6/��������������������������������������������������������������������������������������������������������������eI v probably ))-m)j9'?8l.3v��������������������������������������������������������������������tthe first reticulated pantograph ��������������������, this day to be one of the most important inventions of the 19th century in ?? ? ?  ?  theory�����������


��Is$RN>A�, I am writing this letter 
��N�V????er I bought from you last week. Unfortunately, the a       D�EdA
	GN>O I&? however, ??????????????�HRFRM
OJ� �' is interfering greatly with my pALNI
YP	DR ial purification and I would be grateful if you could rectify this at once. >+|#$  ",oS*v~vxE#A\�

So it looks like the two texts are a biography of some mathematician/inventor, and a complaint letter. Maybe not the juicy enemy communications we were after but let’s stick with it anyway. Incidentally, there’s no way to know which piece of plaintext corresponds to which text. You could end up with them randomly jumbled up together and have to untangle them – I’ve cheated a bit here since I know both of the texts anyway – although I did still make one mistake along the way and have to correct it. The only way to deal with this is through the context in the surrounding text. If the texts were formed of a load of independent characters, de-interlacing them would prove futile. But generally the structure of words and sentences, and the general style of the piece helps with decoding.

Anyway, using this higher level context, we might guess that the complaint letter would begin with a greeting. With a bit of guesswork we find that “Dear Sir or Madam,” fits perfectly. We end up with a good chunk of plaintext of the mathematician’s biography. “Henri Martens was a mathematician from Meche”.

The rest of the solution proceeds in the same way, using the clues given and guessing pieces of the text. Eventually you end up with the two plaintexts:
“Henri Martens was a mathematician from Mechelen, Belgium. He pioneered numerous advances in the fields of string theory and knot theory. He was awarded the Donald Knuth prize for computing in 1842 for the invention of the impersonal computer. This amazing feat, however, was surpassed shortly after in 1843 when Lord Timpson of Skegness demonstrated the the first reticulated pantograph — widely regarded to this day to be one of the most important inventions of the 19th century in the field of applied maths.”

“Dear Sir or Madam, I am writing this letter to complain about the four dimensional printer I bought from you last week. Unfortunately, the arm meant to move it in the temporal dimension is defective and is stuck travelling forward at the speed of light. You are probably aware that this renders my machine into an ordinary three dimensional printer. This is interfering greatly with my plans for peaceful racial purification and I would be grateful if you could rectify this at once. Sincerely, Abratma Gandhler”

The theme of the latter was intended to change to something a little more lighthearted, along the lines of “…my plans for the enslavement of all of lifekind on earth…” but the wrong files got uploaded in the end.

Writing the puzzle

If I recall correctly, this puzzle was written at around 5AM during the 2014 puzzlehunt. Once I had the idea, two of us separately sat down to write 512 bytes of nonsense. Afterwards, I checked to see how they aligned, in terms of difficult sections matching up. The “leapfrogging” technique can reach a frustrating stall if both ciphertexts reach a difficult-to-guess bit at the same time. It’s much harder to figure that “shortly after in” is followed by “1843 when Lord Timpson of Skegness”, but if you see “I’m writing this letter”, then “to complain about the” isn’t a particularly outlandish guess. So by adding/removing filler in one text, I was able to adjust the difficulty of the puzzle. I also decided to give plenty of clues to give multiple angles of a attack for difficult sections.

Here I’ve coloured the two texts roughly by predictable filler text (green) and stupid high-entropy stuff (red). The clues are also marked in cyan.

You can see that the three sections where red occurs on both lines simultaneously have clues relatively nearby.

In the end, nobody reached this puzzle in 2014 and it was rolled over to the 2015 puzzlehunt. It proved quite popular, with several teams eventually submitting correct answers. If this seems like your kind of thing, sign up to the CUCaTS mailing list to get more information closer to the time.


Bullshit on the Internet

You see a lot of bullshit on the internet. Some of it genuine scams, some of it well-intentioned but ultimately misguided.

Well, here’s something I’m not quite sure what to make of: the Eco-Cooler.

The video demonstrates a design of “air conditioning” requiring no electricity: instead just relying on the flow of air through bottles. It’s one of those things where at first glance (at least if you watch their explanation) it looks plausible, then you think about it a bit more and go “wait, what?”

Their snazzy video explains… blow on your hand with your lips pursed and it feels cooler than if you have your mouth wide open.

That, they say, is how their cooler works.


Air blows through a bottle, it gets compressed, and that cools it down. A nice idea, but unfortunately thermodynamics and fluid mechanics don’t work like that.

Let’s look at the first example – air blown through pursed lips. Why does it feel colder? The answer is simply that it’s faster. Faster fluids transfer more heat, because they keep replacing the warm air around you. On a cold day, you warm up a small “bubble” of air around you. When the wind blows, it blows that away and suddenly you’re surrounded by cold air again. The faster that wind, the less of a “bubble” of warm air you can maintain and the colder you feel. But the air you’re blowing on your hand isn’t any colder because you’ve blown it through pursed lips.

Refrigeration Basics

So what about “the bottle neck compresses and cools the air”? This seems like a massive misunderstanding of how a refrigeration cycle works. So let’s start there.

The way conventional air-conditioning works is that it takes some working fluid (1) and compresses it (2), using a pump. The pressure increases and, crucially, the volume decreases. The same amount of heat* in a smaller space is going to get hotter. If you dump this heat somewhere (3) and bring the compressed fluid down to ambient temperature, it now has less heat than it started with. This is what’s happening in the compressor outdoors when you have an air con system and why it blows out hot air. Since the now cool-ish fluid is still compressed, you can expand it again. When you do, that same amount of heat* spreads out across a larger volume, and so you end up colder than you started. This is what’s happening indoors where the cold air comes out.
(*Actually the heat changes a bit because of the work done on or by the fluid, but that’s unnecessary complication and isn’t crucial to understand the general idea. I thought I should say it anyway for the sake of intellectual honesty.)

So is that what’s happening here? No. The key thing to note about a refrigeration (or air-conditioning) system is that no heat is destroyed. The inside’s made colder at the expense of hot air being blown out of the compressor. So, if this eco-cooler does indeed cool air, where does the heat go?

Could we potentially make this work if we extracted some heat at the bottle neck, and dissipated that to the outside environment?

Flow Analysis

Enter everyone’s favourite often-misused equation of fluid mechanics: Bernoulli.

Bernoulli’s principle is simply a statement that along a streamline, energy is conserved. This means that anywhere along the flow, \frac{v^2}{2}+\frac{p}{\rho} is the same. As air travels through one of the bottles, it will indeed have to speed up as it enters the mouth, because of conservation of mass and therefore volume (subsonic air flow is essentially incompressible). By Bernoulli, this causes pressure to decrease and therefore, according to the ideal gas law, the temperature to change slightly.

How much does the temperature change? Let’s say air enters at 5m/s. That’s a fair breeze. Looking at a drinks bottle, i’d guess the mouth is about 1/5 the diameter of the bottle, so the air has to accelerate to 25 times its starting speed to get through. Let’s also assume an air density of 1.2kg/m³ and ignore the obvious ludicrousness of air travelling through a bottle neck at 125m/s (in reality viscosity would mean a lot more energy gets turned to heat).
Plugging the numbers in, the gauge pressure in the mouth is:
\rho \left(\frac{v_1^2}{2}-\frac{v_2^2}{2} \right) = 1.2 \left(\frac{5^2}{2}-\frac{125^2}{2} \right) = -9.4kPa

From the ideal gas equation, air at 45 degrees will be around 110kPa:
P=\rho \overline{R}T=1.2 \times 287\times(45+273)=109.5kPa

Taking off the gauge pressure and doing the same in reverse gives a temperature of 18 degrees. The air in the mouth is actually colder! This isn’t what we want for a refrigeration cycle at all — if anything this colder air would absorb heat from the environment and be even hotter than it started when it leaves the mouth and returns to atmospheric pressure. We’ve made a very shitty heat pump.

What is this thing?

So if this isn’t actually going to do any refrigeration, what is it? And why does it cool the building by 5 degrees as they claim in the video?

My guess is that their test is unfair or unrepresentative. Perhaps they were comparing no ventilation with this, rather than comparing this with… well, just a window. Just the circulation of cooler air from outside is going to help to some extent, but no more than an open window. But they don’t seem to have any information beyond what’s in the video.

I struggle to work out what the motivation is here. Initially it seems like a solar-freakin-roadways sort of thing: well-intentioned but fatally flawed. But although there’s nothing being sold here, the video does mention two sponsors, so perhaps this is just a publicity stunt for them?

They seem to have a website which is under construction at the moment, so perhaps we’ll find out soon…