Not sure if others would have interest in this, but I'm designing a PCB for an easy-to-solder TH node. As presently conceived, it would have 3 surface mount components (an LED on the front and a resistor and capacitor on the back), plus a DIP atmega328p, a header to accept an inexpensive si7021 TH breakout board, an FTDI header, and an RFM69HW. The idea is that it would run at 8Mhz and be powered by two AA batteries, so it's sized to be compact but still easy to solder. The same thing could be achieved with wires and some of the other boards out there, but this might be a little tidier if a TH mote is what you want as either the starting point or the end-point.

PCB dimensions are 0.65x2.55 inches. i.e. it is narrower than a typical AA battery holder, but roughly the same length.

The LED and resistor are optional, and you could forego the capacitor as well if you wanted a truly bare bones TH solution. However, the pads are there if you wanted to utilize them. Also, you could skip the si7021 BoB if you wanted just a generic mote.

I had hoped that someone would make an easy to solder board like this for the RFM69HW, but I got tired of waiting and finally decided to just make my own.

I found a good place to do range testing.... too bad I didn't bring the gear!

Good news! Last night I did some accelerated load testing on the supercap. First I charged it to 3.6v and then I hooked up an RFM69HW mote which woke up once a second to do 3 things: 1. check the voltage level, 2. turn on an LED for 1ms to simulate a sensor load, and 3. transmit a packet containing the voltage data using the RFM69HW.. Bottom line: 14,111 packets transmitted before running out of juice.

Not bad for a first attempt.

Yesterday received the PCB. Today assembled for testing this battery-powered nRF52-based passive infrared motion detector:

Made this 12 button keypad. Requires only one analog pin to read which button is pressed, and any button press can also wake an arduino from sleep:

Consumes no power when no button is pressed.

A pair of lithium AA primaries is hard to beat because:

1. Unlike alkaline's, they don't leak.
2. Have a look at the discharge curve: https://data.energizer.com/pdfs/l91.pdf By the time they drop to 2.4v, if not before, you'll want to replace them.
3. Obviously much longer life, both shelf life (20 years!) and energy capacity.

I think running 8Mhz from the internal RC is a no-brainer: wake up time is less than 4us. So, if your node wakes up often, you'll save a ton of energy over time.

The best time to take your battery measurement is immediately after a Tx. That will give you the most conservative reading. Save that measurement in a variable and then send it in your next transmission. Switch on your ADC just before Tx and take your first ADC measurement during Tx, because you have to throw out the first measurement anyway. That way you can take a fresh (and valid) ADC measurement just after Tx before the voltage rebounds.

Hope that helps!

Here's the version that I most recently assembled:

As you can see, the 15F supercap is now on the board itself. It works fine.

I've since made a few refinements and have sent the new files off to be fabbed. The newest version of the PCB will measure roughly 22mm x 22mm.

Put together this pro mini nRF24 shield for testing...

I have a number of sensor projects that sit within the nice footprint of a Pro Mini. Now I want to find a kind of universal power platform--hopefully small--to power them. So far, what I came up with was this, which runs the Pro Mini from 2xAAA batteries, and where the Pro Mini (and whatever the project built on it) simply "plugs in" to it:

Is there anything better? Ideas? Comments?

@franz-unix said in Coronavirus (way, way, off topic):

a sodium hypochlorite solution is what is commonly known as bleach.

Yes, but take a closer look. I was not interested in bleach (sodium hypochlorite) but instead Hypochlorous Acid. Note the difference in spelling, as they are not the same. I'm aware that some of the aliexpress products, like the one I linked to, may advertise as sodium hypochlorite, but I think that may just be click bait (?).

What are the benefits?

Hypochlorous acid, unlike chlorine bleach, is 100% safe and non-irritant. If it gets on your skin or in your eyes, it will not burn. Even if it were accidentally ingested, it is completely harmless. Yet, it is 70-80 times more efficient at killing microbial pathogens than chlorine bleach.
https://www.hypochlorousacid.com/

It's good for cleaning food surfaces. Other disinfectants aren't safe for food contact, and so they have to be completely washed off food surfaces following disinfection, which to me seems like a major hassle.

There is real science behind it: https://www.ams.usda.gov/sites/default/files/media/Hypochlorous Acid TR 08 13 15.pdf

How I found it: I was looking for a disinfectant with the shortest minimum dwell time to be effective.

I was reading up on Hypochlorous Acid as a possibly superior disinfectant. I was surprised to find little machines on AliExpress which are claiming to make it.
https://www.aliexpress.com/item/4001031093275.html?spm=a2g0o.productlist.0.0.11cd7faa2Rv2Ze&algo_pvid=8e105a6c-589f-49fb-b2b5-aeac6b44f105&algo_expid=8e105a6c-589f-49fb-b2b5-aeac6b44f105-16&btsid=0ab50f6115905983866665115e8103&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_
The typical ingredients are just water plus a little vinegar and salt, plus some kind of reaction which the machines do. I'm not sure how one would test for efficacy, but it sounds interesting.

Anyone heard of these or are using them? One of them sells for as little as $10 and plugs into a USB port. Just about all of them are less than $100. They appear to make copious amounts of disinfectant, such that you could pretty much shower any incoming goods in the stuff. From what I've been reading, Hypochlorous Acid appears to be a capable viruscide that's quite innocuous to humans and has much less odor than bleach. That said, I have no idea whether these machines manage to make it in a useful concentration or not. Concentration levels are directly related to efficacy, and AFAIK, maybe required dwell time as well.

So, why not just buy the stuff rather than make it? Apparently Hypochlorous Acid has a relatively short shelf-life.

110-220V Electric Sodium Hypochlorite Water Generator Disinfection Water Maker Portable Hypochlorous Acid Water Maker Machine 2L

$79.89

I tried soldering wires directly to some femtofet's, but I've concluded it either needs to be locked down to a larger substrate with glue or else properly reflowed in an oven. Otherwise, they behave like super energetic tiddly-winks: even the slightest bump when attaching the first wire will make the femtofet jump great distances, most likely never to be found or seen again.

@MatiasV said in FOTA/MYSBootloader - Documentation enhacement - Make it clear that you must program only the MYSBootloader, without sketch:

Platformio.org for building sketches and programming bootloaders
Out of scope of this post, but way better than using Arduino. Totally worth the pain of learning it's basics.

I'd be interested in hearing the full extent of your thoughts as to why you think platformio.org is better. Feel free to start a different thread if that's what you prefer.

You may be unhappy with your circuit at the moment, but to my eyes it looks like you've got some traction and you've made a good start.

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

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.

Not sure if what I'm finding out about my Vishay SiRA80DP mosfet might be similar to your Vishay mosfet, but I'll mention it anyway as a possible "heads up": I'm finding evidence of a pretty wide discrepancy between Vishay's SPICE model and what's physically true by measurement. Although I don't yet have a picoammeter (I'll be building one soon) for a more definitive test, I was able to do a relevant measurement with the voltage follower I built (see picture in previous post). The test circuit, to indirectly compare leakage of two different mosfets, was this:

If the mosfets were ideal and there were no leakage in either mosfet, then presumably the voltage measured at VF1 and VF2 would both be 3.3v. But, of course, they're not ideal: simulation shows that VF1 would be 3.12v an VF2 would be 1.59mv. I built the circuit and the actual measurements, taken with my voltage follower and a DMM, were almost the complete opposite: VF1=280mv and VF2=3.01v. i.e. the Vishay mosfet measured as far more leaky than predicted by TINA TI spice simulation of Vishay's SPICE model, and the other N-Channel mosfet (one of two mosfets on Diodes Incorporated DMC2700UDM-7) measured as far less leaky than predicted. It's a very easy test to perform... ahem, that is, if you are in possession of a 10 gigaohm resistor and a buffer (voltage follower) with a picoamp input bias.

Edit1: Come to think of it, a better way to test would be to use no resistor and just measure how much leakage there is with the gate set to GND. The leakage would likely be in the nanoamps, so it could be measured with a uCurrent Gold, or equivalent, which maybe you already have in your possession. Setting the DS voltage to whatever is listed in the datasheet for the leakage value would then give a number that can be directly compared to the leakage entries in the datasheet. Much easier and doesn't require high value resistors or exotic buffers! Obvious in retrospect. May or may not require a picoammeter, depending on the mosfet. To cover all the cases, I'll try these measurements after I build my picoammeter (soon!). I'll be building the inexpensive picoammeter designed and already vetted by an EEVBlog user named "Gyro":

https://www.eevblog.com/forum/beginners/static-control-requirements-for-picoamp-measurements-using-ucurrent-gold/msg3068708/#msg3068708
For anyone interested, you can find photos and all the details here: https://www.eevblog.com/forum/projects/picoammeter-design/msg790045/#msg790045
Rather than reduced to a nice simple PCB, the critical parts are air-wired, which is deemed the superior method according to the op-amp's datasheet. In addition, to better ensure accuracy, the DUT and all the measurement instruments should be enclosed together within conductive shielding as a countermeasure against external interference when measurements are taken.

The next test circuit I'll build will be this one:

On VF1 there's a nice 3 volt voltage swing. It's just a small step and will consume around 8na. Not enough current though for blinking LED's to know whether or not it's working, so I configured an MCP6022 as a voltage follower that I'll hook up to VF1 to confirm whether it's working as expected. The MCP6022 voltage follower should have an input bias of around 1pa, so it shoud not disrupt the circuit. If it shows the circuit is working as the simulator predicts it will, then I'll connect the Vishay load switch to VF1 and see if the circuit still works. If so, then the next step will be to swap the 10gohm resistors for 50gohm, and the 1gohm for 10gohm. That will take the total current drain to below 1na. If the circuit still works with the Vishay load switch connected, then bingo. If it doesn't, then I'll need some other low power way for the oscillator to drive a load.

Another possibility would be to build a ring oscillator out of the Vishay load switches. I can't think of any reason why that wouldn't work, and it would kill two birds with one stone. Because it seems so promising, I may even try it before the above. The only drawback is that because of no spice model, there's no way to simulate it prior to building it.

Edit1: Here is the MCP6022 voltage follower:

I air-wired all the connections and UV-glued both the chip and the wires to the protoboard so as not to strain the DIP pins. The protoboard is supported by 1 inch nylon stand-offs. This was the first-pass. I will add some bypass capacitors to polish it off even though initial testing indicates it works well enough without them. It's a two opamp DIP. The sensed voltage (orange wire) feeds the first opamp, which then feeds the input of the second opamp via the white wire. It's the output of the second opamp (yellow wire) which you then measure with your DMM. The red wire is the supply voltage, and the black wires are GND.

Edit2: The question of the moment is: how best to make the low leakage Vishay load switch into a low leakage inverter? One idea would be to have it drive a low-leakage p-channel jfet, such as perhaps the J177, which has a max cut-off voltage of 2.5v. Are there any p-channel mosfets with a lower max cut-off than that? According to the J177 datasheet, the drain cut-off current will be less than 1 nanoamp at a DS of 15 volts, so presumably much less than 1na at lower DS voltages.
Edit3: Looks as though J270 will be better: it has has a max drain cut-off voltage of 2.0v.
Edit4: Although it's an n-channel JFET, the 2n4118 sounds interesting. According to the datasheet, its typical leakage is just 0.25 picoamp at a Vgs of 20 volts: https://www.mouser.com/datasheet/2/676/jfet-2n4117-2n4118-2n4119a-interfet.r00-1649084.pdf
It also has a -1.8v cut-off voltage, so slightly better in that department. It doesn't indicate an Ids leakage current though.

Maybe better is the 2N4339:
https://www.mouser.com/datasheet/2/676/jfet-2n4338-2n4339-interfet.r00-1649114.pdf
It has a cut-off voltage of just -1v, an IGSS of 100pa, and, unlike the 2n4117a, it does list its Id(off) current leakage of 50pa.

 Any other ideas?

There's another class of component that might bear looking into. In December 2019 Omron released:
https://omronfs.omron.com/en_US/ecb/products/pdf/en-g3vm-21mt.pdf
which is a MOS FET Relay that, when switched off, has a maximum leakage current of 1 picoamp. This is a solid state relay, not a mechanical relay. That kind of performance comes at a cost of about $30 per device. However, maybe something not quite as state of the art would still impress at a more favorable cost? I can't say, as I haven't explored the category. I might be happy with with even 100pa leakage, especially if it were available at a much lower cost.

As a ballpark, I'm starting to doubt it's worthwhile to target solar sources which produce less than 7na under dim light. Even if 100% of the current could be harvested without any declines, it would take 8 hours at a constant 7na rate to charge a 100u capacitor from 0 volts up to 2 volts. At that level, I'm sure I can get a wireless node to send or listen for enough packets to be interesting.

I'm estimating my supervisory overhead, if successful, may come out to around 1na. So, in all likelihood, the "worthwhile" lower bound for solar harvesting will be somewhere in the 1na to 7na range for a wireless node. A realistic lower bound would most likely be an even higher range to account for inevitable inefficiencies.

My keychain solar panel can produce 88na (short circuit current) under 1 lux lighting, give or take. So, when it's finally all put together, I guess the only thing that's going to vary will be just how dim it can all still function at.

On the other hand, if the light is reasonably bright, then even just a single photodiode may suffice as a worthwhile power source. In that case the entire device could, in theory, be ridiculously small.

Still, I suppose the most conservative answer comes not from how much energy is required to charge a 100u cap from 0 volts 2 volts, but rather in how much energy is required to sustain that level (while accomplishing at least some minimal amount of work) once it has been achieved. In that case, the minimum harvested energy would just be the supervisory overhead plus storage capacitor leakage plus the energy required to , say, power up an MCU and send one packet once per 24 hours. All the MCU's currently on the market that I'm aware of consume more than 7na even while turned off, so the MCU would need to be turned off by a load switch, or in some other way by the supervisor, to eliminate even that minimal level of drain.

For that reason, I tried measuring the quiescent current of the Vishay load switch using the uCurrent Gold, which is when I ran into the picoamp measurement issues. The measurements I got were all over the map, but they were all less than 1 nanoamp.

That's an outline for getting the most aggressive answer. To finish the calculation I'll need to measure the total energy consumed by a wireless MCU powerup cycle, and that will depend on the particular wireless MCU that's chosen. That in turn will inform whether sleeping the MCU will actually consume less than a full power cycle. In the case of an atmega328p and an RFM69, the combined sleep current is 200na, but that alone doesn't account for the energy expended waiting for the radio's high speed oscillator to come up to speed and its PLL to engage.

Here's a benchmark for comparison: A 5x4mm PIN diode can produce as much as 45ua at an open circuit voltage of 320mv: https://www.mouser.com/datasheet/2/427/vemd5080x01-1767531.pdf
Boosting that to 3 volts might yield 3 to 4 microamps at that higher voltage. Accumlating that electricity over time means that, at least in principle, the entire wireless node could be 5x4mm in size, or even smaller if a smaller PIN diode were used..

I had planned to use a uCurrent Gold with a 500,000 count DMM to measure currents below 1na, but there's simply too much variation in the voltage reading depending on how close I am standing to the DUT to get meaningful measurements. The problem appears to be the multimeter test leads. Even just moving my hands near them changes the measurement. It's not that it's noisy (moving up and down and all around). Rather, it shifts around depending on where I am standing in relation to it. Do I need special test leads, or will I need to use something else (either an o-scope or a specialized circuit) to get a measurement without this problem?

Or maybe it's static electricity and I need to earth ground everything, including myself?

Edit1:
Reporting back: I put everything on an anti-static mat and earthgrounded both it and myself. That cleared up my multimeter going bonkers, at least when it wasn't connected to the DUT. Right now it appears that the Vishay load switch, or more likely my mounting of it, is what's amplifying static electricity or some other stray voltages. So, I'm going to take another pass at grounding the adapter board I soldered it to and removing residual flux to see if that helps at all. Adding a bypass cap will also likely help.

I suspect that a better quality anti-static mat might be worth a try. The one I'm trying is brand new, fresh out of the box, but it doesn't have the conductive rubber like the more professional ones have. Also, the wrist strap didn't work well at all. I got better results from leaning my skin against the mat. Perhaps I need a conductive gel for the anti-static wrist strap to work better? And I don't have a conductive anti-static floor mat, so that's also a weak link in the current setup. Perhaps I should just remove myself from the equation and do the measurements remotely, via wireless link?

Edit2: I tried all of the above (short of getting proper conductive mats), and it all seemed to help, except in the case of the uCurrent Gold. Maybe because it has it's own virtual ground? In any event, I'm not confident I can get better than 1na resolution out of it if I'm physically anywhere near it. I think I would likely need more specialized instrumentation, perhaps remotely operated, to get decent sub-nanoamp measurements.

Edit3: Using a sticky gel electrode pad from a TENS on my wirst insead of an el cheapo plastic wrist strap seems to be a big improvement in terms of grounding my body. I presume it's because the TENS pad makes for a better connection (more conductive interface) between my skin/body and ground.

Edit4: I posted this picoamp uCurrent Gold problem on eevblog:
https://www.eevblog.com/forum/beginners/static-control-requirements-for-picoamp-measurements-using-ucurrent-gold/new/#new
so hopefully that will produce some informed insight/advice.

I had a chance to try both the ABLIC faint signal detector and the Vishay load switch.

Originally I had thought the ABLIC was meant to be a current DETECTOR, but it turns out not to be so in the way that I had thought. Rather, it's more like a current SINK that will sink any and all current presented to it and that will switch if the POWER that's sunk is great than 0.7 nanoWATTS. So, the notion of putting the ABLIC between an NPN transistor and GND in my BJT oscillator circuit isn't going to work, because at that point in the circuit there's very little voltage remaining to drive the current, and hence, not enough power to turn on the ABLIC switch.

It could be made to nominally work if it's supplied with enough power (0.7nW or greater), but 0.7nW is actually quite significant in relation to my BJT oscillator circuit's power consumption. At least it's an option though.

I think the best way to run the ABLIC would be with short but infrequent pulses, because then its current drain could be amortized over the entire cycle. It would need to latch if triggered so that the detection could persist beyond just the short pulse. If the ring oscillator were also made into a charge pump, then I would guess that the the entire setup could be used to detect lower voltages. Perhaps something like: