Best posts made by Mishka

And in less than a week please meet the revision 1.1.

This release addresses annoying NFC bug introduced in rev. 1.0. Such, to cleanup schematic I've moved NFC capacitors to a dedicated space where the C18 was turned upside down in order to improve text readability. This led to C18 net changes which were overlooked for the PCB. After zone refill it resulted in tying the GPIO_P0.10/NFC2 to the ground thus making the whole NFC antenna defunct.

From the good, this revision adds highly requested mounting hole to the main board:

Have a nice time!

@neverdie said in Raybeacon: nRF52 on-the-go Development Kit:

The F-type trace antenna on the esp8266 esp-12F seemed to be the best all-around solution, so I was previously guessing the same would probably be true for the nRF52 as well.

Well, antenna parameters are relevant only to resonant frequency. As long as it matches 2400-2485MHz it should work just fine.

This one on the photo looks like SWRA117 by TI. I used those from KiCad standard library as a simple drop-in solution and they are quite nice.

Then again, the Fanstel BT840X trace antenna seems to be by far the best so far, and it's bigger than the esp-12F antenna:

Yeah, where reduced size is a must, meandered antennas (like the SWRA117 above) are good compromise to straight inverted F-type antennas. Otherwise a straight IFA might show slightly better efficiency.

I'm guessing maybe part of the reason it does so well is because it has less insertion loss than an externally mounted antenna. So, to your earlier point, putting a BT840X inside a cantenna should easily beat externally mounting it to one.

Placing antenna into a can makes to it about the same thing as acoustic guitar body to sound wave - it helps create standing wave and direct it into one particular direction. This simple trick promises to give +10..+15dBi - about the same gain as a power amplifier.

The difference, of course, in radiation pattern. This actually defines the need: do we need long range connectivity in one direction, or it has to be rather omnidirectional? For the first case - use directional antenna, then shape the beam with a can (cheap) or parabolic reflector (affordable), if still low - amplify. Of course, size matters so a small and simple properly placed reflector would make it too. For the latter case just stick to omnidirectional antenna and amplify.

That said, my best experience to date has been with dipole antennas. They're larger still, but they're also very easy to make. Some people have even hacked them onto nRF24L01's and report much better range. I'm guessing that's at least partly because it compensates for the too small ground plane on those devices:

Bigger ground plane usually promises better performance indeed and the module looks very small.

But to be honest, I don't know will the dipole improve anything or not. The nRF24 has balanced antenna feed line so at least it should be possible to connect dipole directly to the module (which is not the case with nRF5). But in order to do this all extra networks must be cut out, and even in this case it's very unlikely the antenna and RF output will match each other. Assuming that both the DIY dipole and the Cypress MIFA are omnidirectional antennas I don't think that the detuned and very likely out of band dipole would beat the original design, sorry.

Luckily, at 2.4ghz the antennas wire length isn't as awkward as it is at sub-ghz. Maybe it could be shrunk down using two squiggle traces instead of just one?

The RF output has 0dBm. Next, the default omnidirectional antenna tries to radiate it with 0dBi gain. Now it's possible either amplify, or shape the beam, or do both. A directional antenna (including reflector as part of it) could help gain up to +20dBi in front (which roughly means you'll have -10dBi in other directions). Amplifier could add +15dBm more in all directions. So it depends.

IMHO the options (inclusive) are as follows:

• Use reflector. Pros: high gain, cheap. Cons: size. Note: results in directed antenna (which may be both good and bad).
• Replace antenna. Pros: better radiation pattern. Cons: needs tuning.
• Install amplifier. Pros: immediate tx/rx gain boost. Cons: extra cost, needs tuning, may drain battery on low-powered solutions.

Hi,

I'm pleased to announce that the first major release of the Raybeacon DK is out. The revision 1.0 offers all the mentioned features and can now be ordered from your favorite PCB service. For highlights please take a look to the OpenHardware description, for details (and sources) please visit the Raybeacon project page.

I've published project BoM on the Octopart where you may easily estimate components. Please note, the aQFN73 package may push you to higher PCB production line. In particular, it suggests ENIG finish and also requires 5 mil track width / clearance, so expect increase in production costs. At the same time I've tried to keep design of the extension slices at the most affordable price so it should be easy to get 10 for $5 and have some DIY fun during Xmas holidays.

The project still work in progress. Such, the radio was tuned for the nRF52840. The nRF52833 may work just fine or may be out of tune. I'm going to order several boards with 833 for that purpose, but it may take a while so please use 833 at your own risk.

And, of course, please don't hesitate to share your feedback, it will be highly appreciated!

Hi @Nca78, I've placed the order on the PCBWay in early January, so it's almost finished. The factory just recently restored assembly services for some kind of boards including this one. Hope to have it in hands within a week or two.

@monte 1) To control all the sensors and 2) to be monitored by the network.

@scalz said in Everything nRF52840:

@NeverDie I think you need to hold it, but it looks there is "coded" holes/slots for helping and correct positioning.

You may want to use TC clip for long-time debugging or odd boards like this:

Thanks!

The board uses aQFN73 package which is kinda two-row BGP with 0.25 balls, so soldering it at home may be quite tricky indeed. I've ordered assembly

The aQFN73 is common package for both nRF52833 and nRF52840. In this particular board you may safely use either nRF52833 or nRF52840 of your choice (need 105ºC and direction finding, or rather more memory and the CryptoCell unit?) The only other thing you have to do is ensure pi network uses correct components for the selected SoC (I'll update the BoM as we can tune it) - the PCB remains the same. Generally speaking, many features are just matter of your BoM. Just do custom assembly by dropping support for USB (C20), NFC (J1, C18, C21), IR LED (or replace it with usual LED), buttons, nRF DCDC, 32KHz crystal, and so on.

On the other hand, if you need an extra stuff for your device (like a MEMS, or microphone, or air quality sensors), you may develop and attach an extension "slice" board. The "slice" supposed to be placed on the back side, i.e. right behind the CR2032 battery. Thus way it will create a kind of sandwich (or probably better said, burger): the Raybeacon board - battery - custom slice board. The red board on the screenshot is an example of such slice developed for my friend. It provides ultralow-power accelerometer, magnetometer, and gyroscope - all in all, we have just CR2032 for that.

@skywatch said in Which vector network analyzer should we buy?:

It will be interesting to see the reviews of the new unit though.....

Big discussion was at https://groups.io/g/nanovna-users/topic/first_pcb_pictures_of_the_v2/68761814. Now it seem continued to the https://groups.io/g/nanovna-users/topic/v2_design/71480430. These topics also explain why some shielding wasn't installed.

Please note, there is also the NanoVNA2 by edy555, but the name has been taken.

@alowhum said in Everything nRF52840:

To what extent Bluetooth has useful smart home profiles
To what extent Arduino devices could present themselves as smart home devices using those profiles.

Well, the BLE has a lot in common with REST. You have to have one or more central devices (clients) which will query peripherals with sensors and actuators (servers). Communication is P2P, but broadcasting is also supported to some extent (various beacons are well known examples).

If thinking about it like a RESTful sevice, there is a number of API which were standardized. They call it profile, like in, for example, the Heart Rate Profile. The standard just assures that any heart rate monitoring device will talk the same protocol as any other one - that's it. I don't think there is many standard profiles for smart home devices (if any). At the same time, there is the Project Connected Home IP.

Of course, there is nothing which may prevent somebody to implement his own interface. If it will be successful enough, it will be later possible to contact the SIG group and promote it to a standard.

Please also note, that for serious boradcasting there is the Mesh. In a nutshell it works similarly to Ethernet switches. Interesting side effect is that it's possible to build a Mesh network physically wide thus delivering packets over long distances. Of course, if you want to go global, the IPv6 will be the right choice to go. It will require a standalone BLE enable router though - it will connect the devices to the Internet.

To what extent older Bluetooth chips can work with newer device profiles (it would seem a software upgrade should be enough?)

As long as they support Bluetooth Low Energy - i.e. version 4.2 or above.

By the way, for hand-soldering version I planning to employ nRF52833 in 5x5 QFN40 package when it will be available. It won't be possible to install anything but 833 there, but the PCB itself should be way more affordable to manufacture and assemble. I think the new board will be 100% compatible with existing "slices" . The current form factor depends on battery size in the first place, and I think it will remain the same. Unsure those 5mm will make big difference if we switch from 2032 to 1632 and loose about 50% of capacity.

Here is a fiber laser machine example: https://mylasermart.com/product/ultra-high-speed-laser-machine/

It uses 50W fiber laser. Minimal track width is 0.05 mm! Space between tracks is 0.025 mm which is limited by the beam size. Precision ±2 µm.

This particular one doesn't look any cheap, but it gives the strong hint what to look for.

@Nca78 said in Everything nRF52840:

A very interesting project, too bad they are using NRF52832 and not NRF52840, the max SPI speed on ...32 is too low (8MHz) and I'm afraid the LCD refresh will be annoyingly slow

It might be also interesting to employ the Sharp LS012B7DD06 which is a round 240x240 6-bit color memory display consuming 11 µW in static mode. It will be far away of the PineTime's $25, but with the CryptoCell, NFC and other stuff it might worth it.

Shall we create a repo?

@NeverDie do you really need it so far? From what I see the BT840XE uses active RF amplifier which boosts it up to +21dBm (BTW may be cumbersome to use it legally in open air). In any case, I'd look for either cheaper alternative, like this - https://www.aliexpress.com/item/32951920205.html. Or use high gain antenna, like this: https://www.gearbest.com/networking-communication/pp_257707.html

@NeverDie That's right. You may want to use open version which is much smaller indeed.

Also, for nRF52 only three pins are required: SWDIO, SWDCLK, and VTref. All the three are very conveniently located in close proximity on pins 2, 4 and 1:

Such, for a tiny custom board a 2x2 pin header, 1.27 pitch can be used w/o breaking compatibility.

Well, looks like the CDEBYTE did great job by placing all required components. From the datasheet (yes, there is one, and that's awesome) I see the only missing part is 32.768kHz crystal which is not an issue either.

The antenna is huge. If it was tuned properly it may give us up to +5dBi easily. Also, the 840 can be boosted up to +8dBm. IMHO this module looks like being serious about long range. I think you may get up to 1 km of it as is.

Another good thing about the module is that it has easily accessible waveguide. Probably it may be possible to unmount the antenna, crop the board a little, and inject an RF front end (like SKY66405-11) to gain some +10dBm more. As I said, the antenna is good so it's worth to reuse it and save little bit more money. Tune it once and then call it a reference design. I don't know can someone make it to 5 km, but would be interesting to see the range.

Anyhow, I've just spend $5 for my couple, thanks for the link!

Regarding the Raybeacon module - it was created as a tiny board for development on the go. I could take a Feather, but this one is smaller and has battery attached. Someone may also consider it as a reference design for own boards (as we in Raytrails do). Long range wasn't the goal here. I totally agree - employing a module is a good practice. Unfortunately, I just found no module with zero clearance for antenna so decided to go this way.

@orhanyor AFAIU you're going to solder it manually, right? Will you use stencil or are you going to presoak pads? For the stencil it might be also reasonable to order a custom PCB pallet which may help to precisely align the board to the stencil. Similarly, another pallet may help to place the chip right into the center. It will be very interesting to hear from you how it went. Wish you best of luck!

I've ordered some boards with nRF52840/nRF52833, but also had to ask for assembly for exactly this reason. The PCBWay did machine soldering of the aQFN chip, and the rest were soldered manually.

By the way, the PCB price were raised two times: first, for 5 mil tolerance (the board uses NFC so I was unable to route antenna differently), and second, for ENIG finish because of the aQFN-73 package (the factory insisted on it in order to avoid soldering issues). I still have no room for the RESET, but perhaps I need to go your way and drop some unnecessary pins - that's nice idea.

Hi,

I'm very gald to post an update to the project. Revision 0.6 features fully round shape for the main board (and hence all extension boards). The battery placement is off-centric now, but it should fit just fine together with the extension socket. Please note, the dedicated ground pad has been also moved and now it's conveniently located right below the USB pads.

And here is the revision 0.7.

It adds 2x4 1.27 pitch pin header on the front side of the breadboard part. The connector is fully matches the Raybeacon extension socket - you may safely break the breadboard part off, and then solder a pin header there in order to use the detached piece as 1.27 to 2.54 adapter.

With this change, need for a dedicated dock board is perhaps even less. If so, flip the SWD socket bottom up.

Hi @neverdie, thanks for your comment! Well, from what I've found in datasheets the SPV1050 was designed for ultra-low power apps and is very efficient in low light conditions, when SPV1040 is capable to gain up to 3W and its step-up converter is more suitable for outdoors. I've also considered bq25504 and LTC3103, but found that they seem isolate battery from the output where the load is expected to be. In contrast, the SPV1050 implements very simple approach by connecting the store and the battery via a FET - this allowed to connect main load parallel to the battery (where by fact it is) and disable integrated LDOs. Another advantage of the SPV1050 is that it's omnivorous - can accept input up to 18V (although I've configured it for 12V) and officially supports TEGs.

But to be honest, never tried anything from the list so will eventually post my findings So far the Raybeacon was finally assembled and sent to Nordic for tuning - you may have noticed updated pics on its page. So hope to advance the project status from preview to beta by the end of the year.

@NeverDie Just received the KXOB25-02X8F panels. In short - you was right, and I was too optimistic about indoor lights.

Generally speaking, the pannel works really not bad and looks extremely handy. However, in order to achieve the desired 2.6V it has to be illuminated with greatly above 1000 lux which roughly equals bright supermarket or factory lights. And yes, it seem differentiates LED lamps from the sun.

Accordingly to my (totally uncalibrated) luxmeter:

• Living room room: 50 lx - 0.4 V
• Overcast daylight, indoors: 250 lx - 1.4 V
• Supermarket lights: 1300 - 2.0V
• Overcast daylight, street: 2000 lx - 2.7V

Therefore, it's safe to say that two of 02X8F would make it acceptably work in buck-boost configuration. By acceptable I mean it could charge an ML2032 battery, but batteryless operation is quite limited indeed.

This pushes me to review use-cases for the Harvester. It has two input sources - first one is spv1050, and another one is MIC5205 3.2V LDO. I'm thinking about switching the spv1050 to boost configuration and cover only PV and TEG sources from 75 mV to 3.6V. At the same time LDO could handle sources capable to deliver 2.0V and above (2.0-3.2V with battery removed, and 3.2V to 16V with or without a battery). The point here is that at higher voltages there is likely a more capable source so MPPT might not be required.

Such, although less flexible, one or two KXOB25-05X3F, 30mW each, and a "supercap" should be enough to power the nRF52 (and even an extra module) without the need of a battery. For more demanding applications, up to five KXOB25-14X1F could be used delivering 150mW in total. With 3.2V, 70mA limit on the spv1050 battery pin it sounds like a good match.

Another option is to make the Harvester configurable via solder jumpers. This will put four more jumpers on top of it (and there are five on the back side already). Well, although possible, sounds too complicated as for one inch board.

Any thoughts?

P.S. I'm going to assembly a buck/boost board anyway so we can compare later.

@NeverDie Thanks for the hint! At first glance, EM8500 looks like a highly integrated solution for embedded devices.

What I like: very efficient for tiny, highly embedded applications, built-in USB charger (cool), can complement primary battery (super cool), programmable (including MPPT and battery thresholds) with EPROM support.

Unfortunately, for the same reasons it will be very hard to use it as an addon module. First of all, it requires the battery to be exclusively attached to the harvester IC. In contrast, spv1050 has much simpler circuit where STORE on the module can be conned to battery on the main board.

Next, ultra low-power capabilities of the EM8500 come at cost of its upper limits - 1.8V max for harvesting source, 20mA for LDO. Choosing PV for such device would be even harder than satisfy 3.2V requirement for boost-only spv1050. Since nRF52 may consume about 20mA at 3V, it might be tricky to run it batteryless, especially when some other devices are sitting on the line.

Also, I2C bus will occupy two of six I/O. Well, fair enough. The spv1050 also has status lines for battery charge and battery connection. But they are completely optional and are off by default. Broadly speaking, the spv1050 is completely transparent to its load.

So my impression is that the IC is very cool indeed, but it just doesn't fit this particular project. At least in its current form.

@NeverDie Use additional battery is an interesting idea. My only concern is that the harvester IC will drain it if connected to input. On the other hand it looks like the IC will work just fine as long the STORE has some voltage. And the STORE is connected to battery, which is charged by the harvester IC - i.e. the rechargeable battery is exactly the battery used to prevent IC from shutting down. But perhaps I missed something - still have no boards for test.

Regarding solar cells from a calculator - yeah, that was my first intent, but have no solar calculator out there. Taking in account that such a calculator costs three times less than this SolarBIT thing I have no excuse for not to buy one from a local store - going to fix that tomorrow

Okay, grabbed cheapest calculator from a store. It turned out to be EATES DC-837, priced below $4. Surprisingly well made, it uses monocrystalline amorphous PV cell and one AAA battery, connected via a dual diode. This means the calculator should be able to work from as low as 1V, and the PV panel should not cause overvoltage at the same time.

The panel size is 12.6x37.8 mm. It looks really inexpensive and is made of glass, protected with paint on the back.

Despite that, the panel works damn good. Such, I was able to get 1.8V from it in a room with dim lights (about 50 lux). At the same time, short circuit current was about 5.9 µA. Exposed to bright sun it resulted in 3.2V open circuit, and 3 mA at short circuit. I reached limit with 1200 lumens torch at 5 cm distance - Voc=3.25V and Isc=7.2 mA.

I did similar measurements for KXOB25-02X8F. This time my torch forced the panel to Voc=5V and Isc=11mA which somewhat differs from datasheet. Results are on the charts below:

@NeverDie You're absolutely right. The cell size is 12.6x37.8 mm which explains 1338. I also agree with your guess on 4V. The CAT is just reflection of my phone.

Well, I'm impressed that a noname manufacturer managed to make such a useful cell. BTW calculator consumption is roughly 5 µW - pretty neat too

Regarding solar watches - they should be fine to charge battery, but I'm in doubt they have enough capacity to power on anything so advanced like nRF52 - I hope should be feasible with the SCNE.

Grr, the boards are out of radar - last track is 2019 December, 29.

@NeverDie said in The Harvester: ultimate power supply for the Raybeacon DK:

That solar cell looks tinier than any that I've ever seen for sale.

https://www.sparkfun.com/products/9541

Not sure where to go with this though

@NeverDie The cell probably is not so sophisticated as the one installed in the fx-260. Also, from four visible sections only three are working. Sigh.

It seems the calculator can acceptably work as long as it supplied with 1.3V or more. This roughly estimates to 40 lux for this particular PV cell. However, the display is the resource hog - every additional digit causes voltage drop. Usable number of digits - five or less. After reset the calculator draws 3µA. Every "8" adds up 0.1µA thus reaching PV short circuit values very soon. The IC seems completely unregulated - at higher voltages it draws proportionally more, up to 1mA.

It's also possible to gain little bit more power (about 70mV) if bypass polarity protection diode. However, it might be rather better to employ 10µF capacitor to compensate for keypress actions. With 92µF it's also possible to enter longer numbers, but you have to type them fast, then store the number in memory, then quickly clear display and wait until the capacitor charges. Now you can type another number, press operation, and, finally, MR and = to calculate result

@NeverDie Well, looks like that is the space mission panel Won't be cheap anyway. On the other hand recently I seen a lot of news about so called nighttime photovoltaic power, who knows how fast it can be delivered to market.

In a meanwhile, I had a chance to play with the Harvester board. Please don't consider everything below as any serious test or a comprehensive review - I just rather tried to understand how the modules are working and what can I get from them.

Following our prior discussion, I've assembled one board in buck-boost mode (the original PCB), and have tweaked another one to the boost configuration. The only solar panels I have are:

• IXYS KX0B25-02X8F. Single panel is annoyingly weak so I connected two of them in series.
• The SCNE SC-1338 from the test above.
• SORBO SB-3077. A big (79x40) panel from solar torch. Didn't tested it like the other two, but noticed about 6V from it.

For the load there is 1500µF 4V tantalum capacitor installed. V_uvp threshold has been set to 2.2V, and V_eoc to 3.2V. Please also note, the MPPT resistor values were set to V_oc=12V (which is too high for all the tested panels) and no adjustments were made to them for any of the tests.

Most of the tests I made in the evening at home where LED lights are the most common source. I used light sensor on my phone to measure light illuminance. Looks like it gives more or less sensible values; such, 50 lux corresponds to a dim light, 250 lux is an office light or indoors illuminance in cloudy day, 1000 lux is outdoors light in cloud day - similar values I've seen in tables over the Internet. After checking illuminance I placed solar panels in the same area and measured most important voltages:

• V_pv - PV panel voltage when it is under load (but see my comments)
• V_store - voltage at the spv1050 store capacitors (2x47µF)
• V_out - voltage on the output capacitor (1500µF)

For low light conditions I waited several minutes for the capacitors to charge and read values only when the system became saturated (or at least look so). Results and my notes are represented in the following table:

The spv1050 datasheet provides quite detailed description on how it works. The only interesting question was - what happens in buck-boost mode if the harvester cannot maintain V_store voltage anymore due to low illuminance. I found that store and the load will be connected as long as V_out>2.2V. After that, spv1050 will shut down the DC/DC and connect PV to the store. I think DC/DC was on because panel voltage raises up from 0.25V to 0.52V when the store disconnects from the load. Why is it 2.2V and not 2.6V as specified in the datasheet, I don't know.

Another interesting test was how much time it will require to charge the 1500µF capacitor to 3.2V. For that I tried different panels with both buck-boost and boost boards. Please note, since the boost board has 4V limit (due to the tantalum capacitor) and pair of the IXYSes are capable to produce up to 8V, I've skipped this couple. The results are in the table below:

You was absolutely right when expressed the concern on the SolarBit panels - they work much better in the sunlight. The SCNE seem also prefers full spectrum to my LEDs.

Evaluate whether this is a good or bad result is possible when considering the load itself. Accordingly to the Nordic, the nRF52840 will consume about 32µC
per heavyweight BLE event, for example, when running at 2.2V (lowest possible value for the spv1050) and sending +8dBm advertisements. For tiny packets it may be as low as 5µC.

The tantalum capacitor stores 1500µF*3.2V = 4800µC - exactly this amount was delivered by the spv1050. This roughly equals to 150 advertisements, or 960 connection packets. If ignore other waste (like sleeping current) and assume 5 minutes as an average result on SCNE-alike panel (indoors, daylight), the system should be able to advertise once every two seconds or send small data packets once every 1/3 second without discharging a battery.

Of course, more light means more power, and there is the night too, but with proper panel and correct location I think it should be possible to maintain positive power balance.

So which configuration to prefer - boost or buck-boost? I still have no simple answer.

The boost is good when you're working in low-light conditions and can keep it low-voltage. I also have impression that the spv1050 is sensitive to the current it can get from a panel. Therefore a couple of single cell panels (i.e low voltage, high current) might be the right choice. On the other hand, it's risky to attach high voltage panels to the boost harvester - neither battery not capacitor will tolerate 6V.

The buck-boost configuration has no such drawback - any panel you may find on attic will very likely to work. Another advantage you might have noticed is that it delivers faster. When fast is fast enough - it depends. But in the low-light when it may took up to 30 minutes to charge a 1500µF capacitor the boost advantage is not so obvious.

@NeverDie What I meant is that in addition to internal LDOs in the SPV1050, there is also the MIC5205 3.2V which is sourced from the USB (can accept up to 16V). This LDO output can be connected to the spv1050 store capacitors via relevant solder jumper on the board bottom, and when connected and not powered it draws about 8µA from the store. So the jumper is kind of requirement.

I've enjoyed the calculators video. BTW my calculator from which I took the solar panel has similar characteristics. Regarding reuse of calculator panels, first of all they must match the application voltage. If speaking about the nRF52, it has to be 1.8V to 3.6V (or up to 5.5V), but a small panel can't supply it under load. Also, the radio can draw up to 16 mA which is possible only with larger panels. Of course, a capacitor can fix online use of a solar panel, but in order to work with lights off a secondary battery required, hence the solar charger.

And now it runs out into a game of coulombs. Today I've tested the boards in direct sun light. They charge the 1500µF capacitor in few seconds and then just idling. It looks like this power excess is more suitable for a battery rather a small capacitor. But a 2032 size supercapacitor could also make it. It looks slightly more interesting strategy to fast charge battery when you can and then work from the battery, than struggle to charge the battery in the lowest possible conditions and hence limit charging speed. Please also note, it is usually easier to build high-voltage panels rather high-current ones.

Anyhow, there are too many influential factors so a practical test is required. I think I have ot try two competitive configurations placed in exactly the same environment:

• 2 x KXOB25-02X8F panels (5.53V, 6.3 mA) and the buck-boost board
• 2 x KXOB25-05X3F panels (2.07V, 19.5 mA) and boost only board

@NeverDie They do review schematic and layout of their SoC and then can do antennas tuning. For the tuning two fully assembled devices (including housing) are required.

@NeverDie Yeah, you're very close. Cypress has mentioned it in the datasheet. The PV panel is AM-1606C and the supercap is DCK-3R3E204T614-E.

It's also interesting to see that they have installed a second pool of ceramic capacitors to workaround high ESR of the supercap.

@NeverDie Wow! Are you sure the lux meter is working properly? 15 lux is about as low as 25 cm from a candle fire which is awfully low.

So, since the ADP5091 is also on the list now, I think it becomes necessary to put all the mentioned power harvesters side by side so we can compare at least basic parameters. Please take a look to the Google Spreadsheet listing some of the tiny harvesters. Please note, The Analog Devices has about ten harvesters which may be described as "tiny", but so far the sheet is covering only ADP509x series. I'm going to add the LTC series later.

From brief analysis, it looks like the Startup Input Voltage and the Startup Input Power are placing the major constraint on PV panel. Of course, the panel should be also able to supply voltage required to cold-boot the harvester. Such, under the low-light conditions (50 lx to 100 lx in a dim room) power capabilities of both of my panels are simply not sufficient to bootstrap the SPV1050: the IXYS is too weak and works starting from 150 lx, and the SCNE is good for boost, but still require 150 lx to reach 2.6V at STORE (the data is for the charts I've posted earlier):

Therefore any panel which can reach required voltage and provide enough power should make the harvesting IC work. Please also note, that after cold-boot almost any harvester can work on a lower power (usually 1.5 - 3 times lower than it was required to start).

Also, in the table there is a class of super-tiny harvesters, namely the Cypress S6AE102(3)A and Ricoh R1800K. They can charge a store by harvesting source with less than 1µW capability. At the same time, the EM8500 looks like the most sophisticated embedded solution with lots of features. The rest of harvesters are quite nice ICs for a modular system.

Summarizing the work on the SPV1050 (irrelevant to other ICs mentioned in this topic) please let me publish revision 0.9 of the Harvester board. It addresses some issues found on the previous boards, and introduces number of important changes.

The most noticeable one is that the board now supports both boost and buck-boost DC-DC configurations of the SPV1050. After reviewing some tiny PV panels it was indetified that the maximum working voltage for a tiny panel is about 3V which means the boost mode is more suitable to do the job. Also, tiny high voltage panels (like some SolarBIT models) have very limited current capabilities and in low light conditions simply can't supply enough power in order to make the harvester chip work. It's also important to note that the cold-boot voltage for boost DC-DC is 0.55V which is only about 20% of maximum 3V voltage a panel can gain - please compare that with 2.6V and 4.4V respectively for some most advanced cells. Finally, if in the boost mode the SPV1050 supports TEG modules.

Therefore the BOM was adjusted to the boost configuration with the following thresholds:

Please note, in the boost mode the SPV1050 will effectively set V_eoc = V_in for all V_in values greater than 3.1V which may cause damage to the battery or the nRF52 SoC. To prevent the negative impact please carefully consider the source.

If the only source you have is a high voltage solar panel, it's possible to adjust the R1-R3 resistors ladder (please refer to the SPV1050 datasheet) and switch the Harvester board to buck-boost mode as follows:

Hint: If the solar panel is really big (like 2W / 12V or so) and you don't need MPPT, it's possible to close the USB Charge jumper and connect the panel to VBUS and GND solder pads in order to employ the 3.2V USB LDO.

The MPPT fixed voltage ratio is set to 78% with resistors R2=2.2M and R3=8.06M. For a TEG module with MPPT ratio at about 50% just replace both R2 and R3 with 5.2M resistors.

Other notable changes included into this release are:

• Fixed some silk layer errors
• The MIC5205-3.2 LDO got missing input filtering capacitor
• The current limiting resistor between the SPV1050 and the tantalum capacitor has been removed
• Connection to the ground plane in some isolated areas was improved

And last, but not least, I'd like to thank the MySensors forum and in particular @NeverDie for his tremendous contributions. It's pure fun to discuss tiny boards with tiny harvesters working from tiny power sources

@NeverDie That's all true.

Please also note the AEM10941 can regulate up to 5V of input when ADP5091 upper limit is 3.3V.

Again, when speaking about BLE, a beacon (as a low-power application example) has to advertise at least once every 1000ms to be generally usable. For nRF52840 it approximates to about 50µW of power consumption. By adding up some microwatts of the harvester itself it may be safe to expect a panel should be able to produce 60-70µW of energy in average. In turn, this means that those 3µW or 15µW are rather an edge scenario, and there is must be a timeframe when the system can collect all the required electricity. Such, when running from a daylight it should be expected that in February the harvesting will be efficient for at most 8 hours a day. The system must be able to offer minimum (24h/8h)*70µW = 210µW during the light period of time. For the reference, a couple of my IXYS panels can provide only about 150µW when located in 1m from window on the north side.

From the experiments, to me it currently looks more not about possible minimums, but rather about higher efficiency at nominal values. But I admit the difference between 3µW and 50µW doesn't look big either. The sleeping current of the nRF52 is already less than 3µW, so perhaps some time the source and the load can converge.

@iiibelst There is one. The R7=549Ω is limiting current between the tantalum capacitor and the extension socket. Its purpose is to keep it under 2mA for ML2032. You can bypass it with relevant solder jumper on the board bottom.

The dropped 50Ω resistor was previously located between SPV1050 and the tantalum capacitor.

A little update. The nRF52833 test boards assembly was just finished. Now waiting for delivery.

@NeverDie My thought was that amorphous silicon (a-Si) cells have better spectral response to artificial light than crystalline cells (c-Si). However, after investigating this a little bit I've found that this doesn't seem to be true. Instead, it's shown everywhere that c-Si cells have better response to every wavelength:

Moreover, when the light source has wide spectrum (like the sun or an incandescent bulb), c-Si panels take the advantage and produce significantly more energy from the same source, and this all explains why a-Si cells are almost two times less effective than c-Si (roughly 8% vs 20%). Please note, because of narrow spectrum a LED lamp will be obviously inefficient for a PV panel.

But at the same time, there are reports of a-Si cells being 4x more effective in low light than crystalline. Indeed, both crystalline and poly-crystalline cells may degrade a lot:

The seem happens due to low parallel resistance of c-Si type cells. Shunt resistance of amorphous cells is naturally higher which results to less degradation of Vmpp and hence higher efficiency in low light conditions. Some paper show the shunt resistance rather low, when other mentions it relatively high, but at extremely low power conditions even 20 kOhm may be too much.

In short, a-Si cells are tend to produce fairly better results in very low light environments. But they can't leverage from wide spectrum sources, yet are subject to the Staebler-Wronski effect when exposed to direct sun (which can be reversed to some extent by heating the panel). In case if the light source is bright enough (around 1000 lx and above) a c-Si pannel should be preferred.

Finally, there are some other kind of solar cells, in particular those made from III-V semiconductors compound and promising even better low light performance.

@NeverDie said in The Harvester: ultimate power supply for the Raybeacon DK:

Only one way to know for sure, but I would guess that the crystalline one would drain off the current produced by the amorphous one (based partly on your theory as to why amorphous is better in low light). Worth a shot though: maybe as a compromise solution you can have the best of both worlds.

I mean connect them in series with bypass diodes so the amorphous cell can be used to bootstrap the harvester, and then crystalline cell will be workhorse during the day. Unfortunately, can't check it right now - left all my cells in the office.

Thinking out loud here, I have read about some research solar harvesters where they use a separate "pilot" solar cell to power the control electronics past the cold boot threshold.

That's a smart idea. Perhaps connect a dedicated tiny charge pump and an amorphous panel parallel to the buck-boost harvester storage capacitors?

The ideal solution would be if there were some way to re-configure multiple cells in series or parallel depending on the lighting conditions. It could default to series to push past the cold start and then switch to parallel.

A mechanical device?

@NeverDie Going to order 1 x AM-1606, 2 x AM-1456, 1 x AM-5610, and 2 x KXOB25-05X3F.

@NeverDie said in The Harvester: ultimate power supply for the Raybeacon DK:

At very dim levels I notice that my Fluke 87v multimeter actually draws too much current off the solar cell to get an accurate open circuit voltage measurement. So, I'll have to rig up some kind of voltage following op amp buffer as an aid to doing these measurements.

Heh, we seem dived below 1 µA / µW level here

@NeverDie said in The Harvester: ultimate power supply for the Raybeacon DK:

I built the op-amp circuit, and now the open circuit readings on a solar cell are much higher than when I was taking the readings with a regular multimeter. As long as I can keep the control logic current at just a couple hundred nanoamps or so, I think I'll probably have enough voltage under even very dim lighting that I doubt cold start will be an issue.

There is the Ricoh R1800K which consumes just 144nA and can start from a 0.72 µW source. It requires at least 2V to operate, but schematic is very miniature - only three more components needed.

@NeverDie Wow, I'm really surprised with so high voltage of the panel. Thank you for sharing the measurements!

My understanding is that OC voltage is defined by amount of free electrons in the depletion zone, and hence by the width of the zone. When in the light, more electrons will fill the zone, but there seem to be some saturation threshold limiting the max voltage. It would be interesting to somehow measure the electric field in the full absence of light. Also, capability to emit new electrons in the depletion zone defines the max current from the cell. It looks like crystalline cells can do it more effectively than amorphous, but the latter have wider depletion zone in the dark.

I don't know how to use so ultra-low current sources. The harvester should be able to work from 100 nanoamps or below. This limits design to a linear charger only (at least at frontend) - anything more complex (like a boost or buck circuit) would require higher quiescent current which will collapse the cell.

A MOSFET may draw as low as few nanoamps so virtually it could be possible. The PV cell needs to be isolated from the load to prevent voltage drop on the FET which may cause it defunct. Perhaps an isolated capacitor will be required to sustain the FET state while input capacitor releases its charge. The FETs may require resistors to shift voltage level, but again, they need to be hundreds of megohm. This will also impact switching speed. Perhaps some sort of hiccup switching circuit may make it. Also, I see some similarities with how dynamic RAM implemented.

For a usual solution, there are some ideal diode like the MAX40203 with 300 nA quiescent current. I suspect that the charge pump of the SM74611 may draw microamps when in ON state - it's unclear from the datasheet.

All in all, it looks like a puzzle

Found a nice paper on charge pumps design: https://www.mdpi.com/2079-9292/8/5/480.

@NeverDie Oh, nice! It's interesting that the full circle including oscillator consumes so low power. It seems really possible to build a discrete harvesting circuit which can collect enough charge to execute a single duty cycle of an MCU.

Such, assuming (88-35) nA/s = 53 nC charge it will require less than 5 minutes and 22 µF capacitor in order to shot a single BLE event from an nRF52 MCU. And that's at so ridiculous low light. Quite awesome, I think.

The only issue is that the timer can't optimize it for faster charge, but a voltage driven latch could.

Hi @NeverDie, thanks a lot for doing the experiments!

Basically, the circuit needs to remain inert until enough charge builds up and a trigger gets tripped. And, it needs not to bootlooop even though it ramps up using just very little current. A tall order, I know. Not sure if the right kind of circuit exists, but that's what I'm in the hunt for.

Yeah, this is would be very interesting to achieve indeed. Looks like the water bucket from aquapark. Unfortunately, have no practical ideas at the moment. Maybe a FET + BJT combo where the FET generates spike of the current which activates the BJT which then drains the input capacitor? The idea here is to utilize the inrush current from the FET before it will be stabilized. A comparator may have higher quiescent current, or may not.

8mv was as low as I could take it, but I suspect even then there may have been some slight amount of light getting at it. The room was very dark...

8 mv is fair enough. So it seems all about the structure of the amorphous cell. Interesting!

Should I start a separate thread for this, or continue it here? It seems that your project is completed, and although this is all relevant, maybe it would be better to split it off? @Mishka Since you're the OP, what's your preference? Continue as is, or fork your thread and continue in a separate thread? I'm enjoying the collaboration, and hope you feel the same. I'm fine with either choice.

Although the discussion went beyond the original project, the topic is very interesting. While most of existing harvesters are aimed at low-voltage sources, it seems that we're trying to address the unique property of a-Si cells to have high-voltage bias in the extremely low light. This is not only enjoying, but might also have (and I hope will do) some practical extension. Of course, if there is a better place for the discussion - it's completely okay to move it, I'll be glad to follow-up there.

Regarding the project, it wasn't finished yet. I'm currently waiting for newest PCBs - they're still based on SPV1050, fully configurable, the components selection is for the boost. Appropriate solar panels are also on the way.

BTW, I had a chance to try the SPV1050 (buck) and nRF52833 with a single one SolarBit I have, no battery attached. In the direct sunlight it works without any issue, even with 1 mA red LED blinking 50% of time. This is definitely not the best setup, so the mentioned PCBs and panels should make it more useful and especially for a cloudy day. Also, for the version 2.0 I'm considering to replace the harvester IC with the AEM1094. I also have somewhat different idea about form-factor, but that's for another topic.

And here is Version 2, which uses an Arduino UNO

Perhaps the right thing would be to charge the capacitor first, and only after that connect it to the Arduino. The Arduino has to read the ADC often so it should be possible to determine highest voltage before it decays.

Unfortunately, the AEM10941 breakout board can't seem to rise above 0.352 volts when tested with the same solar cell and same dead-bug op amp assembly under the same 1 flux light source, so I'm afraid I have to label it a FAIL for use in boosting, just by itself, from that particular low light scenario.
...
Despite a promising start, the BQ25504 peaked at 0.812v and couldn't seem to pull itself above that.

From my (perhaps not too careful) review I did earlier in this thread the AEM10941 requires 3 µW input, and the BQ25504 requires 15 µW. Either of those are far beyond the 3V*80nA condition.

Unfortunately, by most of manufacturers a nanoamp source seem usually considered as zero current.

if a silver zinc 8mah SR416 primary battery, which is just 4.8mm in diameter and 1.6mm thick, were used to continuously drive a 35na TLV5110 timer, then assuming all 8mah could be extracted and ignoring self discharge, it would last for 26 years.

Hmm... taking in account those 80 nA collected in 10 seconds will be wasted in one millisecond, and then the next 9.999+10 seconds it will wait for another portion, it sounds like bargaining 35 nA for 40 nA. Well, fair enough!

@NeverDie This. Is. Stunning!!!

I must admit that I were stuck with a CMOS driven circuit, but there are BJT circuits with amazing level of practicality. The decision to employ a LED is simply brilliant. I don't know shall we put it into a SPICE, perhaps to facilitate selection of real components, but taking in account the Hackaday project it should simply fall into place. Going to examine the project. It's definitely worth implementing it, thank you very much for finding the project!

The AP4470 looks very interesting too. With reported 7µA current consumption when boosting starting from 0.2V, and fixed high to low output voltage from 2.6V to 3.55V, it looks like a strong competitor to the AEM10941. I'd still stick to the latter though, not only because of availability (including documentation), but the e-peas product also has very appealing buck-boost configuration.

I'm also thinking about even more modular design of the boards (details will follow later), so having two harvesting circuits targeting different scenarios is the right way to go.

Thanks again for your interest!

@NeverDie said in The Harvester: ultimate power supply for the Raybeacon DK:

Here's another one: https://patents.google.com/patent/US20170133938
He claims the startup power is just 100nW. For contrast, TI says their BQ255xx chip requires 15 uW. i.e. an entire order of magnitude more. Sounds too good to be true, doesn't it?

What's interesting about the circuit is that it uses the self-resonant converter together with a MOSFET (HEMT is recommended) which is closed at low voltage. There, the 1:1 transformer is used to bump the gate voltage and thus fully open the MOSFET when it reaches the threshold value Vth (the paper notices it at 120 mV, but for the a-Si cell it might be at 2.6V). The more it opened - the more voltage at the gate. This results in discharge of the input capacitor to the load until the gate capacitor voltage + the second inductor voltage won't drop below Vth. The input capacitor cut-off voltage could be configured to 1.8V so it will charge faster on the next cycle.

The patent mentions 0.1V x 1µA = 1nW startup power. Upon charge of the input capacitor, the leakage current will be at about tens on nanoamps. Perhaps rest of the harvester circuit consumes something too. Obviously, when it's going to discharge the inductors will cut some efficiency, but it's worth it anyway.

Looks interesting!

Which leads me to wonder: just how well are patents vetted before they're granted? Might it still be granted even if the author never made a a circuit that performed anywhere near as well as the patent claims? Is anyone even checking?

Well, a patent is just an exclusive right to the invention, and AFAIK there is no practical consideration neither verification of the patent subject. All that's checked is the invention wasn't patented before.

By the way, on a different topic, this might interest you: https://www.mouser.com/ProductDetail/426-DFR0579 It's a $12.90, 30mmx30mm, fully assembled breakout for the SPV1050, configured as a boost converter.

Yeah, it's nice! Thanks for the link! I think we here will be able to offer something interesting too: both boost & buck-boost combo board with USB and LDO, 25 mm diameter. Now in trendy corona-shaped profile from the OSHPark

Unfortunately, can't assemble them due to the quarantine

@NeverDie said in The Harvester: ultimate power supply for the Raybeacon DK:

I'm confused. Aren't 315 and 445 simply different aliases for the same physical component? 315 at a higher abstraction layer and 445 at the detailed layer? Except... isn't an E-PHEMT different from a PHEMT? So, they aren't aliases for the same part after all? Or, maybe they are the same, but 315 refers to a different potential embodiment than 445? Or... do HEMT, PHEMT, and E-PHEMT all mean the same thing?

I see the components are numbered through all the figures in the form XYY where X is the figure number, and YY is the component number. Such, 115, 315, and 415 are referring to the energy harvesting circuit. The circuit contains x20 resonant DC-DC converter, and x45 do reference the transistor or crystal oscillator.

An enhancement mode transistor (N-channel MOSFET or an E-HEMT) is required because it has to be closed at zero bias.

By the way, I mispoke in my earlier post. 100nW is actually two orders of magnitude lower than TI's 15uW cold start minimum for TI's flagship energy harvester. If the patented circuit now under discussion here really does perform as well as it claims, then that makes it all the more impressive.

Oh, my, it's 100 times different, rght. I'm still not used to the numbers and feel that if we take a couple more steps, we will go to the quantum level

Unfortunately, can't assemble them due to the quarantine

You mean their automated assembly is off-line, or that you can't source all the parts you need due to the quarantine, and so you can't DIY the soldering even if you wanted to?

Just can't get to the soldering station, it's closed in the office with some other components until May.

BTW, I like your PCB homage to the caronavirus. Subtle, yet amusing!

All credits go to OSHPark which didn't bother to remove the panel tabs

@NeverDie Well

What can I say? Only that the PDF is here. They seem achieved this ridiculous leakage with careful transistor selection. Very nice!

@NeverDie said in The Harvester: ultimate power supply for the Raybeacon DK:

Fortunately, Figure 5 in the paper you linked shows an equivalent transistor layout for the voltage detector. It lacks a BOM with part numbers, but I'll nonetheless take a quick run at trying to simulate it in LTSpice--maybe I'll get lucky. If you were in my shoes, exactly which simulated transistors/mosfets would you be trying?

That's true. The components selection is the hard part. I din't find anything, but the MOSFET arrays by ALD, and I see you've found them already.

It seems the most of discrete elements are tied to nanoamps and only few are diving to picoamps area. For example, the Nexperia settled it to 25 nA, as well as the TI does. But for some selected integrated circuits there are the picoamps, and some opamps may draw only femtoamps which is impressive. There is also the nice article on possible design issues - quite surprising - when building such a uber-low-power circuit - https://www.edn.com/design-femtoampere-circuits-with-low-leakage-part-one/

As for alternatives to the UB20M, the nearest I could find is this:
https://www.ablic.com/en/doc/datasheet/photo_ic/S5470_E.pdf
...
The last option I can think of would be to try these special mosfets from Advanced Linear Devices:
https://www.aldinc.com/pdf/ALD110802.pdf

Yeah, that's it. And the cool part is that the ALD offers 2V*200nA=400nW energy harvesters which work very similar to those we're trying to design here - http://www.aldinc.com/pdf/EH300.pdf

Unfortunately, still not sufficient to run your a-Si 80nA solar panel.

Edit: I put Figure 5 into LTSpice. I could get it to generate the ~100mv reference voltage, but it doesn't appear to switch anything nor "detect" and then switch anything either. So, maybe there is more to the circuit that what they are showing. Given the circumstances of not being able to acquire their UB20M, it's a bit of a let down.

It has to switch the VOUT on as soon as the VINL will be high enough to close the MN5 and pull down the VREF thus resetting the triggers and causing them to produce the VOUT.

I've put it into KiCad and immediately failed with component selection. In addition to issues with the search of a low-current MOSFETs, the ngspice has incomplete support for the modern PSPICE models. And create own models is a cumbersome task

After trial and errors I've ended up switching to ngspice internal models. After some trivial tuning the circuit started to work. I've just added input (storage) capacitor and have attached a simple load (switched with an additional N-MOS) to get the simple harvester work.

On VinL≥2V input capacitor is discharged to load R2 until VinL will drop below 1V. Both voltages are configured via MOSFET gate thresholds.

For details please take a look to the eeschema file - https://drive.google.com/file/d/1O8aVj7ZzjG1TNdTJOce4i2P65X-aRLgB/view?usp=sharing.

Voltages:

Input current I (via R1) in dependency of input voltage. I(R1) = 3V/100M = 33nA to simulate the a-Si cell.

I don't know how much current the circuit will draw in real life, but taking in account low voltage source (please note, datasheets mention 25nA as upper threshold) perhaps there are some chances to fit into the a-Si cell current budged.

@NeverDie said in The Harvester: ultimate power supply for the Raybeacon DK:

Have you run across any other parts that might fit the UB20M role?

The UB20M is hard to beat. But there's another sub-nanoamp option: https://www.vishay.com/docs/66597/sip32431.pdf

@NeverDie I've finally managed to make a readable image of the circuit. For our convenience, here it is:

The circuit self consumption is comprised of MOSFETs leakage current, and current required to charge the C1 capacitor. Regarding C1 the startup current may be arbitrary low, but sufficient to charge it eventually. After that it won't require too much to sustain the circuit. For this particular ngspce model (where MOSFETs leakage is really low) those are picoamps indeed:

Those 30nA you've mentioned in my previous post are due to charging the C2 storage capacitor and is actually limited by R1=100MOhm resistor installed solely to emulate the weak a-Si panel. I.e. for one gigohm resistor it will not go higher than 3nA.

Of course, the model itself is far from being optimal and could be improved.

@NeverDie said in The Harvester: ultimate power supply for the Raybeacon DK:

@Mishka said in The Harvester: ultimate power supply for the Raybeacon DK:

@NeverDie said in The Harvester: ultimate power supply for the Raybeacon DK:

Have you run across any other parts that might fit the UB20M role?

...But there's another sub-nanoamp option: https://www.vishay.com/docs/66597/sip32431.pdf

How would the vishay fit into it? Are you thinking you would switch it on-off using an ALD mosfet, or ...?

I have no idea. Bipolar based opamps have similar characteristics, as well as the UB20M which seems build using FETs only.

@NeverDie Oh, right. For the voltage source there is the 3V pulse defined as follows:

PULSE (0 3 20m 1u 1u 60 0)

Reads like "start pulse from 0V to 3V, after 20ms timeout, 1us raise time, 1us down, keep it on for 60 seconds". This helped to examine how the circuit starts. But the circuit has to start with flat 3V input anyway.

All MOSFETs are defined with MOS level 3 model, zero-bias threshold (vto) set to ±2 V, transconductance (kp) to 50 mA/V^2 to minimally reproduce a real transistor. Both drain and source has 1 Ohm resistance. Please note, the controlled NFET has lower voltage threshold at 1 V - this defines lowest VinL voltage. The rest of parameters can be derived from the ngspice manual, section 11.2.

I expect that the ngspice and the LTSPICE may have different directives to setup the circuit

Also, you may want to drop the C3, M8 and R2 thus leaving the circuit very similar to that one in the paper. The R1 still be used to limit input current, and the C2 will help to model raising voltage.

Also, does the .tran 10k means 10k milliseconds?

@NeverDie That's true, discrete transistors have not too thin parameters on paper. But assuming quite low voltages (about 3V) I'd expect those Nexperia and TI transistors may draw picoamps indeed. I think this is better to check with real devices. But assuming how small they are an evaluation board will be required. There are some, but it might be way cheaper yet more flexible to assemble such a module manually.

For convenience, the custom PCB may also contain circuit required to measure the leakage. Perhaps a low-leakage capacitor may be charged to some known value, then a MOSFET (or number of FOSFETS in parallel) would discharge it for some time, and then the voltage drop can derive leakage.

@NeverDie Wow, my congratulations on making so much progress! Diving below 1nA is serious achievement!

The paper on the batteryless sub-nW Cortex-M0 shows a state of the art circuit built with FETs in super cut-off state. Thanks for finding it! But this makes me think that from all the networks we've considered here - the 3nA oscillator by David Johnson, the ready to use 10nA solar-powered motor driver by Stepan Novotill, and the UB40M chip by Bristol - due to some leakage elimination techniques a MOSFET based circuit might have the best performance anyway. The essay on femtoampere circuits also suggests FETs for lower leakage. (But I admit I still extremely impressed about those two BJT circuits.)

The relaxation oscillator with 230 fJ/cycle efficiency is also voltage driven, i.e. based on MOSFETs with proper threshold. Similarly to the UB40M it uses capacitors for time adjustment. By chance, detailed description is available for free as part of the Ultra-low energy electronics for synthetic biological sensors paper, please see chapter 3.

Components selection still play important part here, but assuming no femtoamp level I don't think package size will be too important. Enough clearance around elements may work similarly well. Regarding threshold values, the bigger voltage should be applied to the gate in order to close the transistor, the less leakage current between source and drain may be expected. This is exactly the reason why super cut-off state gates are working so well. The nice overview of this and other leakage elimination techniques described in the Design and Modeling of Low Power VLSI Systems book the relevant chapter from which is available as a dedicated paper. Perhaps it could help optimize current leakage later when the concept will be more or less ready.

The pragmatic question still how the desired circuit should work. I think the major finding was that a-Si cells are capable of high voltage in very low light conditions. This allows to avoid overhead and complexity coming from extra charge pumps or other regulators, and drive MCUs and sensors (usually working in 1.8V to 3.6V range) from a store capacitor charged directly from the a-Si cell. Another big advantage is that such a circuit will have nearly zero cold boot current.

How low should it go? The measured cell you use for experiments produce current in nanoamps, and when the cell current goes down to picoamps the voltage level seem also drops significantly. Also, waiting for the capacitor to charge from a picoamp source may take forever. IMHO targeting sources with few tens of nanoamps, and therefore maintaining the harvester self current less than 1 nanoampere should be considered a great success. Of course, the less quiescent current the harvester has - the better.

For such a low-power source I'd expect the energy will be harvested for a long period of time (tens of minutes), and then dumped it all at once into the MCU and a sensor. Using the collected energy to charge a battery is impractical: modern devices require microwatts of energy, and the harvester simply can not withstand the consumption. For online operation a conventional harvester like the AEM10941 should be used.

@NeverDie In SPICE the ring oscillator works just fine. I don't see why it may be hard to have it oscillating with any FET. However, due to the fact it's perfectly balanced (at least in it's current form) across all the three transistors it may be somewhat reluctant to start.

Here is how I get it in the light of the power consumption. For convenience, I've re-enumerated all components left to right.

Phase 1. I'm not going to cover the circuit boot and will start at the moment when the Q3 is closed. In the closed state VF3=0 it will tie the Q1 gate and the bottom pole of C1 to the ground. The Q1 is closed, and the C1 will be charging via R1 resistor. The R1 will set the charge time for C1.

At the same time, due to the closed Q3 the voltage source will be grounded via R3 and this results in extra leakage Vin/R3. At this phase, the overall current consumption will be about I ≈ Vin/R2 + Vin/R3

Phase 2. The phase 1 will last until C1 will be charged to the Q2 gate threshold voltage. After that, the Q2 which was previously open due to the C1 voltage drop, closes, and this will tie VF2 to the ground and open the Q3.

The opened Q3 will shift ground level for the C1 thus doubling the VF1 voltage. The C1 will start discharging down to zero until VF1 won't balance VF3. Actually, the C1 it will be charged from the opposite side via R3. The R3 defines period of the stage 2. Consumption current should be about the same I ≈ Vin/R2 + Vin/R3. Please note, since the Q2 is not participating in C1 charge/discharge it should be safe to keep R2 resistance much higher than R1 and R2, like 300g or so, and decrease the circuit consumption.

However, it seems slightly more complicated than that. Due to FETs non-linearity, at marginal gate voltages there is some drain–source resistance. Despite the Q1 is open with VF3 voltage, it's not enough to have the C1 grounded, and this is why the circuit works at all. Instead, both C1 and the voltage source will be leaking via the Q1. While voltage source is limited by R2, the leakage for C1 may be quite high and require extra attention. Luckily, this is not true for the SiSA40DN - the C1 slowly discharges via Q1 and this compensates for leakage from the voltage source via R1.

Repeat. Upon C1 discharge it's going to close Q2 and raise gate voltage on Q3. The Q3 opens, VF3 voltage drops, then Q1 closes too, and the C1 starts charging back again.

Well, as for an astable oscillator the circuit looks cool, definitely would be interesting to build. But for energy harvesting purposes I'd prefer some UB40M variation - IMHO it's much cleaner from the point of parasitic leakages. Especially if taking in account those leakage optimization techniques like we've seen for the super cutoff gates.

@NeverDie said in The Harvester: ultimate power supply for the Raybeacon DK:

Glad you asked, but it turns out not to be happy news. As it stands, it's a very constant 28.73ua leakage, which is obviously pretty terrible.

The circuit has so low leakage mostly due to the gigohmic resistors. In particular, those 300 pA bumps on AM3 may be due to the R4 limits (3V / 10 gOhm). And it looks like the right thing to do for a BJT. Oppositely, MOSFETs can be used as ultra strong resistors themselves. I.e. should you put several in series and the leakage drops.

I.e. it might be reasonable to drop a MOSFET in the place of T5+T6, and yet another gigohm resistor will cut the T4 leakage down.

Hi @NeverDie,

I also have the strong feeling that a couple of voltage detectors could make it much easier, yet – due to picoamps leakage – more effective.

Such, in the couple of last days I've tried to reproduce the UB40M circuit with real transistors. Perhaps, when you build a die you can construct every transistor with the parameters you need, but I admit it was not a trivial task to pick anything suitable from a catalogue. I defined no strict constraints to the application, rather tried to be opportunistic and use whatever works. There were some assumptions though:

• Input voltage is defined by the solar cell and is somewhere between 2V and 3V. In the deign below it's set to 2.5V.
• The short circuit current for the cell should not extend 50 nA. I've limited it down to 25 nA with the Rcell = 2.5V/25nA = 100MΩ.
• The harvester should be able to charge 22µF storage capacitor - this capacity should be enough to send a single non-connectable BLE advertisement.

I haven't bothered to find the low leaking MOSFETs and chose something small, handy to solder, cheap, and in stock. That turns out to be power MOSFETs in PowerPAK SC-70 package from Vishay. The nomenclature is SiAxxxDJ where the xxx is what you may see in the circuitry below. For example, 421 stands for SiA421DJ.

The core of the circuit is the pretty much of the UB40M reference design. The series of MP5-MP7/MN6-MN8 triggers will pull Reset line high when VREF will become low. This will happen when MN5 will pull it down, and depends on the Vdet voltage. At the same time, the MP4 will be turned off by VoutL (Reset) thus preventing VinL from being unintentionally pulled to the GND. The MP3 is used as a diode there.

I failed to create the VREF voltage in the way it was defined in the Bristol paper. With the circuit powered by the very low-power source, it suffers from transient processes a lot. To address that, I went for more complicated solution with couple of triggers controlled by Rdet1+Rdet2 divider. The divider also allows to tune the circuit to better match source and storage capacitor.

Finally, there is a 1k load attached to the Vin line. It discharges the Cstore capacitor as soon as the Reset will be set high. Upon discharge, the voltage detector will went reset the Vdet and the Cstore will be charged back.

In my KiCad ngspice it looks as follows. With Vcell=2.5V, Icell=25nA the Vin oscillates between 1.4V and 1.95V. Although this is way below required 1.8V for most of sensors and MCU, raising solar cell voltage to 3.5V will shift the voltages to the usable range.

Discharge current is limited solely by the Rload=1k. At the same time, average current consumption of the harvester is about 3nA - the blue line I(Rharv). Solar cell load is below 10 nA - the yellow line I(Rcell). However, due to non-linear nature it's hard to predict how the line will look like with a real cell. Probably, it's better to simulate it with a current source, I don't know. The red line Vdet shows how the voltage detector works.

To be honest, I'm quite unhappy about the circuit.

First of all, it uses a lot of transistors, please compare this to the BJT harvesters you're working on. Also, many of them work on subthreshold voltages, and this makes it relatively hard to to tune. But worse, real devices will likely to suffer from the voltage interference which makes the whole circuit too fragile. Taking in account the money to build it (even with $0.40 per FET), it turns out the circuit shall be considered rather impractical.

@NeverDie Yeah, this is awesome hack! I'll followup in the CNC thread