I bought this noname kit, from somewhere in China. I may want two of these in the end, to compare radiation near one site compared with the general background, so getting cost down1 is worth having, and I can’t moan about the price, delivered for < £25 including tube. But I can moan about the total absence of documentation! You pays yer money, you takes yer choice. I looked at some of the offerings from Banggood to see if I could match it. Maybe I should have paid an extra £5 and got someone in China to assemble it for me but the board looks the same. Bangood’s board says RadiationD v1.1 (CAJOE).
It’s a tough life in Chinese electronics, even the copiers get copied. The write up is on Github – it’s a little bit obscure, the PDF overall schematic isn’t that useful but the more detailed schematic in blocks is.
Not a kit for beginners
If you are new to electronics, this kit is not for you due to the amount of guesswork and component ID required, buy it built, the difference isn’t that much. There’s nothing that special or earth-shattering in the parts however, all are easily available, and the board is a reasonably decent construction and easy to solder. There was some satisfaction in second-guessing our Chinese friends but really, how hard would it be to provide the link in a slip of paper or even on the original ad? Investigation of the original designer CAJOE with the Wayback machine at the URL indicated on the PDF didn’t deliver much enlightenment, but there’s enough in Github to give it a go. I used the block schematic with values, thankfully the screenprinting reference numbers on the board match the components in the PDF, which the exception of C5 which is MIA on the schematic, at a guess a 100nF decoupling cap.
The kit prep is a bit slapdash. The classic way to drive a mechanic bonkers is tossing some spare screws in his tin of screws taken out of the engine, and here they lob in some spare components – on populating the resistors I was left with a few over, same for the caps and a couple of transistors. Continue reading “CAJOE ebay geiger counter kit”
I wanted to take an IR photo at dawn. IR photos are easy enough these days, you can either butcher the camera you already have taking out the IR filter, or you can use a Raspberry Pi NoIR camera where they have taken the filter out/not fitted it at the factory.
The dawn part, however, is a problem. I am night owl 😉 I don’t do dawn, if I can help it, and since the target is static in my case, and the Raspberry Pi is the camera of choice, it seems a nice idea to get the Pi to do all the work of getting up early. Why keep a dog if you have to bark yourself?
There are no end of shims and gizmos that will up the 3.7 to 3.2V of a LiPo battery to 5V, for putting into the Pi. I’ve used a wide input shim to power Pi’s off a CCTV power supply rather than have one mains PSU per Pi.
In this case I favoured the piZero variant, and didn’t have aims to be connected to t’internet in the field. So I need a real-time clock, and all of a sudden a simple requirement has turned into dongle hell. This is where I wanted to be:
Pi running off a battery
and at first considered a 5V LiPo plus RTC device like this only to discover it won’t start the Pi on a schedule. I then considered using a 16F628A PIC with one of the DS3231 dongles, a Chinese noname clone of this. It turns out the stock Raspberry Pi driver can support one single wakeup alarm on the DS3231 – the gory details are here. That will pull down Pin 3 on the alarm, although pin 3 isn’t brought out on the connector it’s easy enough to tack a wire onto that. Some Pis have a setting where you can pull a wire to ground to start the board; fortunately the DS3231 pin3 is open-drain so ti will work with that. The PiZero (not W, mine is a 1.3) draws about 30-40mA powered down, that’s much better than a real Pi but still a bit much for a battery. Continue reading “Running a Raspberry Pi off a LiPo battery”
This is for a piezo contact mic that is powered off a field recorder’s plug-in-power supply. That is for small recorders like Olympus’s LS10, LS-5, LS14 series, which are typically unbalanced audio inputs, stereo on a 3.5mm jack, not for a P48 phantom power from a mixer or pro recorder1. You also get an opportunity to drink beer.
Batteries are a pain in field recording. One battery to rule ’em all is best in my book, the fewer things with a battery required to make a system work the better. The recorder generally has to have a battery, and Sony agreed with that many years ago, back in the days of Minidisc, and came up with plug in power (PiP). to eliminate the battery in the microphone. An electret mic doesn’t need much power so battery life is weeks not hours, but it has a switch you often forgot to switch off.
Plug-in-power
Plug in power is about 3-5v supplied through a series 6.8kΩ resistor, with the output developed across the 6.8kΩ resistor. It so happens that electret microphones were coming in at that time, and typically had a FET to buffer the high source impedance of the capacitive electret mic, an audio source through a few tens of pF. The 6k8 resistor is the drain resistor of the electret mic. You can read more about plug in power here.
Ideally a contact mic would work with this just like an electret mic, after all the problem and the solution are the same – a high source impedance 2 requiring a FET to buffer it.
there’s not much power available in plug-in-power
The trouble is that both the FET piezo amplifier and the opamp version need a 9V battery, which is a pain in the backside. An Olympus LS10 needs two AA cells and will run hours, and then the piezo contact mic wants a box with a 9V battery all to itself. That’s not good, and you tie yourself up in wiring.
The opamp version will never run on plug in power. The basic FET buffer is optimised for 9V, so it won’t run off PiP either.
The design
Plug in power contact mic
Initially I tried this with R2 omitted (replaced with a short) but there is not enough current to get the drain off the 0V rail. That’s fair enough – the IDSS is specified as 2-20mA and the most current you are going to get from a 3V supply through a 6k8 resistor is a little bit shy of 0.5mA. I bought a job lot of 2n3819s and experimenting with the value of R2 showed 470Ω put the drain voltage about 1.7V, with the source voltage about 0.5V, dropping about 1.2V across the transistor and 1.3 across the 6k8 drain resistor, which is good enough. Surprisingly, though they were from Ebay, nominally Fairchild parts, all five I tried biased about right with 470Ω up to 680Ω. Ordinarily you’d bypass R2 with a capacitor to maximise gain, but you are rarely short of signal level with a piezo mic, so I omitted that, it will slightly reduce gain, and also limit the current slightly if the piezo puts too much through the gate/source junction if I drop it. I have decided to take a chance with this omitting the diodes of the original FET version, because they’re hard to fit in. I expect peaks to be lower because I have mechanically damped the mic a lot. Time will tell if I get to regret that and the mic goes noisy or fails in service if I drop it once too often.
Your optimal value of R2 may be different due to the spread in FET Vgs- put a multimeter on the drain and aim for about half the open-circuit voltage3 ±10%. Mine is chosen purely for the supply through the left hand channel. If you parallel the inputs you may need a different value. I have hopes of using two to record stereo, else for mono paralleling the inputs makes for easier monitoring in the field.
Plug-in-power contact mic construction
First open a bottle of beer, trying to preserve the shape of the crown seal rather than bend it in the middle cavalierly. Obviously you can’t leave an open bottle of beer in the lab, so drink it. A 2cm piezo disc fits in a typical bottle cap, as shown
2cm piezo disc and bottle cap
Now pick out any plastic seal in the bottle cap, and then clear the residue. I used a Dremel and the face of a small stone attachment to get this
Dremel-cleaned bottle cap
You are going to solder to that surface, it will be the ground, and the cap provides shielding on that side, the brass disc shields the other side. This is a good time to drill a 3mm hole in the side of a cap, I used a small modelling pillar drill. If you don’t have a pillar drill you are probably going to give blood and turn the air blue with cursing. So better file a U shaped slot with a round needle file instead 😉 A hole locates the cable better in the later glue stage, but it’s not essential.
Now assemble the amplifier Manhattan style over the ground plane of the bottle cap, which also takes the shield of the cable. Use relatively thin cable for the output signal, because else handling noise will drive you bonkers. Some cable protection against the sharp metal would be wise, heatshrink in my case. Solder the piezo to the relevant place. You don’t want too much of the red and black cable going to the piezo element because you will have to lose this in a load of glue. I had too much.
the amplifier constructed on the cap
Now would seem a good time to test this with your recorder, handling the piezo should be easily audible.
I wanted to keep a continuous solid interface to both sides of the piezo. Air is quite high impedance mechanically, and a contact mic detects signal in low-impedance solids. The sharp resonance of piezo discs is because they are a solid in air on both sides, and I wanted to damp this and match the disc to the solid medium. I used glue for that.This takes out a lot of the dreadful 2kHz honk in the sound, which is the natural resonance of the disc in air. In an ideal world you would put the contact mic in a solid medium with the same impedance as the ceramic either side to pick up the longitudinal vibrations, which would get rid of that resonance, but getting rid of the piezo-air interface on both sides helps a lot, even if epoxy and hot-melt glue aren’t a great match for the ceramic – they’re much better than air! The cap and glue mass-load the rear of the disc somewhat, which usually improves the sound.
In the final device, you want the disc to sit slightly proud of the bottle cap so the brass disc is in contact with the sound source, but the bottle cap isn’t. For that you can use hot melt glue all the way, though probably in two stages.
the amplifier as the glue is setting
I wanted to glue a neodymium magnet to the other side of the piezo disc. Many things that are interesting with a contact mic are steel surfaces, and a magnet saves you trying to clamp or stick things to such a surface.
I discovered the hard way that hot melt glue is above the Curie point for neodymium magnets, and the loss of magnetism is permanent, turning it into an expensive metal disc. So I used epoxy resin to stick the magnet to the back of the piezo disc. I also got to resolder the red lead to the friable silvering on the back – best done quickly.
magnet glued to piezo disk
It pays to manipulate the magnet with non-ferrous tools, the best way to hold it in place is put it on the disc over a steel surface covered by masking tape. The next stage is glue this to the cold hot-melt glue encapsulated amplifier in the cap – the depth of the glue plus the magnet should set the brass disc proud of the cap. I also had to lose all the red and black wire in the gap and fill this with epoxy. I did this glue operation is two stages, because otherwise it was going to be a very messy job.
first the magnet and disk are glued to the hot melt glue surface
This stage is used to prove the brass disc is clear of the wires and the bottle cap. Only just in the case of the wires.
The second stage infills the gap between the disc and the cap, by mixing the glue on a piece of paper and using a fold in the paper to pour the epoxy into the gap.
The final product
You will get to scrape some epoxy off the disc. Aim to make nothing stand proud of the brass disc, which will then be held on steel surfaces with minimal gap.
Field testing – what does it sound like?
This is the mic attached to galvanized farm gate, the excitation is entirely the wind. I could not hear this sound standing next to the gate, it was a strong breeze, not a hurricane. I found it was very easy to overload, because you can’t really hear those low frequencies and metering isn’t fast enough for the transients. I record the main output on the LH channel, and put a series 15k resistor to the RH channel, which gives me about 10dB attenuation, but also more noise. I had to take this recording from the RH channel because I made a mess of the recording level.
Experimenting with stereo
I made another one of these, to see what stereo sounds like in a contact mic. Using a Y cable that connected the tips of two TRS jacks to tip and ring on another, I can combine these. This was on a farm gate, tapping the gate to test the sound
this was recorded on the opposite sides of the middle vertical strut of the fence below
this was recorded on adjacent struts
and this final one was recorded as shown below
Farm gate recording setup
You can hear a bit of the wind impacting the cables, perhaps I will have to clamp them to the gate to get rid of that effect. The speed of sound in steel at nearly 6km/s is much higher than the speed of sound in air at 330m/s o although the spacing of the third sample (pictured) would be outrageously wide for a spaced pair of microphones in air, the stereo effect is clearer for me on that than on the first two.
typically presented on three-pin XLR, or five-pin for stereo. ↩
The piezo disc doesn’t need anywhere near as high a source impedance as the electret. 1MΩ is fine for the piezo, the electret wants nearer 1GΩ ↩
That’s o/c without the FET present, typically 3-5V on a PiP input ↩
I bought this must’ve been 2016. It was a bad move from the get-go, because the hard disk is only 32Gb. And it had Windows 10, and 32Gb is only just enough to get Windows on. Pretty soon I had to use an outboard hard drive to be able to update windows, and by about 2019 even that didn’t work. It’s a shame, because it’s otherwise serviceable, but totally non-upgradeable – the ‘hard drive’ is an eMMC soldered to the board. It lasted me three years. I did like the light weight and silent operation, but the overall gutless performance and slower and slower startup was bad.
I could use a linux laptop
I had been tinkering with a Raspberry Pi4 for amateur radio field use, but wrangling a Pi in the field for things like SOTA is a mess, because a Pi plus all the odds and sods you need to make it work is a collection of parts flying in loose formation, and unlike a DC3 they don’t always work well together. It’s bad enough connecting the computer to the radio via analogue audio connectors 1, having to connect the Pi plus screen to a Bluetooth keyboard plus some sort of battery to USB-C power contraption gets a bit much in the field although it all works fine on the bench. I had already run the FLdigi and WSJT-X software on the HP Stream in Windows so I knew it was capable of decent performance, better than the Pi4 which struggles a bit to decode WSJT-X in a reasonable time.
However, I had heard bad things about trying Linux on the HP stream, because the Wifi card is very proprietary. The Ubuntu drivers seem to have fixed that now
It was surprisingly easy to load once I quit trying to install on the UEFI BIOS. I uses Xubuntu LTS 20.04, downloading the iso and putting this onto a USB stick using balenaEtcher. I found the install instructions for Xubuntu hard to find and sketchy, but they are good enough to feature this hint
If you don’t already know how to install Xubuntu, then please read this great tutorial, which applies as much to Xubuntu as to Ubuntu.
which is indeed great, and took me from there. But first I had to switch off the UEFI BIOS. It’s not that obvious to me what advantage UEFI gives me with a machine with a whopping 32Gb of disk space which is far from the 2Tb limit UEFI is supposed to fix, so legacy is fine with me.
A disadvantage of linux on a laptop, apart from the general gangly geeky oddballness of linux on the desktop as opposed to on the server is battery life is not optimised so well.
I bought this Chinese HW-131 breadboard power supply, which seems to be a Chinese clone of another Chinese product, the YwRobot MB U2. There are reports of unreliability with that device run off 12V, the suggestion is to run it of less than 12V if you are drawing notable power from it because heatsinking is marginal, using the small board. And definitely test all the output voltages before wiring this to something valuable.
It uses a AMS1117 3.3 and 5V regulator, and the minimum input voltage is 6.5V, due to the regulator dropout of 1.3V, so I will look around for something more suited to this. A breadboard tends to get shorts easily, and I could see the AMS1117 getting shirty trying to dissipate 12W into a short at 1A current limit 😉 The schematic matches this Addicore one
Absolute maximum junction temp is stated at 125C, and their example calculations say a good board will get thermal resistance of 45C/W, so if we start at 20C the chip can dissipate 2W max. I favour 12V since I can split the 12V off an existing LED lamp power supply, and sockets are a premium around the computer.
It’s hard to ignore some of the dire warnings on the Net about this, it always seems to be the 5V regulator that gives itself to the cause, failing short, which is bad in a series pass device, though typical. So before I put this into service I tested its resilience.
I could reduce the short-circuit voltage with a power resistor or a PTC current limiter. I only had a 12 ohm power resistor, so I used that. Voltage drops to 2.5V into the AMS1117-5 when I short the 5V rail, but it recovers OK, and more importantly the 5V rail recovers to 5V. However, when I replaced the 12 ohm resistor with a PTC 550mA current limiter the series pass device failed short and a wisp of the magic smoke escaped. There’s not much of it. I can see where these get their bad rap from – show me the experimenter who never shorts the power supply rail with the scope probe 😉 As Bob Widlar said in his paper on the design of the LM109 voltage regulator “Actually, the dominant failure mechanism of solid-state regulators is excessive heating of the semiconductors.” And it happens pretty damn quick.
Magic smoke output port. Single use only
So I ordered five of the AMS1117 +5V which set me back £21 and set about the spare board with a Dremel to break the + connection from the pin on the socket, to insert the resistor. There’s no need for the PTC 550mA device because it clearly heats up slower than the AMS1117. I shorted the 5V rail, and while you can smell the overheating resistor the +5V rail came back fine, and I diddled about with the short to see if I could catch it out. After about 20 seconds and a wisp of smoke rises from the resistor, although the picture shows it doesn’t discolour too much, and everything is still serviceable. I measure about 10V across it into a shorted 5V rail, across 12 ohms that is about 10W, pushing what is probably a ~2-5W resistor a bit. Resistors tend to fail open, not short, which is good in a series pass device. It was time to introduce this to the old National Semiconductor method of testing voltage regulator short circuit tolerance, dragging the 5V rail over a grounded rough file. As Bob Pease used to say
Don’t just see if it oscillates — see how it RINGS when you tickle it with a pulse of current. In other words, BANG ON IT.
output of 5V dragged along grounded file
Analogue shorted load test equipment
That has this sort of effect on the rail going into the AM1117 5V regulator. Since the DrDAQ I am feeding into the PicoScope only has one channel it is a different instance
12V input to AMS1117-5 with output dragged against shorted file
Modded board – you cut the +12V centre contact of the barrel jack and wire the resistor in its place
It’s not subtle, and it’s not clever, but it seems to save the board from swift destruction, at the cost of limiting power drain to 500mA. This one has taken a fair bit of abuse now. I am still tempted to desolder the great big USB socket and bridge a 5.6V 5W Zener across the 5V rail as a better use of the space.
It beats using a big lab power supply on my desk. My lab bench is in a garage, and it gets cold in winter, so for some PIC development I wanted to run a solderless breadboard on my desk. Two reasons – it’s warm in the house, and secondly the lab computer is an old Sony Vaio 32-bit machine. Microchip’s MPLAB X IDE demands 64-bit, because it is shocking bloatware built on Netbeans and Java under the hood. Until recently I’ve used MPLab 8 and assembler, but, well, they ain’t making any more time and assembler is time consuming as complexity goes up.
Tinkering with PIC programming on a breadboard tends to involve a couple of LEDs or perhaps an LCD display, so a few hundred mA is good enough for that sort of thing. If the project needs less than 30mA a decent alternative is to use the PicKit 3 programmer to power the board, but 30mA won’t power many LEDs.
Chinese electronics seems pretty hit and miss. I find buying components like power transistors there is a crapshoot, because these things are terribly easy to relabel2, which is probably what did for my SkyTec power amplifier, over and above the rotten thermal design 😉 I’ve had better luck with their modules, which are often the only way to tackle some SMD devices. There seems to be a mix of pick and place and hand soldering – looking at the reverse of the board
It’s a tad on the scruffy side. The jumper wires I got with the kit are OK, I normally use stripped Ethernet cable for jumpers so more colours are welcome. The breadboard is disgusting. The grip on wires is weaker brand new than my 20 year old main lab unit. Life is too short to muck around with janky solderless breadboards, so I will throw it way to save my future self hours of debugging it.
Update – I replaced the faulty 5V regulator and fitted the resistor mod, and that board now survives the 5V rail being shorted. To change the 5V reg snip the three SMD legs of the old one, which lets you lever up the chip and desolder the tab, and then clear the snipped residue. It wasn’t too painful, but I am glad I found this failure mode out before losing something valuable. ↩
I couldn’t find an ISO9001 supplier of the 2SA1941 and 2SC5198 transistors. Digikey carry the latter, on a quote required basis, so you probably have to buy 10,000. So I bought my transistors from ebay, and they probably got them from a Chinese relabeller. I should have put the effort in to identify a suitable substitute that I could get from a reliable supplier ↩
I was fascinated as a teenager by biofeedback, which was big in the 1970s and early 1980s. It’s called neurofeedback now, at least in the EEG guise. Technology and digital processing has made this easier, though some of the fundamentals remain. Wherever you see a puff piece about the latest and greatest dry electrode technology, be that from Muse or from some games gizmo, you are not getting optimal signal quality, because the Holy Grail of the messless EEG pickup has never been found 1. You can get some sort of signal using dry electrodes or capacitive tech, but the EEG signal is weak, in the order of microvolts, so things like Muse and EEG games controllers are frustratingly inconsistent, sort of serviceable but not great IMO. Colour me a cynical bastard but I suspect poor signal quality is why it seems to be the devil’s own job to get the raw EEG data out of Muse, although this and this indicate it might be possible. You’re stuck on a F7-F8 montage with Muse, although that has the advantage of being outside the hairline.
I found Muse a mildly expensive mistake/rathole. I could get somewhere with it, but it was frustratingly inconsistent, I found it stressful using a phone as the interface and the dumbed-down interface grated. I was glad to give it to someone who will use the product as it is designed.
I was intrigued way back then by the Dragon Project, an attempt to measure effects around ancient sites. The physical monitoring part of that project didn’t yield anything of note, but one device they did use was called a mind-mirror, a transportable EEG, there are some pics in their gallery.
This was designed in the late 1970s by the late Geoff Blundell of Audio Ltd, a heroic piece of analogue design to make a multichannel audio spectrum analyser using hardware.
I managed to get one second-hand since publishing my first article on the Mind Mirror. This didn’t work properly – the right-hand side didn’t display right, one of the LED channel boards was down and there was an odd output from the lowest frequency LED display. It’s challenging trying to fix something with no circuit diagram, particularly when it is something that quite this one of a kind, you can’t draw parallels from other designs.
Mind mirror display daughterboards
However, what made this easier is the display is made up of plug-in daughter boards fed in parallel.
one of the Mind Mirror daughterboards
This made it easier to isolate faults and by swapping boards trace whether the issue was on the board or the common backplane drive.
At first this was a sick puppy – the left hand channel didn’t work at all. I compared this with the right hand side, discovering the quiescent signal voltage was 0.82V as opposed to 2.5V on the right hand side. The 5V power line on the RHS was mirrored by 1.7 on the left.
So I pulled display boards till I found the offending board dragging down the power supply. The LHS still didn’t work, so I traced the input signal to a 4016 CMOS analogue switch which had failed on one section. Changing the chip cleared this fault, so I replaced the daughter board till I found the one that pulled the power supply down, which turned out to be a faulty CA324 quad opamp.
The last fault was a weird display on the lowest RHS channel. That turned out to be a duff LED gone short, which due to the odd Charlieplexed display on the UAA170 made me first suspect the UAA170. These are still available NOS on eBay, but swapping the chip didn’t fix the problem. Modern LEDs are much more efficient and a slightly more orangey red than the 1970s ones, so I had to shunt the replacement LED with a resistor to balance brightness.
The unit was originally designed to work with two 6V SLA batteries, but the strip on the PCB joining the mid-point of batteries is not connected to anything else. This is a 12V unit, though the system ground is not connected to the battery 0V.
Tracing out a daughter board was tiresome. an example active filter is
Mind mirror active filter schematic
and simulated in LTSpice this is
Mind Mirror 16 Hz filter LTSpice simulation
This reasonably matches the expected display. Bear in mind the display is linear steps up to 16 levels, so the difference between minimal display and full-scale is about 1:16 or about 24dB, so if all LEDs are lit by the peak the display will extinguish (show the lowest LED) for the same amplitude frequencies < 12.2 Hz and > 20Hz.
The output of the filter goes to a pin on the DB25 socket, and is rectified and low-pass filtered before going to the UAA170 16 LED display IC on the same daughter board.
I have set this on soak test for a few days. In the video the 26Hz channel is off on the LHS, this was due to an unsoldered joint.
unsoldered joint. The messy flux residues aren’t my work, but I figure if it doesn’t cause grief after 50 years I will leave well alone, though I did solder this input to the high side of the level pot.
To feed the signal in I made a special differential driver from a quad opamp and padded the output down. I did test the input impedance which was of the order of >100k, though it got noisy with 100k source impedance. I suspect there’s another one of those CA324s on the input stage. There’s nothing that special about the CA324 nowadays. The datasheet is silent about noise performance, speed is similar to a 741 opamp. It is specified to work down to 5V , and the input common mode goes down to the negative supply, which has the edge on a 741. Looking at the internal design, there’s much in common with the nasty2LM358 and indeed Texas Instruments lump the LM324 and the LM358 together in this application note.
You can do a lot better now, I’d be tempted to run it on the 100uV range and use a preamp to get a higher Zin, though I should test first. Perhaps the high noise is the 100k source being amplified so much – the specification is for a 10k typical contact resistance. You can only achieve this with wet electrodes, which is something I have yet to wrangle.
The spaces top left and right was originally to take two 6V sealed lead-acid batteries, nowadays the same capacity can be had in much less space in NiMh or a 3S LiFePo drone battery.
In the meantime I also got the Olimex EEG-SMT to tinker with. Although I feel the openEEG antialiasing filter leaves something to be desired I didn’t observe shocking levels of interference so perhaps I was overthinking that. Reading the archives of the openeeg mailing list I was impressed with the care taken over the analogue design, to the extent an easy win would be to use the EEGSMT in the LHS battery slot and break out the analogue signal from C51 and C52 to go into the MM. The driven right leg grounding scheme of openEEG works very well, and I verified that messing about with the EEGSMT and a pair of Olimex active electrodes used dry.
Sadly I screwed up buying only two active electrodes, since the channels are differential you need two active electrodes per channel, four in all. Since the UK has left the EU there is a whole world of hurt associated with buying from the Bulgarian company Olimex that I didn’t have when I bought the original devices a couple of years back.
However, I have a working Mind Mirror EEG and a serviceable Olimex OpenEEG system. After a frustrating foray into the dry electrode world of Muse, I can return to tackle the problem I never faced up to, which is eschewing the mirage of decent dry contact solutions. There aren’t any, because you cannae change the laws of physics. Dry contact solutions means higher contact resistance, which associated with a weak signal coming through a high resistance means more noise and less signal. I need to suck that up, because I have wasted too much time on that sort of thing.
Nasty because these damn things are responsible for a lot of audio crossover distortion when used by tyros drawn to the low cost and low voltage performance. See TI application note page 17. If you really must use these at audio frequencies, pull the output down to the negative rail with about 10k to bias the output push-pull NPN Darlington into Class A. The TI app note preamble The LM324 and LM358 family of op amps are popular and long-lived general purpose amplifiers due to their flexibility, availability, and cost-effectiveness. It is important to understand how these op amps are different than most other op amps before using them in your design. The information in this application guide will help promote first time design successes. should warm you up to ‘here be dragons’↩
These Chinese fairy lights cost less than £5 somewhere on Amazon – you can get 3 for £8. In the ad there’s this lovely golden glow
but in practice the damn thing is dimmer than a Toc H lamp
not very bright – in design and in output
These things are basically a throwie upscaled to a 50 LED string. Powered by two CR2032 lithium cells in series, the LEDs are in parallel, Current is limited by the internal resistance of the batteries. The whole thing is a disposable hazard to the environment, intended for a single use at someone’s wedding or party. It shouldn’t be allowed 😉
They make quite a nice distributed light in an outdoor shed, where I can fix the wire along the ceiling, just as well as the solid enamelled wire is going to break if moved too many times. I was surprised that you could put 50 LEDs in parallel. They are all fed from one end, and in the original configuration you couldn’t see any gradation along the string. However, putting 700mA through them generated a very welcome increase in light, and a slight gradation down the string, due to the voltage drop.I feared that would be bad for LED life, so I ran a third piece of enamelled wire through the string and fed one side of the LEDs from the far end and the other side from the near end, the drops along the string sort of cancel out. Using a light meter with the LED taped to it the original version as received gives a single LED output of 10EV, with 700mA it’s 15EV, a gratifying five stops more light – about 30 times more.
The 700mA is split among 50 LEDs, about 14mA per LED. I’ve never come across a LED (designed for illumination, rather than an indicator segment) that’s rated at < 20mA, so I figure I am OK. I was looking to upgrade this to 12V, powering off three 18650 LiIon batteries. The obvious solution was a Chinese LED current limiting switchmode supply, but the obvious solution comes with a nasty wrinkle for battery powering. Current rushes up as the voltage drops
a constant current driver is a very unkind battery load
Run off 12V it worked a treat. I used three LiIon cells scrounged from laptop packs and bits, and I found that this is a weapon of battery destruction – first I wrecked two cells out of three, then another two. Hmm. On the upside, at least I have now selected the strongest cells. On the downside, four LiIon 18650s have met their demise.
What’s up? The constant current LED supply is one of these
5-35V 3W LED Driver 700mA PWM Dimming DC/DC Step-down Constant Current. From these guys on ebay
and I really should have been paying attention to that 5-35V spec, because as my Li-Ion’s fall from 11V down to 5V, it will say gimme, gimme, gimme more current, NOW.
Gimme all you got – particularly as the battery dies
And you don’t get to see that the batteries are running down from the LEDs dimming until it reaches less than 5V, because the driver is good for 5V. Oops. My bad. That’s why I am four 18650s down. Most things you run off batteries tend to draw less power as the voltage fades, but these suck the last dregs out of the battery in double-quick time, giving up just as it discharges the second weakest battery to below recovery.
I was imagining low-voltage disconnects and mucking around with P mosfets and PICs, then I spotted the PWM pin. You either leave this open, or ground the sucker to disable the output, the basic chip is the XL4001 from XLSemi. The EN wants to be < 0.8V to turn the thing off, and > 1.4V to turn it on. I had a vague recollection you could use a TL431 to get an active low power is high enough output, and Google delivered inspiration from ON1AAG on electro-tech online. TI also have a rather nice app note Using the TL431 for undervoltage and overvoltage protection which goes into some of the trials and tribulations of such misuse. One of which is quite a high Low condition voltage of about a volt or so – to wit
A lower bandgap reference voltage as seen in the TLV431, allows for a lower logic”low”output voltage without the need for external hardware.
Testing this with a LED showed it basically worked, but feeding the signal to the EN pin did ‘owt. As TI say, the resting Low voltage is over a volt. They’ve also got a Understanding Voltage References: Using a Shunt Reference as a Comparator blog series which points you at the TLV431 for this sort of thing. I needed to pad the output down with two diodes to get it just below 0.8V
Low Voltage disconnect for XL4001 PWM LED regulator
There’s no hysteresis in this. I did first consider a 5.6V Zener instead of the two diodes, but that introduces a nasty pathology. The LVD turns the chip off at about 10.5V, but switches it back on again at about 5V, and the XL4001 goes way-hey, let’s suck the maximum current out of these dead and dying batteries. At least with the diodes it has to get down to < 3V and the XL4001 doesn’t draw half an amp like it does at 5V since 3V is out of its operating region which is 4.5V to 40V.
I’d be better off with the TLV431, but TL431 is what I have to hand. I’ll get some TLV431 next time I order some parts.
Recycling Neato 4/5AA batteries.
Looking for an alternative I hit on an old Neato XV robot hoover battery up for recycling. These get thrashed in this application, but the problem is there are two 7.2V battery packs with six 4/5AA size NiMH. To me these look pretty much like 18650 size. One of the cells has gone high resistance, but the remainder charge well on my MAHA battery charger/analyser
A decent capacity of 3700 mAh, typical of all but one cell
Although no chemistry appreciates full discharge, NiMh will tolerate the odd deep discharge. I’ve learned my lesson running constant current LED drivers off LiIon batteries, and while I have a LVD now I’m not taking the risk again.
I’ve also got a chance of trickle charging these via a solar panel. Battery University say you shouldn’t trickle charge NiMH at > .05C which is ~180mA or less. Mine is an old Maplin 1.5W @17.5V solar panel which would theoretically give 86mA in blazing summer sunshine. Which is not the time of year when you want lights in a shed, so I’m not going to be anywhere near endangering these batteries 😉 11 of them will give me a nominal pack voltage of 13.2V
A few years ago I did a couple of piezo contact mic amplifier designs, because people often moan that these things sound tinny and crap. There’s a wrong way and a right way to use these – they want to work into a high impedance. Using Piezo Contact Mics Right sets you right. Trouble is these use a 9V battery, and it seems world + dog want to use 5V, because that’s what they had. Time was when power supplies were +/- 15V for analogue and 5V for digital, but that’s a different story for later.
So what can you do with your piezo contact mic at 5V then?
Not much. If you are looking for low signal level performance an emitter follower biased at an output of 2V would work well, but if you only have 5V available it’s likely you are trying to digitise this signal and bung it in an Arduino or something. In that case, think laterally. Toss the power supply. I developed those amps because as a field recordist I wanted to hear faint signals from the contact mic. You know, like the whispering in the rails as a distant train approaches, though you need to avoid the Fredzania Thompson ending.
These days people would look at you funny if you attach a box with wires to the underneath of the rails. Don’t try this at home and all that.
Turns out many people want to use their contact mics on an instrument, or drum pad, or generally something they bash seven bells out of. Life is a lot easier for you. As established in Using Contact mics right, you want an input resistance of about 330kΩ so the bass doesn’t roll off with the typical series tens of nF capacitance of the sensor. 330kΩ is a damn sight more than your typical plug-in-power audio recording doohickey, which usually feeds the electret mic power from 3V via about 6.8kΩ. I measured my Olympus LS-14 and even the line input is 10k.
So stick the 330kΩ resistor in series with the input. Even writing that makes me cringe, because it will lose a hell of a lot of signal level, making a potential divider with the input resistance – for a 6k8 input you’ll take a loss of 33dB. That translates into a direct worsening of your noise figure by that much, that’s a lot of performance to throw away1. OTOH it works perfectly well down to 1.8V, it’ll be OK down to 0V as it doesn’t use power 😉
how much signal do you get from a piezo contact mic?
UK Ordnance Survey maps are lovely, particularly the Landranger and Explorer series, but they are dear, and you haven’t been able to use them online for a while. As taxpayers we paid for it, but unlike the enlightened approach the US has to government collected data, which is generally in the public domain 1, the OS has had a Gollum-esque relation to letting the great British public use their map data, and didn’t let their precious out of expensive pricing plans.
The OS used to have a way for people to feature Landranger maps on websites which was called OS Openspace. I used this for mapping on a website about standing stones. OS Openspace is no more, it is now called OS OpenData.
Avebury stone circle in Wiltshire. Zoom in and you get Explorer 1:25000 detail.
You now get to zoom in and get Explorer-level detail, and the free data usage is easily enough for a hobby website or blog unless you get slashdotted. Nice. Well done OS. The documentation is pretty rough and ready, and note that at the time of writing if you simply implement their code example with a non-premium API you get a blank maps with just the OS logo in the bottom left corner, which can lead to much head-scratching and WTF?
Initially I thought they had stiffed freebie cheapskates like me by demanding a Premium API, but no. You can do useful stuff without providing payment details 🙂 Well done Ordnance Survey!
Unless it’s collected by the NSA or CIA I guess ;) ↩
The weather was various shades of atmospheric as we came to Carn Gluze barrow on the Penwith peninsula in Cornwall. To be honest, it was foggy as hell. Let’s look on the bright side, it wasn’t actually raining at the time.The subdued light and damp really made the heather glow
Heather on the side of Ballowall Barrow
but it didn’t really make for an inspiring picture of the site – a nearby derelict tine mine was lost in the misty gloom Continue reading “Ballowall Barrow”
