My second solar setup did not fare well:

Started during the day, with the supercap charged up in bright daylight behind a window, it was able to power the JeeNode for about 16 hours – and then in the middle of the night it gave up:

    L 02:36:44.743 usb-A600dVPp OK 17 30 4 0 0
    L 02:37:48.074 usb-A600dVPp OK 17 31 4 0 0
    L 02:38:51.402 usb-A600dVPp OK 17 32 4 0 0
    L 02:39:54.725 usb-A600dVPp OK 17 33 4 0 0

Times are in UTC and we’re in the CEST time zone, so this was two hours later – i.e. around 4:30 AM.

I left it there for another few days, but unfortunately once dead this setup never recovers. The main reason for this is probably that the RFM12B starts up in a very power hungry mode (well, relatively speaking anyway) with a 0.6 mA current consumption – because it starts with the crystal oscillator enabled.

Maybe the self-leakage of the supercap was still too high, and would be (much) lower after a few days in the mostly-charged state, so I’m not ruling out using supercaps just yet. But as it stands, not getting through even a single night is not good enough – let alone being used in darker spots or on very dark winter days.

More experimentation needed!

Capacitors have a “leakage current”, i.e. when you charge them up fully, the leakage current will cause them to slowly lose that charge. I was wondering about this in the context of an ultra-low power JeeNode, which has a 10 µF buffer cap right after the voltage regulator. Does its leakage affect battery lifetimes?

Time to do a quick test – I used the 47 µF 25V cap included with JeeNode kits these days:

So how do you measure leakage currents, which are bound to be very small at 3.3V? Well, you could charge up the cap and then insert a multimeter in series in its most sensitive range. This multimeter goes down to 0.1 µA resolution, although its accuracy is specified as 1.6 % + 5 digits, so the really low values aren’t very precise.

A simpler way is to use the RC time constant as a basis. The idea is that a real-world cap can be treated like a perfect cap (which would keep its charge forever) plus a resistor in parallel. That resistor merely “happens” to be situated inside the cap.

What I did was charge the cap from a 3x AA battery pack which was just about 4.0V, then disconnect the battery and watch the discharge on the oscilloscope:

As you can see, it took 500 seconds for the charge in the capacitor to drop by some 2.5V – note the exponential decay, which matches the assumption that the leakage comes from a fixed resistance.

Can we derive the leakage from this? Sure we can!

The formula for RC discharge is:

    T = R x C

Where T (in seconds) is the time for the cap to discharge by 63.2 percent, R is the discharge resistor (in ohms), and C is the capacitor size (in farads).

Above, it took 500 seconds to drop from 3.98 V to 1.48 V, which by pure accident is almost exactly 63.2 %, so T = 500 and C = 0.000,047 – giving us all the info needed to calculate R = 500 / 0.000,046 = 10638298 ≈ 10.6 MΩ.

Using ohm’s law (E = I x R), that means the leakage current at the start is 4 V / 10.6 MΩ = 0.376 µA.

The good news is that such a result would not be of any concern with ultra-low power JeeNodes – the regulator + ATmega + RFM12B use an order of magnitude more than that, even when powered down.

But the bad news is that this result is in fact completely bogus: to measure the charge, I placed the oscilloscope probe over the cap, and it happens to have 10 MΩ internal resistance itself. So basically the entire discharge behavior shown above was caused by the probe i.s.o. the capacitor’s own leakage!

So it looks like I’ll need a different setup to measure real leakage, which is probably in the nanoamp range…

As you may know from posts a short while ago, I’ve been working on creating an ultra-low power supply, providing just enough energy to a JeeNode or JeeNode Micro to let it do a little work, report some data over wireless, and then go to sleep most of the time.

I even designed a PCB for this thing and had a bunch of them produced:

The good news is that it works as intended and that I’ll be using this circuit for some projects.

The bad news is that they won’t be available as kits in the shop. Ironically, this was the first time where I actually had a batch of kits all wrapped up and ready to go, ahead of time.

But the reality is that I can’t pull it off. For two different reasons:

• The circuit is connected to live AC mains @ 230 VAC and that means there is a serious risk if you build this stuff, try it out, and then hurt yourself because of some mistake. And even after that, there is the risk that the whole circuit is not properly protected, exposing these voltages (even just humidity and condensation).

• The other risk is that once everything works, it gets built-in for permanent use and becomes part of your house. What if it gets wet or malfunctions for some other reason, and your house burns down?

As supplier, I’d be liable (rightly so, BTW – there is no excuse for selling stuff which might be dangerous).

The hardest part of all is that even if an accident has nothing to do with this Low-power Supply, I still have to prove that this stuff is safe under any circumstance and that it complies with all regulations!

I’m not willing to go there. Life’s too short and I don’t have the pushing power to go through it all.

Having said this, I do intend to use this supply myself and create all sorts of nodes for use here at JeeLabs. Because I know the risks, I know which failsafe features have been built into the supply, and I’m ok with it:

The design is available in the Café, to document what I’ve done and for others to do whatever they like with it.

I’m not happy about this decision, in fact I hate it. I’m really proud of finding out that it is possible to create sensor nodes which run off just 12 mW of AC mains power. But the right thing to do is to stop here.

Out of curiosity, I’ve been doodling around with the AS1323 boost regulator chip, as suggested by someone in the comments. It’s not super efficient, but it does claim an extraordinarily low quiescent current of 1.6 µA.

Hooking up such a tiny chip is quite a challenge:

The trouble is that this stuff can’t be spread out too much – the specs say that both input and output caps have to be within 5 mm of the chip. So I tied it to a pcb – mostly for stability – and then wired everything up around it.

Without any load, this thing operates in quite an efficient mode:

The yellow trace is the voltage drop across a 10 Ω low-side resistor, the blue trace is the output, but AC coupled, i.e. showing only the ripple voltage.

The red line is the math function to integrate total power use in Coulombs – more on that later.

If you look at the top overview graphs, you can see the current blips which are over 160 ms apart, i.e. this thing is activating only 6 times a second. And each current pulse lasts only about 150 µs. The peak current on the input side, i.e. drawn from the battery, is about 12.5 mA.

Let’s put a 1 kΩ load on the 3.3V output line:

I’ve adjusted the scale a bit. The switcher is now operating at about 11 KHz. The peak current drawn is almost 18 mA, but note also that the current never drops to 0 anymore – the baseline of that yellow trace is 2 divisions down from the top, so that’s about 6 mA (as expected, since the load is always drawing current).

Now let’s push it and change the output load to 100 Ω, i.e. about 33 mA @ 3.3V. To make that work, I had to change the input sense resistor to 1 Ω:

The baseline for the yellow trace is now halfway, same as for the blue line.

I also added a third probe, the green line monitors output voltage, which is indeed steady at 3.3V (both red and green lines are based at the bottom of the screen). Note the huge peak current draw on the battery: over 290 mA!

Let’s try to understand what’s going on in this last case. First of all, with a 1 Ω sense resistor, a 190..290 mA current draw creates a voltage drop of around 0.25V – which means the battery voltage isn’t really reaching the switching regulator. The battery was measured to be at 1.37V, so the switcher is getting only about 1.1V on average. The datasheet says that it will only be 70..75% efficient on such an input voltage, when generating the 3.3V output.

The 100 Ω output load draws about 33 mA. That’s at 3.3V, so a perfect step-up regulator would need to draw 3x as much when running off 1.1V, i.e. 100 mA. A 70% efficient switcher would draw about 150 mA (0.70 x 150 ≈ 100). What I’m seeing here is more like a 40% efficient switching result (0.40 x 250 = 100) – hm, not sure why…

The other way to determine average current consumption, is to do some Coulomb counting…

In the first screenshot, each blip uses about 900 nanocoulombs (the red line rises 4.5 divisions over the entire width of of the screen). With 6 blips per second, we use 5.4 µC each second, i.e. 5.4 µA average current draw (not quite the 1.6 µA I expected, but still very impressive for an unloaded step-up regulator).

The second graph is trickier. We need to figure out the Coulombs increase per repetitive cycle. My guess would be around 820 nC. Multiply by the switching frequency of 11.25 KHz, and you get 9.2 mC per second, i.e. 9.2 mA average battery current to deliver about 3.3 mA @ 3.3V.

Gotta be a bit careful here. It turns out that the battery (which is a bit old), still delivers 1.44V at this lower power level. Also, since I’m using a 10 Ω current sense resistor in this case, there’s 92 mV wasted in that resistor. That leaves about 1.35V going in. A perfect switcher would draw 3.3V / 1.35V * 3.3mA = 8.07 mA. We’re pulling 9.2 mA, which is about 87% efficiency. That seems a bit optimistic, since the AS1323 doesn’t really go much further than 80%. Oh well, there are probably several measurement errors in my quick and dirty test setup.

For the third case with the 100 Ω load, I end up with a figure of 215 mC/s, i.e. an average current draw of 215 mA. Better than before but still under 50% efficiency.

One more datapoint: with a 100 kΩ load, the switching frequency goes to 120 Hz, while still using about 800 nC per cycle, i.e. ≈ 100 µC per second, or 100 µA. Again, pretty good for what is essentially a 33 µA load @ 3.3V – even if all these estimates are off by perhaps 25%.

So this chip might work quite well for bursty ultra-low power contexts such as a mostly-sleeping JeeNode!

Now that my analysis capabilities have improved, it thought it’d be interesting to see the power consumption profile of a Room Node, running the roomNode.ino sketch. Here’s an annotated capture of a motion event:

The yellow line is the current consumption of the entire room node. I’ve added some annotations, although there are still a few things I’m not so sure about, such as the 2.5 ms delay between SHT readout and XMIT start.

The red line is the integral of the yellow line, i.e. the total amount of energy consumed as time progresses. It took about 3 ms between the end of the packet transmission and the first ACK packet header byte coming in – this thumb-twitching with the receiver enabled accounts for about 1/3rd of the power consumption!

There is almost always room for improvement with this sort of stuff. The closer you look, you more you find things to optimize. For example, I noticed that there were blips every 32 mS or so (it’s slightly irregular):

This is zoomed in on both axes. The noise level is a bit higher, perhaps due to the 2 mV/div setting of the scope.

That’s over 1 ms @ 0.8 mA, at about 30 times per second on average. I don’t know what it is – it’s not the PIR sensor, which was removed during this measurement. Not a lot of energy per blip, but it does add up: ≈ each uses 1.1 µC, i.e. some 2,000 µC per minute. Whereas the first screen shows that a transmission takes only 130 µC.

So there you go: vampire power at microwatt levels!

Update – Ah, wait – of course! – it’s the Scheduler class in the Ports library! When idling, it lets time pass in steps of 0.1s (or rather 96 ms), and there’s no other way to do this than with 32 and 64 ms watchdog timer steps. I told you – there are no show-stoppers in this game, it works on logic and insight, all the way down!

The Low-power Supply described previously on this weblog now has a PCB – and it’s about as small as a JNµ:

Here’s an assembled unit, ready for testing and hooked up to power a JeeNode (w/ disabled bootstrap):

And here’s the whole test setup I had to create to check this thing out:

Lots of stuff involved, including high-voltage probe and 230V isolation transformer (to the right, out of view).

Here’s a demonstration of how it works – summarized as one elaborate scope capture:

Green is the mains voltage (235 Vrms), purple is the charge building up on the 100 µF reservoir capacitor, yellow is the regulated output, and blue is the JeeNode’s current consumption (measured as voltage drop over 10 Ω). Note how some of the voltages measured here differ more by than four orders of magnitude!

Anyway, that zoomed-in image is the clear signature of the second 8-byte RFM12B packet transmission. Current consumption varies from 23 to 26 mA. It’s a relatively coarse image, since it has been zoomed in 4,000 times.

The 3.3V supply level is reached ≈ 2s after power-up, with another 2.5s needed to fully charge the reservoir cap. You can see from the purple dips that this supply could sustain at least one packet transmit per second.

No surprises here, but it’s good to see that the PCB design works as intended. Next step: implement deep sleep on the ATtiny84 – hopefully this’ll take just some minor adjustments to the Sleepy::loseSomeTime() code.

Now that everything is working, I want to have a ready-to use printed circuit board for it. Came up with this:

It’s tiny – about 48 x 12 mm – even though it’s based entirely on through-hole parts. The idea is to build it up, add wires, and then encapsulate the whole thing in heat-shrink tubing to reduce the number of contact points.

It can not be repeated enough: when tied to AC mains, the ENTIRE circuit carries AC mains voltage levels!

I have a couple of configurations in mind (see yesterday’s post for the schematic):

• with C1 a 10 nF X2 cap and C4 replaced by a wire, this delivers an average of 0.3 mA on 230 VAC mains
• with C1 a 22 nF X2 cap and C4 replaced by a wire, this delivers an average of 0.3 mA on 115 VAC mains
• with both C1 and C4 22 nF caps, this supplies 0.3 mA on 230 VAC with no direct connection to AC mains
• with C1 and C4 replaced by a wire, this supply can be used with 10..24V DC in – which is great for testing
• with C1 and C4 each replaced by a 220 kΩ resistor, and R1 by a 1N4007 diode, this becomes a somewhat less efficient (but lower-profile) resistive version, again delivering up to 0.3 mA at 230 VAC

With C4 replaced by a wire, this circuit will have its “GND” output tied directly to the “N” input. This is important when powering the AC current monitor, which needs to have one side of its shunt at 0V relative to AC mains.

Note that no matter what, even with C1 and C4 both included, faults can develop in this circuit which cause the “low voltage” output of the circuit to end up directly tied to an AC mains “live” line. This is not, and will never be, a “safe” circuit. It can only be used safely while enclosed in a plastic box, with no contacts or parts sticking out.

In all cases, the on-board regulator will be activated once the input voltage rises to about 6V. This is the key to being able to power up a JeeNode or JeeNode Micro with their on-board RFM12B module.

It’s been a challenge to get all the bits of the AC current monitor ready, but the last hurdle has now been taken.

In a nutshell: the AC current monitor is a small unit based on a JeeNode Micro, which periodically broadcasts information about the current consumption of an attached appliance. It’s hooked up to 230V, so it’d be a bit silly to run it off batteries. It would be equally silly if it were to draw lots of power, and since it has to be permanently on, I wanted to get its power consumption really, really low – under 0.1 Watt. That goal has now been reached.

According to this calculator, the following setup draws only 12 mW @ 230V and will supply 0.3 mA @ 3.3V:

Here is the schematic:

It’s a transformer-less capacitive power supply, combined with an LT1121 low-power 3.3V linear regulator. C4 can be omitted. This regulator has a shutdown pin, which is tied to the input voltage via a voltage divider. As a result, the output of this supply switches on only once the 100 µF reservoir capacitor has charged up to 6V (it’ll continue charging to 12V, at which point the zener diode kicks in). Here is the power-up behavior w/o D2, and no load:

The blue line is the voltage over the reservoir cap, and the yellow line is the regulated output. If you look very closely, you can even see the 50 Hz cycles “pumping up” the reservoir once every 20 milliseconds.

By itself, this isn’t good enough yet to drive my test JeeNode (no bootstrap, brief wakeup activity every 10s):

Very odd behavior, as the regulator and the RFM12B start pulling more current than is coming in, preventing the output from ever reaching more than 1.8 V (I used a 470 µF reservoir cap in this test).

The final trick was to add a diode from the regulator output to the shutdown pin. This positive feedback causes the regulator to very quickly snap out of shutdown mode. So once the reservoir cap reaches about 6V, the regulator switches on, at which point the shutdown pin is quickly pulled high to finish the job and keep it on:

This is running from 150 VAC using the new AC power box. At lower voltages, the trickle current becomes too weak to reliably turn on. At 230V, on the other hand, the whole startup process is even quicker and very robust.

I have not yet been able to measure the power draw of this supply. Due to its design it will always draw the same amount (predicted to be 12 mW), regardless of load. The feed capacitor (C1) is a 10 nF X2 type.

Here’s the final proof – a JeeNode, powering up in a few seconds and sending out a test packet every 10 seconds:

Many thanks to martynj – his weblog comments and great suggestions by email made this result possible.

Good – now I can sprinkle dozens of these around the house and still use no more than one Watt extra!

I’m going to switch to a different type of graphic LCD once the next batch of Graphic Boards arrives. Instead of a white-on-black type, the new GLCD will be a blue-on-sort-of-blueish color:

The reason for this is that it is still quite readable without the backlight:

There is a solder jumper on the Graphics Board, to choose between powering the backlight continuously via PWR, or having it powered under software control, via the not-often-used IRQ pin. This not only lets you turn the backlight on and off, but even dim it under software control, because the IRQ pin supports PWM, i.e. analogWrite().

The above display draws about 280 µA without backlight right now, which means it will keep running for over a month on a single AA cell, using the AA Power Board. There are probably ways to reduce that current consumption further, as I’m not doing anything yet with the ST7565′s standby and other low-power modes.

And although it may not be practical for a permament display, this definitely is useful for something I’d like to have around and use on a regular basis. By sending it stuff to display over wireless, this setup can support what is quickly becoming my favorite mode of operation here at Jee Labs: no wires cluttering my desk!!!

The new AA Power Board is working very nicely, but it turns out that the ferrite-core inductor is a little fragile.

This is the 10 micro-Henry inductor core, as it should be:

… and this is what happened to one of the shipments “all the way” to the UK:

(photo courtesy Steve Evans – great close-up!)

Whoops, not good!

This happened to two of the five boards shipped as part of a “5-pack”, where I placed all the boards together in a single plastic bag, along with the metal AA clips. The fact that the inductor is placed near the outside edge also probably doesn’t help…

I’ll package the boards differently from now on. But if you got these boards, please check them and let me know if they got damaged (they may still work, depends a bit on the breakage) – I’ll send out replacements for any bad boards out there, of course.

Live and learn!

C’mon, admit it… you’ve got a pile of discarded AA’s somewhere in your drawers as well:

With all the JeeNodes and room nodes I’ve been trying out around here – and the modest results with the rooms.pde sketch w.r.t. battery life so far, I’ve gone through all these much faster than I would have liked to:

Went through over 60 here at Jee Labs, in the past year or so. So much for the environment!

Enough is enough. I’m switching to the Apple charger with the Eneloop NiMh’s. And with the new AA Power board, it looks like a single AA cell per node might be enough.

But wait! Are all those AA cells really empty? Time to find out!

Since the AA Power board is so efficient, I though it’d be interesting to see how many of those “dead” AA cells are truly empty. Note that the AA Power board can pull juice out of a battery and generate 3.3V even when it’s supplying less than half its original voltage:

So let the battle begin: which cells really can no longer drive a JeeNode as wireless test node?

The result surprised me quite a bit. These 10 were completely dead:

But the rest – which is ALL the batteries shown in the first picture – still worked!

This doesn’t mean that any of these batteries will last very long. But still – they drive the JeeNode and its on-board RFM12B transmitter well enough to send out a fresh packet once a second. Which means that even with an output voltage less than 1.1V, they are still able to a briefly deliver a 80 mA peak current once a second (i.e. 3x the current required @ 3.3V) !

Hmm, now what … charge an Eneloop with all that residual energy, perhaps? :)

Today might be a good day to set aside your dislike of Apple for a moment…

There is a new battery pack + charger option on the market:

In Europe, this combination costs €29.95, and it gets you 6 rechargable batteries plus a charger.

The specs are what make this thing particularly interesting for low-power battery-powered devices:

• each NiMH AA cell holds 1900 mAh charge (rumored to be rebranded Sanyo Eneloops)
• the batteries are reported to retain at least 70% of their charge for 2 years
• battery lifetime is reported as being at least 10 years (with recharges, of course)
• the charger drops to 30 mW “vampire” power draw once the batteries are charged

To start with the latter: I’ve verified it. When charging, there’s a small yellow LED and it draws less than 4 watts. When done, the LED turns green, and power consumption drops to 0.18W. Then, after 6 hours, the LED goes off and the power consuption drops to… zero! At least on the very sensitive SBC-500 I used.

This is a very big deal!

It means you can permanently leave these chargers plugged in, with a few spare AA’s. No cost, no waste – simply a way to keep a couple of cells fully charged and ready to go.

It’s also perfect for JeeNodes, using the standard 3x AA battery holder:

That’s 4.1V when fully charged. And JeeNodes will work with them all the way down to 3.3V, i.e. 1.1V per cell. Also, unlike LiPo batteries, running NiMH batteries down won’t do them any harm.

At last, I can stop wasting alkaline batteries… I’m probably going to replace all of them in the months ahead!