The P1 port connection I set up recently for picking up data from the utility company’s smart meter isn’t working reliably yet, so I dove a bit deeper into it.

Here’s how I now think the P1 interface is implemented inside that smart meter:

There’s no way to ascertain whether a PNP or NPN transistor is used, without opening the box (impossible without breaking a seal, so definitely not an option!). But given that NPN is more common, the above circuit is my best guess.

The resistance was measured between the DATA and GND pin. The resistance between DATA and REQUEST is usually over 1 MΩ, which indicates that the phototransistor is open. Makes sense: pull the data low regardless of the REQUEST line state, and pull it high when the internal IR LED in that optocoupler is lit. That also explains the awkward inverted TTL logic levels provided by this interface.

What you can see is that the REQUEST line is really nothing but the power supply to the isolated side of the optocoupler. But the surprise is the value of that pulldown resistor!

Let’s do the math: when REQUEST is 5V, and the phototransistor is conducting, it’ll have about a 0.2V collector-to-emitter voltage drop, leaving 4.8V to feed the 180 Ω resistor (I measured 4.75 V, so yeah, close enough).

Whoa… 4.8 V over 180 Ω is 26 mA, a pretty hefty current draw in µC terms!

My conclusions from all this are: there’s no external pull-up or pull-down needed after all, and my hunch is that it’ll work just as fine with REQUEST powered by 3.3V (reducing the current somewhat). All you have to make sure when working with this P1 interface, is that your REQUEST pin can supply those 25-odd milliamps.

If the above is accurate, then nothing forces us to actually use that resistor, by the way. We could simply connect the REQUEST and DATA pins and leave GND dangling. In fact, by re-using the phototransistor in a different way, one could even make it work in active-high mode again (only if no other P1 devices are connected, evidently).

Note that this is for the Landis & Gyr E350 (PDF) smart meter I have – others may differ!

Here’s a nice little power supply:

It’s one of many LED drivers currently available on the market.

These supplies are designed to provide a fixed current – not voltage. What this means is that if you “overload” them, then they will regulate the voltage down to the point where the current reaches a preset value (700 mA in this case).

In itself, this isn’t really useful for powering anything but power LEDs, but this unit also specifies that as long as the load remains under 650 mA, it will supply a fixed voltage of about 6V. So the other way to interpret these specs, is to see this as a 6V power supply with automatic current limiting to 700 mA.

This means that if we add the optional diode to a JeeNode v5 or v6, then the voltage would be just right to power one or more 5V relays.

The nice thing about these units is that they are fully “potted”, i.e. encased with proper high-voltage isolation.

This might offer a practical way to set up mains-powered JeeNodes, with perhaps a Relay Plug to switch simple AC loads, or a triac for switching / dimming incandescent lights. Another option would be to use a MOSFET to switch 1..3W power LEDs.

PS. Unfortunately, I don’t remember where I got this particular unit (yeah, I know – I feel pretty stupid!)

Electronics design is trivial compared to mechanical design, I tell you!

I’ve been looking at creating a Touch Screen option for the Graphics Board. In theory, it’s simple – just combine these two parts:

But how?

What I’d like to end up with of course, is that the touch screen is firmly held in place in front of the GLCD display:

The width of this touch panel is perfect. The touch surface is slightly higher than the 128×64 GLCD, but that could actually be used to create a couple of extra “fixed” buttons, perhaps even with some LEDs – right above the display itself (or below, if we turn the whole thing around).

It’s not hard to create a one-off mounting setup: just hot-glue the thing together, with plastic, wood, or foam-board. But that’s hardly a solution for others to replicate. And this really is one of those cases where the end result needs to look sharp, IMO. I’d really like to use this as a small self-contained unit for display and control of a few variables as part of a home-automation setup. So it better be robust. And battery powered, if possible.

The headache is the mechanical part… suggestions would be most welcome!

The power-line isolation saga continues…

Ok, so the idea of adding an extra RCD device which trips on differences in current turns out to be a mistake. It adds no extra protection when used in the secondary winding of such a setup. I will leave it out.

Next step was to hook up the circuit as described in the above post. Soldered evertyhing in place, used heat-shrink tubing to isolate the loose ends, lots of hot-glue to fix things, and then:

POP, said the 5A “slow” fuse – and the next one too!

That’s not just a molten piece of metal, BTW, that’s a vaporized coating on the left. I suspect that the current was substantially over 5 amps. But no ill effect on the rest of the house, no lights dimming, and no other blown fuses (the main ones downstairs are all 16A).

Uh oh, something is wrong. Well, at least I now know the fuse works!

Here’s the plan again:

Ah, wait, not quite! … I have a transformer with dual secondary windings:

What I did was wire them up in parallel:

And that’s where I went wrong. Each winding generates an AC voltage. There are two ways to put them in parallel. Connect them right, and you get twice the power – connect them in reverse, and one will generate an AC voltage which is exactly opposite in phase w.r.t. the other winding. The result is not just a short circuit, but two power sources actively counter-acting each other:

Hence the blown fuses. I did not expect a 300 VA transformer to blow a 5A fuse, which implies its power draw was over 1100 watt – but that’s exactly what happened.

The solution is simple, once you step back and think about it. Instead of connecting the windings in parallel, I need to connect them in series:

(and the same on the other up-converting transformer too, of course)

Here’s the key: it’s still possible too hook them up the wrong way. But doing so will simply lead to no ouput, i.e. zero volts, instead of the desired double voltage output. But no blown fuse.

Once I made this change (which required a lot of undoing), I found out that the transformer secundaries were indeed wound the other way than their wires had initially led me to believe.

Problem solved!

I now have a working isolation setup which turns a 230.3V input voltage into a 230.9V output voltage under no load. An 80 watt test load worked fine.

P.S. I also gained more respect for AC “power”: such circuit mistakes are a different ball game from fiddling around with an AA Power board and getting all sorts of microcontroller- and sensor hookups messed up!

Remember the weblog post about mains voltage and my plan to work on some circuits involving high voltage?

Well, with fear and trepidation I can announce that the missing link has finally arrived:

That’s an “Aardlekschakelaar” in Dutch, i.e. a Residual-current device.

People are probably either very thankful or very afraid of this thing, judging by the number of acronyms under which it is known: RCD, RCCD, RCCB, GFCI, GFI, or ALCI – take your pick!

An RCD continuously compares the current in with the current out, and breaks the circuit once the difference exceeds a certain threshold. I decided to get the most sensitive one I could find, which trips at 10 mA. I’m terrified of electric shock – especially the non-lethal kind… (no idea about the other variant, and I intend to keep it that way).

Ok, so it took over a month from ordering the RCD to getting it delivered – from a mail-order company here in the Netherlands, go figure. I’ll build the RCD into my super-duper isolation jig as soon as I find some time. I sure don’t want to fiddle with 230V while distracted or in a hurry.

Speaking of which…

There are a large number of people waiting for orders from the JeeLabs shop right now, for which I offer my sincere apologies. The reason is not really lack of time (well, maybe a little…), but the fact that stocks have run out on several items. I’ve got just about everything that’s missing on order right now, but in some cases my hands are tied because things don’t always arrive as quickly as promised or expected.

I’ve been refraining from sending out partial shipments, because it doesn’t really make much sense most of the time (sending out a JeeNode without the matching JeeLink, for example), and because it adds substantially to my overhead. Which is of course my own concern, but it also raises the chance of mistakes, something which I really want to avoid.

Not everything is in yet, but I hope to have just about all shipments ready by this weekend. Please get in touch if you’d rather have a partial shipment sent out soon, rather than waiting until next week. There will be no extra charges for such split shipments.

Crazy times around here. And the long-overdue server migration isn’t making it any easier!

P.S. Don’t get me wrong. I’m still working on quite a bit of fun stuff on the side, so let me finish this slightly panicky post with a little smiley anyway :)

There’s no escape… I have to get into mains voltage stuff and try some things out. All 220 volts of ’em!

I want to experiment with power measurement, power control, dimming, an such – but, ehm… preferably in a way which will let me live to tell the story! (how’s that for dedication to this weblog, eh?)

Now the thing about electricity is that it’s not the voltage that kills you, it’s the current – after all, if this weren’t the case, all the birds on high-voltage wires would be dropping dead out of the sky…

Current means: electricity going from A to B. And the problem with us human beings is that we are conductors – which isn’t too surprising, given that we’re mostly made of water anyway (the rest is hot air, I’m told).

So the first thing to do is to isolate myself from AC mains, by not allowing a ground path to carry any current. This can be accomplished with an “isolation transformer”, i.e. a transformer from 220V to 220V which – like any normal transformer – creates a galvanic isolation barrier:

Trouble is, these transformers are relatively expensive. One reason for this could be that you want to get a fair amount of power across, so these are hefty units with a lot of copper, iron, etc. in them!

Fortunately, there’s a simpler way to accomplish exactly the same using two back-to-back normal transformers:

The first one is a normal step-down transformer, producing 10.5V – and then we simply take a second one of the same type, and connect it the “wrong” way around.

There will be losses, so the output voltage won’t be exactly 220V, but I’m fine with that. In fact, it’s a nice test to see what a mains circuit will do with a somewhat lower voltage.

I’m going to use a pair of beefy 300W transformers I have lying around here, which will let me test some control and measurement options up to at least 250 Watts or so. Here are the main ingredients, which I’ll put in a wooden box with slots for natural convection. This dual wine bottle unit looks like a good fit:

(I told you they were hefty tranformers – each of them weighs several kilo’s!)

What this gives me, is an isolated source of mains-like AC voltage. It is still just as lethal when touching both power lines. The only safety this adds, is that touching a single high-voltage output wire is now harmless.

The other benefit is that an accidental short will limit the amount of current that can flow through this setup, thus also limiting heat and damage somewhat. But 300W of power is still a huge amount, so I have no illusions that things won’t break. It’s mostly to prevent fires and burnt-out cabling.

Evidently, this thing needs a switch with indicator light, and fuses: a 5 Amp fuse on the input, and a 4 Amp resettable fuse on the output (lowest I could find):

Scary stuff. I could set up a pair of much lower-amperage transformers for loads up to 25 Watt or so, but it’d still be potentially lethal, so that would only lead to a false sense of security.

The low-voltage / high-amp intermediate circuit does provide an option to insert a fast-acting fuse. A 100 mA fuse @ 10.5 VAC would translate to 5 mA @ 220 VAC, which is no longer lethal, but I’m not willing to “prove” that! Trouble is that such a setup would blow its fuse with anything over 1 Watt of power, even if only a power-up spike.

Here’s the whole contraption (not wired up yet) – with thanks to Ikea for the “front panel”:

I’ll finish this isolation setup for some experiments, but it won’t alleviate much of my fear. Unfortunately, there’s simply no other way to get going…

There are lots of projects and schematics floating around the web for encoding and decoding IR remote-control pulses. Many MPU-based ones use one of the built-in hardware timers for this, which is understandable since you can’t easily bit-bang your way out of a 13.158 µs time between switching flanks.

But I don’t want to be constrained to a specific pin, nor dedicate a hardware timer to this task.

Instead, I’ve been experimenting with the venerable NE555. Its normal operating voltage is 4.5 .. 16V, but there’s a CMOS version which will easily go down to 3.3V, as preferred by JeeNodes.

So here’s the circuit (from the ICM7555 docs by NXP):

The RESET pin can turn this thing on and off. It’ll also need an extra transistor to drive a bright IR LED.

Had to try it out, of course (the chip I used wants at least 4.5V, hence the USB power):

Great. It works, but at 35.1 kHz the frequency is a bit off.

Here’s a Logic Analyzer screen shot (warning: MDI toolbar madness, flash from the past!):

(FWIW: my USB Scope and Logic Analyzer now both work properly on Mac OS X under Parallels 6 emulation – I no longer need a dedicated Windows setup.)

Let’s do the math for the components used in the above test setup, with RA = 1.2 kΩ, RB = 18 kΩ, C = 1 nF:

f = 37.09 kHz  
∂ = 51.6 %

Let’s use the 1 nF capacitor, smaller would make the circuit more sensitive to stray capacitance. Then I get:

RA + 2 RB = 36.315 kΩ

I don’t want to raise RA much, because that would change the duty cycle in the wrong direction: more LED-on time (i.e. higher current draw). I also don’t want to lower it much further, because that would draw more current in the discharge phase (pin 7 pulled low).

Using 1% resistors, RA = 1.5 kΩ and RB = 17.4 kΩ will do:

f = 38.02 kHz  
∂ = 52.1 %

The 1 nF capacitor will have to be 1% too, of course.

I think I’ll try this. What remains – as always! – is the software, to send out the proper IR bit patterns and to decode the ones coming in. It’d be nice if that could be done fully interrupt-driven like the RF12 driver, because then the whole real-time aspect of IR communication would be taken out of the loop(), so to speak.

LED lighting is efficient right? Oh, sure… the LEDs are, but the way they are connected isn’t necessarily!

I’ve always been puzzled by those resistors you see on RGB LED strips:

My impression is that these strips consists of segments which all have (essentially) the following schematic:

Measuring the 60-LED/meter RGB strip I have here, I get the following readings for 0.5 m, i.e. 10 of the above segments:

• RED draws ≈ 180 mA, the voltage over its resistor is ≈ 5.5V
• GREEN draws ≈ 170 mA, the voltage over its resistor is ≈ 2.2V
• BLUE draws ≈ 190 mA, the voltage over its resistor is ≈ 2.4V

Total current draw is roughly 540 mA, or 1.08 A/meter @ 12V. So the total power consumption is 13 watts for each meter of LED strip.

But…

• the red LED’s resistor consumes 0.18 x 5.5 = 0.99 W on each 0.5 m
• the green LED’s resistor consumes 0.17 x 2.2 = 0.37 W on each 0.5 m
• the blue LED’s resistor consumes 0.19 x 2.4 = 0.46 W on each 0.5 m

That’s 3.64 W of the 13 W pumped in per meter – turned into … heat!

Is LED lighting a good idea, in terms of efficiency? Yes, probably. A meter of RGB LEDs at full power draws 13 watts and looks a lot brighter to me than a classical lightbulb, halogen light, or even them new fluorescent “long-life” bulbs that are all the rage (but oh so ugly).

Note that I’m not talking of “power LEDs”, i.e. the 1W, 3W, or more LEDs that are used for very bright lights and which need te be mounted on a cooling fin (Infineon, etc). These are usually driven by a (more complex) constant-current source, and not a plain 12V supply. The reason for this is that at those power levels, you couldn’t possibly adjust the current draw via resistors – these would become scorching hot and cause lots of problems of their own!

The are a couple more figures to be gleaned from the above information, btw, but you can also measure these values directly on the RGB strip:

• the red LED resistors are 300 Ω each, each LED gets 2.2V
• the green LED resistors are 120 Ω each, each LED gets 3.3V
• the blue LED resistors are 120 Ω each, each LED gets 3.2V

That voltage is the real reason for all these resistors. If you were to feed exactly 6.6V to the 3 red LEDs (9.8V for green, 9.6V for blue), then you wouldn’t need any resistors to pick up the slack. But that’s not feasible in a practical / cheap way, so instead we drive these strips with 12V and have the resistors eat away 28% of excess energy in the form of generated heat. Note: it would be more accurate to say: “if you were to feed exactly 180 mA, etc” … because the voltage is a “side effect” for each type of LED, the current is what determines their brightness.

Another source of inefficieny is the 12V power supply – let’s assume it’s about 90% efficient. Then 72 watts of power going into the LEDs will draw a total of 111 watts from the power line.

The good news is that these ratios won’t change when the LEDs are dimmed via PWM. A 50% pulse reduces the total amount of energy used by the same percentage (again: 28% of that is gobbled up by resistors).

Sooo… if you’ve got 8 meters of RGB strip going at full power, then that’s about 100 watts – 28 of which are turned into heat before they even reach the LEDs. And the power supply used another 11 to do its job.

I still think LED strips are a good idea – if I can get the white balance right and if their color is very even.

Whoa, great news – the Mac Mini is getting even more energy efficient:

It was announced today. Here’s what the new box looks like (and no more power brick!):

We’ve been using the Mac Mini as our TV + audio system for several years now. It’s connected to a master/slave power switch to switch off the other components when in sleep mode (sat tuner and amplifier). Works great, although I had to replace the hard disk recently. The new one would be a great successor… one day.

Might also be an excellent option for a home server, although a SheevaPlug or GuruPlug uses even less power in idle mode (approx. 4 vs 10 Watt). But that’s more of a software decision really: Mac OS X vs. Linux.

Looks like the FS20 remote-controlled system from Conrad and ELV has gained a few new units. Here’s a 60-euro 2-channel power-strip, controlled from 868 MHz, hence also from a JeeNode:

Also a 42-euro repeater to get through multiple layers of reinforced concrete.

Great options to avoid having to deal with line voltages. Not ultra-cheap, but still.

The one drawback worth noting with FS20 is that’s it’s not replay-safe. If someone picks up these signals, they can re-send them at a later time, even from outside the house. This means there is essentially no security at all with this approach. But for things like lighting and office equipment, that’s probably not an issue.