John Dee, Queen Elizabeth I Astronomer-Royal’s magickal scrying mirror in the British Museum

The British Museum in London is full of all sorts of oddball items. Some of the things on the ground floor are much more in the line of the cabinet of curiosities than the themed collections upstairs. As you turn to the left after coming through the main entrance there is a  Victorian room dedicated to the Enlightenment. Although the cabinet of curiosities approach is frowned on nowadays, it appeals to the randomly curious part of me. It’s home to a small collection of John Dee’s magical items

Here we have John Dee’s obsidian scrying mirror. Obsdian is apparently a natural volcanic glass formed from solidified lava, and can be polished to a smooth dark surface. I didn’t notice from the description, but apparently this comes from Mexico via the Spanish.

John Dee’s obsidian scrying mirror

It’s the devil’s own job to get a picture of this in the dim light of the Museum, and taking a picture of a black object is never tremendous fun. John Dee entered Cambridge university at 15 and graduated two years later. He taught at various European universities before returning to England when he was 24 to teach navigation and mathematics to captains of the British Navy, playing a key role in the fight against the Spanish Navy.

Astronomy and astrology were linked disciplines at the time. Dee was imprisoned in 1553, allegedly for casting a horoscope for Elizabeth, Queen Mary’s sister and heiress to the throne. It’s easy to see how he got into hot water – the horoscope indicated Mary would die, and Dee was charged with attempting to kill Queen Mary with sorcery. He was released in 1555. Elizabeth became queen in 1558, and Dee’s fate improved.

Instead of scrivening mirrors we have computers and they operate not on unseen angels but on unseen electrons. Because of his failures Dee remains modernity’s dark and forgotten twin. We are able to live in a world that he could conceive of, but one which he could have never invented.

Ed Simon, Notes on John Dee’s Aztec mirror

Enochian Magic and Angelic contact

Dee is best known now for his work on occult phenomena and contact with the spirit world, which he began in earnest in 1581. He found this contact hard, so he starting doing this work third-hand, by employing gifted ‘scryers’ – people who could see the spirit world directly. Using scryers enabled Dee to take comprehensive notes. He first worked with one individual, Barnabas Saul, until Saul was burned out from some disturbing encounters, and Dee searched for a replacement to help him.

In 1582 he found him, in the form of the imperfect rascal Edward Kelley. Kelley was a sensitive, but also a charlatan, whose ears had been cropped for forgery, so he was difficult to use as a witness. Dee’s notes indicate that in November their spiritual research encountered the Angel Uriel, who instructed them to create a talisman that would make communication with the spirit world easier. Dee and Kelley constructed  these and other magickal tools.

John Dee’s wax tablet with Enochian script

This breakthrough led to Dee using a novel language called Enochian script, and the pair made significant progress, and their fame spread. This led to sponsorship from a Polish noble, who was dazzled by Kelley’s scrying ability when he came to England, and who funded the pair’s attempts to discover how to transmute iron into gold. When the noble’s money ran out, Dee and Kelly were sent to Prague with a letter of introduction to Emperor Rudolph II. This began auspiciously, as Rudolph was also attracted to the possibility of the Philosopher’s Stone, but naturally their experiments always remained on the verge of success without actually delivering.

Dee’s reputation as a wizard caused problems at home in England, where in 1583  a mob attacked his home in Mortlake and destroyed his books and instruments.  His fortunes started to decline when the Pope instructed Rudolph to expel Dee and Kelly, and they sought sponsorship for their work from King Stephen of Poland and another noble, Count Rosenberg, who accommodated the pair in his castle.

Kelley’s mischief meant that they parted their ways when he made some unreasonable demands of Dee, who then returned to England in some style in 1589. However, without Kelley he now relied other scryers who were even less reliable than Kelley. Dee held a few more posts before retiring in 1603. Dee died in relative poverty in 1608 – Kelley had perished in 1595 jumping from a window escaping from a Prague penitentiary.

A fascinating diversion into Elizabethan intrigue and mystical inquiry, brought on by a chance visit to the Enlightenment room. A few cabinets down is fellow Cambridge alumnus Stukeley’s romanticised picture of  A British Druid in his 1740 book “Stonehenge, A Temple Restor’d to the British Druids”

Picture from William Stukeley’s Stonehenge, A Temple Restor’d to the British Druids

which is why Druids have been associated with Stonehenge ever since. All part fo what makes the British Museum a wonderfully diverse way to pass a couple of hours in London.

Sparrows of Ipswich survey 2016 results

The RSPB Ipswich Local Group surveyed the town’s sparrows last year, and we have the results now in. It’s a little bit better than I had thought – strongholds to the north-west and south-east. It’s tough to compare this with the 2006 survey because that wasn’t controlled-effort, although at first glance they still seem to be losing the fight around the Valley Road area relative to the earlier survey.

Skytec PRO 600 PA Amplifier repaired but bad design can’t be fixed

tl;dr – to fix the problem throw the Skytec Pro 600 away and buy something better before the Skytec blows your speakers again. Don’t buy Skytec, and if you have it throw it away before it fails on you.

Skytec is cheap rubbish made in China for kids who are wannabe DJs but have little money. This is not quality – I had to repair this amplifier because of a fundamental defect in the engineering design. These are fine for background music, say in a pub. They’ll go reasonably loud in a modest party setting ,say 30 people, but it’s rough, and it’s nasty. You’ll save on the amp and pay in bass drive units if you DJ with this at any scale 😉 And get a limiter if you can, but if you can afford that you won’t be down at the Skytec end of the market.

I made the dumb mistake of buying one of these used from Cash Converters for £30 a while back. I bought it purely on price, I wanted something basic for parties of about 30-50 people. I knew nothing about PA, but I figured a hifi amp wouldn’t cut it for that sort of usage. What I hadn’t anticipated was people shift junk onto the PA market with design defects that were solved in the 1970s. They don’t even need any new parts, just put the Vbe multiplier on the heatsink rather than on the circuit board.

Skytec 600s are sold as 600W and the manual claims 600W output. They are absolutely away with the fairies on that, to the extent they should be done under the Trade Descriptions Act. I guess they hide behind the fact they don’t say RMS power, so they probably mean peak power, though that’s still only 280W. I measured 80V p-p, which is about 28Vrms. Run that into 4Ω and you’ll get V²/R≈200W. Do that for any length of time and it will blow because of the inadequate heatsinking and bad thermal design.

about 80Vp-p (I am using 10x probes) into 6ohms, 130W per channel. Don’t do it for too long, though

HiFi tower talk glowingly about the MOSFETs

Skytec’s PA-600 gives you the extra power you need with exceptional bass. All sound components are co-ordinated carefully and captivate their longevity. The modern MOSFET transistors and extra large power transformers give great sound and dynamics. The high build quality makes it the ideal amplifier for tours and gigging. For use on stages, for DJs, monitoring, parties and conferences.

but there ain’t no MOSFETs in this, simply a pair of paralleled bipolar junction transistors in the complementary pair output stage, 2SA1941 and 2SC5198. Toshiba described the transistors as suitable for 75W amps, you have two in parallel so 150W tops, okay times two for stereo = 300W. The toroidal transformer isn’t over 600W, I’d guess 200W from the size.

It worked OK for me for a couple of years, but then I let someone use it unsupervised for live music. Which brings me to the first warning

Do NOT use the Skytec 600 for live music unless you are aware of the risks you are taking!

I wasn’t, there, and the result was a blown output stage and blown woofer. It only cost me £11 to service the amp and £50 to change out the woofer, so I am now down £91, and I still have a junk amplifier, though it works now. Now that I know the ghastly horror of the circuit design I am not sure I have the balls to use it again, but at least it works as it was meant to originally 😉

Why not for live music then?

After all, the promotional blurb says this:

The high build quality makes it the ideal amplifier for tours and gigging. For use on stages, for DJs, monitoring, parties and conferences.

so what’s the problems then? Dynamic range – live music has a higher peak to mean ratio than recorded music. You end up pushing the bugger harder, so unless you limit the live source in the mix you’ll clip the output. At least that’s what I assume happened, I wasn’t there when It failed 😉 The Skytec is fine for prerecorded music, but the basic problem is that this amp has zero protection for the speakers or the output stage. Worse still, the VBE multiplier that biases the output stage isn’t thermally coupled to the heatsink on the output stage. Let’s hear it from Rod Elliott why this sucks

Thermal Stability

It can be seen that in the Darlington configuration, there are two emitter-base junctions for each output device. Since each has its own thermal characteristic (a fall of about 2mV per degree C), the combination can be difficult to make thermally stable. In addition, the gain of transistors often increases as they get hotter, thus compounding the problem. The bias ‘servo’, typically a transistor Vbe multiplier, must be mounted on the heatsink to ensure good thermal equilibrium with the output devices, and in some cases can still barely manage to maintain thermal stability.

If stability is not maintained, the amplifier may be subject to thermal runaway, where after a certain output device temperature is reached, the continued fall of Vbe causes even more quiescent current to flow, causing the temperature to rise further, and so on. A point is reached where the power dissipated is so high that the output transistors fail – often with catastrophic results to the remainder of the circuit and/or the attached loudspeakers.

I got to find that out the hard way. I’ve actually managed to do a fair number of parties with this fine, but I was always careful to keep the bouncing LEDs of the output display under control by controlling the master gain.

How does the Skytec PRO600 do thermal stability?

On a wing and a prayer.

PC case fan blowing on the internal heatsink

They run a PC case fan 100% of the time  onto the main heatsink, sucking air out of the case, inflow is through the front. There’s no margin for error – although I didn’t trace the circuit it’s a complementary pair of paralleled output transistors driven by a driver (effectively making a Darlington output)  so you got four VBE drops reducing with temperature at 2mV/deg C, asking for thermal runaway. There’s no fight against that with the VBE multiplier because it’s not thermally coupled. Get the die temp of those output devices hot enough, say 40C above ambient and you have 40*2*2 = 160mV less bias than you started with (the drivers are conveniently mounted on the heatsink to make sure their VBE drops too).  This is designed for thermal runaway and the only thing standing between you and a blown output stage is the hope the heatsink and the fan keep the temperature rise down. You can get a little bit of an idea of the architecture from this thread and this PDF of a similar noname PA amp which gives a rough idea of the architecture on the output

Schematic someone has traced out of a similar piece of junk. In my case the four o/p transistors are 2x 2SA1941 (BG7,8) and 2x 2SC5198 (BG 9,10). BG4 is the offending Vbe multiplier that isn’t on the heatsink.

How to fix a Skytec 600 blown output stage

Change the 2SA1941 and 2SC5198 transistors 😉 I buzzed these through with a DVM on diode setting and found them all short, traced back to the drivers expecting them to have gone but they were OK, traced back a further stage of BJTs but they were OK too. The 5A fuse saved the other passive components.

It’s quite repair-friendly – unscrew the three screws on the base holding the heatsink, unplug all the connectors after taking a photo to remember where they go back. Lift the PA module out, snip the duff transistor legs to save the PCB while desoldering the pins one at a time.

snipping the duff transistor legs lets you unsolder the legs one at a time, saving the PCB from overheating.

I powered up the repaired stage on a 30-0-30V bench power supply set to limit at 100mA, I know it’s meant to work off 60-0-60V but I got a signal through and confirmed it wasn’t still duff, before getting it onto the main supply. I also compared the quiescent current (10mA at 30-0-30V)  with the good side, which was the same, so I figured the VBE multiplier was still set about right. Easy win for about £11 in parts. In fact one of the old output transistors was still okay, presumably saved by it’s parallel buddy shorting across it, but I’m not chancing it.

I also went round and tightened the output transistors a tad. It’s easy to overdo this, but the still- working side was about finger-tight like the failed side. I wonder if this also led to the early demise. You just can’t risk the transistor die heating up to any great amount with this design.

Having fixed it I started to test it looking for why it blew. I got a couple of 50W 6Ω wirewound resistors. These are sold on Ebay to people doing LED upgrades to their lights, to put in parallel with the LEDs and draw 24W so the automotive CAN bus filament blown detector doesn’t keep going off. I figured 6Ω is a nice compromise between typical 8Ω and 4Ω speaker loads; real speakers present complex loads anyway. It was the cheapest way of getting a power resistor up to the job. I then dunk the resistors in a pan of water.

Low cost high power load

since I don’t have a heatsink/fan combo up to dissipating 300W. I know electricity and water don’t really mix, but I figure the water isn’t going to shunt my 6 ohms too much. Worth heatshrinking the ends of the resistors though 😉 The reason I used a pan is because the failure mode of these type of power resistors is to violently eject the ceramic slug out the end. So a Pyrex dish or a jam jar isn’t really desirable.

Running both channels full tilt at 130Wpc for two minutes the transistors get up to about 50C at the hottest part of the plastic case. If fairness to the amp I’ve been able to fill a rented Scout hall with music without ever taking it up that high even on peaks, so I ran it for five minutes at 33 watts per channel (~40V p-p). And got the transistor cases up to 110C. The manufacturer’s spec for the junction temperature is 150C peak. If you thrash this like that for a long time I guess  the heatsink/case fan combo is hopelessly inadequate, and it blows.

Sadly I battle tested the inadequacy of the design a second time. Five minutes after running the second test, after I had brought the signal down to 0, I was greeted with this, telling me the right hand channel has gone DC, presumably thermal runaway again.

failed again

While I know how to repair this, I don’t know how to fix it to make it fit for purpose because of the fact the Vbe multiplier isn’t on the heatsink. It’s probably true that my needs don’t push it that hard, but an amplifier that blows after running a steady 33W for five minutes isn’t something I’m going to risk ever using, so it’s time to scrap it.

Skytec Pro600 – Avoid. Just say no.

Sonar ranger for the visually impaired

This project was for someone I know who is blind. If you can’t see your surrounds then coming into contact with things is always a surprise, she is elderly so it’s not easy to use a cane, which is the low-tech surprisingly effective way of orientating yourself if you can’t see.

the fisihed ranger - the glue stick on the side is to give a softwer contact and avoid damaging the ultrasonic sensors

the fisihed ranger – the glue stick on the side is to give a softwer contact and avoid damaging the ultrasonic sensors

Initially I thought the idea was original, but a little Googling shows it certainly isn’t and more sophisticated versions are available commercially, like the minigude and K sonar. But for the low cost ~ £15 of a PIC and a few bits it’s worth a go to see if the basic principle works, assistive tech seems very variable in effectiveness depending on the user.

Bats use ultrasonic pulses to locate things by emitting a pulse of high frequency sound and listening for the echo. More recently ultrasonic ranging has become a big thing in the robotics field. These modules turn the analogue interfacing into a microcontroller-friendly length of pulse digital signal. I bought a SR04 from ebay for less than £3, which does much of the hard work.

SR04 ultrasonic ranger module

SR04 ultrasonic ranger module

You apply 5V, pulse the trig for 10μs and get a pulse of varying duration from Echo. It’s surprising easy to turn that into a tone rising in frequency as you get closer. Start a timer on the leading edge of the echo return, and when the training edge comes, copy the count into the duration control of another timer (copy into the PICs CCP module which controls the period of TMR1) Then toggle a pin when the CCP module resets TMR1.

You have to do a little error checking to catch timeouts or when the distance is too large, the signal gets more reliable as you get closer to an object, which is good. I was able to find doors and follow a wall using it. It works better when the ultrasonic sensors are vertical, the beam spread is narrower. It does not help you find things on the floor.

I was surprised how little it takes to make one of these now – all you need is the HC04, a 16f628 and a piezo speaker, and it runs 5mA off a 9V battery regulated down to 5V.



PIC code on Github


This works better in real-world application held vertically, not horizontally. but I didn’t know that when I made the video.


Olympus LS10 and LS14 DIY wired remote control

tl;dr – the schematic

Olympus LS10 (and LS14) wired remote schematic

Olympus LS10 (and LS14) wired remote schematic

A new approach to a timed recorder

For the last year or so I’ve been trying to make an timed start recorder using a Raspberry Pi and the Wolfson/Cirrus audio card. I was able to make it work, but never eliminate some rattiness in terms of overruns on record – I confess I couldn’t hear them, but it didn’t give me a good feeling. Then I added up the costs –

£25 – Cirrus Audio card
£27 – Raspberry Pi B+£10 – case and odds and sods to make it work
£20 – PCB, time and bits to make a preamp to get from mic to line level

so I’m looking at £80 to get off the ground, and that gives me a seriously power-hungry SD audio recorder, although I can use a timer to save the power drain for active service.

Alternatively, if I could crack the remote control for them I could go on ebay and get a secondhand Olympus LS10, or one of the similar models (LS-5, LS-11, LS-12, LS-14) and use my own LS10 to start with. I can feed a mic straight into the LS10, no extra preamp required and the audio spec is good.

Reverse engineering the Olympus remote control protocol

This cost me £90 on ebay, and it turned out I didn’t need it. You get the info for free, but then I got a natty nearly new LS-14 with an RS30 remote control, so I’m not too unhappy. Unfortunately the RS30 doesn’t work with my Olympus LS10, don’t know why. I’d have been hacked off if I’d just got the RS30 1. Works a treat with the LS14 it came with, on their own  a RS30 seems to go for £50, so I got an okay deal.

my Olympus recorders

my Olympus recorders

Google first – I owe dashanna of the naturerecordists’ list for inspiration, I vaguely recall seeing that post go through on the list. Their solution is this


The connector is an evil little 2.5mm four-pole jack, and these are a bear to solder

nasty connectors to solder, though easier when you realise you only need t wire to two parts. You can pick up 3.3V on the tip, which may be of use...

nasty connectors to solder, though easier when you realise you only need to wire two parts. You can pick up 3.3V on the tip, which may be of use…

I can’t help wondering if life would be easier using a three-pole jack, since only sleeve and ring are needed. Now I didn’t like that battery in dashanna’s version – I mean who the heck would make a wired remote for a machine offering you a 3.3V supply on the tip of the plug and demand you go fit a battery in your remote? It’s just not a clean engineering solution at all. But apparently it works.

So I rigged the cable in series with the RS30 and sniffed the signals. Of the TRRS the tip had 3.3V, the second ring seemed open circuit, the first ring had the wanted signal and the sleeve was ground. Presumably the IR receiver and LED driver are powered off the 3.3V on tip. The signal on the first ring rests high at 3.3V.

Record is this funny little signal

Record is this funny little signal, 100ms at about 1.5V followed by a low

Stop is this signal, pull to ground for 100ms

Stop is this signal, pull to ground for 100ms

In practice you can ignore the second pulse. For all I know it could be an ack back to the receiver to light the LED. I tried using a couple of diodes to pull the signal down to 1.2V but that didn’t initialise record. I then figured this is one of those analogue resistor chain remotes, so I look for what resistor would give me ~1.5V. Turns out if you replace the 1.5V battery in dashanna’s schematic with 100k you get about 1.5V and the recorder starts recording. You don’t need the second pulse at all, and the debouncing seems to be done in the recorder, it takes a little while, up to about half a second to start recording. I guess that means inside the recorder there’s a 100k resistor to the 3.3V rail in series with the first ring.

That works with both the LS 10 and the new LS14, although the RS30 only works with the LS14. So now all I need do is mod the timer to pull down a couple of pins, one through 100k. If I make the stop command the open-drain pin to ring and the rec command a normal pin resting High via 100k to ring, and pull the relevant pin down for 100ms I should be good to go.




  1. I’ve just got onto the Olympus RS30 website and if you scroll through the models that is compatible with it includes the LS-3, LS-5, LS-11, LS-12, LS14, LS-20M, LS100 so perhaps my LS10 was never compatible with it and Olympus have changed their mind since writing the LS10 manual which says on p65 “Exclusive remote control RS30W (scheduled for Spring 2008)”

Compost research moved to

I’ve moved the study into Elaine Ingham’s techniques of soil restoration using microorganisms and compost over to where it’s made more readable and set in it’s horticultural context by Joanne.

a compost heap tracked all the way from start to thermal equilibrium. The sensr for the green trace was reallocated to ambient about 1/4 way through

a compost heap tracked all the way from start to thermal equilibrium. The sensr for the green trace was reallocated to ambient about 1/4 way through

I recently tracked a compost heap from start to reaching thermal equilibrium. Partway through we started another one, and one of the sensors was allocated to tracking ambient. While a single data point isn’t conclusive, it was interesting to see the week 37 (12-18 Sept 2016) second breath of this heap after the second turn coincided with a week of particularly warm weather. This points to a potential ambient sensitivity and it being worth tracking ambient in future and perhaps insulating the heap more in the later stages.

I will keep the technology of remote temperature sensing here, because I’d imagine horticulturalists aren’t that into electronics 😉

How to use an external GPS with a smartphone

The best GPS for a Brit searching for prehistoric stones is a GPS which has OS maps built in.

GPS with OS maps

GPS with OS maps

The trouble with these is the sticker shock, you’re looking at about £300-400, which is still a bit stiff. If you start with nothing, it’s probably still the best way, and you will undoubtedly get a better moving map experience, particularly with a GPS including an electronic compass, which will orient the map correctly for you.

I had a smartphone and Viewranger. I’d bought the Landranger 1:50k set of OS maps on viewranger for about £70 – once you buy digital mapping you’re locked into that provider unless you want to pay up again.

Smartphone GPS is awful and power-hungry

The big problem with a smartphone is that GPS performance is dreadful. Quite how dreadful I hadn’t realised until I got out on Dartmoor and tried to use Viewranger, which made no attempt made to track current position.Well, pretty much until I was on my way back to the start point. I had a paper map anyway, although the smartphone version was easier to control in the wind!

A-GPS doesn't help you here

A-GPS doesn’t help you here

The problem with a smartphone GPS is that by design it will fail you when you need it most, on a featureless moor with no signal. It is new-born each time you start it up. Rather than maintaining the ephemeris (knowing where to look for the satellites) when the phone is off, smartphones use A-GPS – getting the rough location from the network connection and using this to simulate the ephemeris.

Which is OK in towns, and no good to man nor beast in rural areas, because there’s no network connection. So you get to do a cold start of the GPS which can take over half an hour. No fun at all when you are out on Dartmoor. Even in towns the performance of smartphone GPS is dire, compared to a handheld GPS, as I found out looking for birds. Plus it’s power-hungry – running about 43mA @ 5V with continuous GPS on, compared to 25mA with a BT GPS.

Go for a separate Bluetooth GPS

A secondhand CoPilot GPS3 The default Bluettoh code for one of these is 0183 (from NMEA 0183 protocol, I guess)

A secondhand CoPilot BTGPS3, 2003 vintage
The default Bluetooth code for one of these is 0183 (from NMEA 0183 protocol, I guess)

and use an app to get the location signal into the phone, something like Bluetooth GPS to set this as a mock location provider. Then shut off the internal GPS to save power. Start the hardware, then start the app before starting Viewranger, and everything will work better than before. The CoPilot battery is good for six hours, ebay has many more modern equivalents which probably have better battery life. You can save more smartphone power in the sticks by putting the phone into flight mode and specifically re-enabling Bluetooth, this shuts down the power-hungry wifi and phone data systems. Plus it stops Google knowing where you are in real time 😉

I still hanker after a Garmin GPSMAP64 because while this sorts out the poor GPS performance, it is hard to see the smartphone display outdoors, even under a wide overcast sky, and impossible with sunlight falling on it. Nevertheless, the smartphone app is a lot more practical now.