The “EPAD” MOSFET circuit described in an earlier post is nice, but as Ronald recently suggested in a comment, LEDs also have a nice high forward drop – so why not take advantage of that instead?

Much simpler and cheaper!

LEDs act a little bit like zeners, but they too have a somewhat round “knee” ar very low current levels. I’ve done some experiments, and have come up with a blue LED in series with a red one as suitable voltage reference:

As you can see, even with 1 mA of current, they are clearly visible (especially the red one, which is a low-current type). So this also makes an excellent power-on indicator – at no extra charge – if you pardon the pun. There’s also a 1N4148 diode to a 470 µF buffer capacitor. Here are the voltages this thing seems to stabilize on:

• 3.69 V @ 2 µA
• 3.80 V @ 5 µA
• 3.86 V @ 9 µA
• 3.97 V @ 25 µA
• 4.09 V @ 98 µA
• 4.13 V @ 150 µA
• 4.18 V @ 251 µA
• 4.22 V @ 399 µA

Taking the extra diode drop into account, this leads to a very acceptable 3.04 .. 3.57 V supply voltage for a JeeNode.

So for a capacitive AC mains supply, this could be doable with 7 components:

• a 10 nF X2 capacitor as reactive component
• a 4.7 kΩ fusible resistor to limit the inrush current
• a blue LED plus a red LED to create the necessary voltage drop
• a 1N4148 across the LED for the reverse current
• a 1N4148 from LEDs to feed the capacitor
• a 470 µF 6.3V electrolytic capacitor for energy storage

This circuit is dangerous when directly connected to AC mains, but if a direct reference to one of the input pins is not needed, then it can in fact be made a bit safer: replace the 10 nF cap by a 22 nF unit, and add a second 22 nF cap on the other power line input (plus a second 4.7 kΩ fusible resistor for extra security). Touching the “low-voltage” side limits current to 1 mA – this should cause at most a slightly tingling sensation when touched.

I don’t know about temperature sensitivity, but in a case like this where voltage stability is not so important, this circuit might in fact be the simplest way to build a 0.1 .. 0.4 mA ultra low power supply!

Update – as pointed out by Martyn in the comments below, this circuit is not safe in case of a fault. It’s still transformerless, so it has to rely on caps to do its work – both of which can fail by shorting out. Fusible resistors are not a good enough security in terms of safety, because they don’t blow at current levels below 1 mA – they only protect the circuit from large currents in case something goes wrong.

As always: be careful with 115V and 230V AC mains!

Yesterday’s post described a new test setup to be able to safely experiment with low- to medium-range AC voltages. As a first check when working on new AC mains powered circuits, and to allow me to poke around.

The main idea is to use a few small transformers to create relatively weak AC power sources with a range of different voltage outputs. Here’s one way to connect their six secondary windings together:

The key safety feature is the connection to ground. Anchoring the middle of the output signal to ground effectively halves the voltage when you touch one of the outputs. It also (sort of) centers the sine wave around earth ground.

Note that touching both ends of the 6 series-connected secondaries will still cause a scary 148 VAC jolt.

To make this more convenient to test with, a double-pole 11-position rotary switch will be used (the best I could find for a reasonable price). This will allow me to switch between each of the above voltages, as well as 0V.

Every position up to 92V will have at most 56V to ground, the highest two positions will make that 74V and 84V, respectively (through secondary coils designed to supply at most 20 mA). I’ll consider up to 46V to be safe.

Here are the parts I got from DigiKey – and how I would like to arrange it all:

Next step is to come up with a nice box for it – the work never ends!

(No, not comic strips or movies…)

With 230V experiments becoming more commonplace here at JeeLabs, I’m worrying about safety again. It doesn’t take much to get sloppy once you do things over and over. But sloppiness and 230V don’t mix well!

Given the recent trials, and some great comments, I’d like to get back to a safer voltage level for day-to-day testing. Using the isolation transformer only for incidental cases because no matter what, 230V is tricky.

According to Wikipedia, a “safe” voltage is 50 VAC or less. According to another page, it’s even lower: 30 VAC.

Until now I’ve been using my lab supply, which goes up to 30 VDC. But that’s limited, and more importantly it doesn’t let me see the effects of AC rectification and ripple. It’s time to come up with a better test setup:

A couple of small well-insulated PCB transformers, one rated 2x 18V @ 45 ma and two with 2x 28V @ 20 mA secondaries, respectively. The 18V and 28V secondaries are completely safe (by themselves) – they might damage a circuit, but they won’t harm me.

When combined, these can produce from 18 to 148 VAC. Note that 148 VAC is still over 400 Volts peak to peak, so this is serious stuff. The best way to keep the risks down is still to use low voltages as much as possible, of course.

One reason why these voltages are less risky than AC mains, is that the output current is very limited. I picked the weakest transformers I could find, meaning that their secondary coils have a high internal resistance and small magnetic cores. That and magnetic saturation should keep maximum currents limited to 20..30 mA.

Tomorrow I’ll describe a convenient hookup for all these little power sources.

In yesterday’s post, I reported how the 4.3V zener diode wasn’t quite cutting it – it wasted too much power.

But the new setup described in that post (12V zener w/ 100 µF cap from a 0.4 mA input trickle) works great:

Note how the supply voltage recovers from a packet send within a mere 0.2 seconds. What I’m hoping now, is that with a capacitive supply on 230V, the average power supply consumption will end up well under 0.1 W.

This experiment was done with almost the same sketch as before – running on a standard JeeNode for now:

Every 10 seconds, a dummy packet gets sent out – as this receiving node shows:

    OK 4 0 0 0 0 0 0 0 0
    OK 4 0 0 0 0 0 0 0 0
    OK 4 0 0 0 0 0 0 0 0

And it keeps on going as long as the 0.4 mA trickle is on. It turns out that the energy consumption is so low that it will keep running for quite a while on a single 100 µF charge, before crashing into some indeterminate state:

Note the time scale: 10 s/div – enough to send out six valid packets!

There’s still some nastiness to get startup reliable. I’ll probably need to mess with the BOD fuse setting and perhaps also power up with the 8x prescaler enabled, to be absolutely sure that the JeeNode doesn’t start drawing too much current before there is sufficient energy in the 100 µF capacitor.

For kicks, I tried pushing things to the limit: with a 10 µF cap, plus the 10 µF on board the JeeNode, the sketch still works, but each packet transmissions now drops the supply to a dangerously low level of about 3.7V:

Eventually, sailing this close to the edge causes the node to fall into some sort of zombie state. But as you can see, it now takes only 50 mS to recharge this cap @ 0.4 mA. So could that trickle feed be reduced to – gulp – 0.04 mA? This is getting silly, but first I must get that power-up logic fixed – right now, the JeeNode isn’t starting reliably.

Anyway. It’s mind-boggling how little energy is needed to get a little bit of info across the house!

PS. I’ll stop posting oscilloscope snapshots for a while. There’s more to this weblog than scopes and signals…

If you recall the 6800 µf supply test, I used the following setup:

The problem was that this uses a 10 mA supply current as test, which is 10x higher than I want for the final 220V capacitive transformer-less version.

Trouble is, increasing the resistor to 22 kΩ didn’t work, there wasn’t enough juice to keep the JNµ running.

In a comment, Koen remarked that it might be due to the zener soft “knee”. Zeners are not perfect, they tend to cut off the voltages at a specified point, but it really depends on how much current flows. With too little current, the zener-like properties are in fact far less pronounced. Here’s what I measured, roughly:

• 2.7 V @ 0.25 mA
• 2.9 V @ 0.5 mA
• 3.2 V @ 1 mA
• 3.8 V @ 5 mA
• 4.0 V @ 10 mA

That’s a lot of variation. In the case of the ultra-low power supply where very little current flows once the capacitor has been charged, it looks like a 1 mA “trickle” feed is not getting the energy to the right spot. Aha, found it:

Those zener diodes under 6V are pretty leaky! Unfortunately, I can’t seem to find any with better specs.

Time to try something else. How about the forward voltage drop of a regular diode? After all, that’s supposed to be somewhere in the range of 0.6 .. 1.0 V.

Here’s what I got with 5x the 1N4004 diode in series, and connected in conducting mode:

• 2.6 V @ 0.25 mA
• 2.75 V @ 0.5 mA
• 2.9 V @ 1 mA
• 3.1 V @ 2 mA
• 3.3 V @ 5 mA
• 3.45 V @ 10 mA

Hm, looks like that’s already a lot better! Next test is to replace the zener in the above circuit with 7x an 1N4004. With a bit of luck, that might provide a voltage in the proper range for the unregulated JNµ. Here’s what I get:

Not bad! The 470 µF 6.3V cap charges up to 3.8 V in about 2 seconds.

Even better is that with a 4.7 kΩ simulated 0.65 mA load, the voltage drops a bit but still stabilizes at ≈ 3.15 V.

There is a major drawback, though: the forward drop over a diode tends to be very temperature-dependent :(

Here’s an idea for a different approach: add a regulator and let the capacitor charge up to say 12V. That would give a lot more energy to draw from, even if a sizeable portion of those milliwatts get “wasted” as heat. Also, it turns out that the 12V zener I used (1N5242) has a considerably better behavior – less than 10 µA when I drop 0.1 V under the zener voltage (11.8V in the unit I tested), whereas the 4.3V zener eats up most of a 1 mA trickle feed.

And lastly, there’s the idea of tracking the supply voltage with the ATmega/ATtiny itself, to let it decide when to delay a power-hungry transmission. After all, the micro-controller is able to turn itself from a 10 µA to a 30 mA power consumer, just by changing its own mode and the RFM12B from mostly power-down to full-power.

Theoretically, an MPU could in fact regulate the power supply voltage by “modulating” its current consumption!

Ok, so the new target I’d like to aim for as ultra-low power source will be: 0.01 µF X2 cap (a mere 0.4 mA trickle), 12V zener, MCP1703 regulator, and an even smaller 100 µF 16V cap, since there’s a lot more voltage drop available if you start from a full charge @ 12V. Will it work? Well, there are some surprises ahead – stay tuned…

One of the things I ordered recently was a High-voltage Differential Probe. The reason for this is that I want to be able to view signals which are tied directly to 230V AC mains, both isolated and directly connected.

That’s – d a n g e r o u s – stuff, unfortunately.

And I intend to do this absolutely safely. My own safety first, of course. But also the safety of all the circuits I’m messing about with. One important trick is to simplify: the shorter the checklist of things I need to deal with, the more concentrated I can stay on the task and its risks.

With a multimeter, measuring 230V stuff goes as follows: disconnect all power, attach the multimeter, put it in the right mode, check that nothing touches anything it shouldn’t (and that all connections are solid!), stand back, turn on the power, read out the measurement, and power down again.

That’s already more than enough to worry about. Now imagine hooking up an oscilloscope and adjusting its knobs to get a good readout. Too many risks, potential for mistakes, and wires going all over the place!

I use the isolation transformer as much as possible. The voltages are still just as high between the two mains pins, but at least an accidental single connection to me or anything else isn’t a problem. The worst that can happen is a blown fuse, which in the case of the isolation setup is resettable and is set to go off at 4 amps.

Trouble is, an oscilloscope normally measures between its probe input pin and ground. Yes, ground!

One way to deal with that is to power the scope through an isolation transformer. But that’s actually one of the most dangerous “solutions” one could think of, because that means you lose all safety nets of having a safe ground attached to the instrument, its case, its probe’s BNC connectors, etc. I want more safety, not less!

Luckily, there’s a much better way to do this – but at €200 it’s not cheap:

This is an isolated differential probe, which I got from BitScope. The box is 20 cm long, so it’s quite large.

Looks like there’s a fair bit of circuitry in there, too:

It’ll run off an internal battery (which I just added) or off the included 9V power brick.

The point of all this is that you can put any voltage up to 1000 V between the two input pins, and that both of the pins can be up to 600 V “away” from ground potential. Yet the output will still stay within 6.5V of the scope’s ground level. The differential aspect is that it doesn’t care what the common-mode voltage excursions are, i.e. it’ll ignore any voltage which happens to be present on both inputs – only the difference is passed through.

With this probe, all you need to think about is the high voltage on the circuit and on the test leads. And as you can see, it comes with some very well-isolated cables, (huge!) clips, and test hooks. The specs are as follows:

The 20x range is nice for low-voltage measurements on AC-connected stuff, such as the output of a switching regulator: a scope set to 1 mV/div will be able to display signals from this probe at 20 mV/div, which is enough to view power supply ripple, for example.

Here’s a first test, simply viewing the 230V mains:

Note that the vertical scale is 200x higher than indicated on this snapshot, i.e. 100 V/div (230 VAC RMS ≈ 325 VAC peak). The voltage shown here is definitely not a 100% pure sine wave – I have no idea why.

Onwards! (with caution)

In a comment on the daily weblog, Jörg pointed to a very interesting chip which can directly switch 220 V.

All the parts are available as through-hole, so I decided to give it a go:

I used the LNK302, with a 2.00 kΩ / 2.32 kΩ 1% resistance divider to select the output voltage. At the left there’s a fusible 100 4700 Ω resistor, a diode, and a 3.3 µF (400V!) electrolytic cap for (high-voltage) DC input.

The circuit officially only works with input voltages above 70 V, but that’s a conservative spec. It actually works fine from my 30 VDC lab supply, which means I can safely poke around in it and see how it behaves.

Time to fire up the scope again. Here’s the output with a 1 mA load:

Channel 1 (yellow) is the output, but AC coupled, i.e. just the fluctuations, while channel 2 (blue) is hooked up to the same pin but in DC-coupled mode.

As you can see, the output is roughly 3.8V with brief but fairly large spikes of almost 0.3V. Basically, the switching chip periodically connects the input voltage to the output (through an inductor, and charging a 100 µF cap).

The fun begins when you start loading the supply a bit. Here’s what it does at 10 mA:

Similar spikes, at roughly 10 KHz (quite a bit of variation in timing). Now 25 mA:

More of the same, the repetition rate doubles to around 20 KHz, and the voltage drops a bit. Let’s go for 50 mA:

It’s getting a bit jittery now, doubling its frequency every once in a while. And here’s 75 mA:

Nice and steady output, the ripple voltage is under 0.2V now. Still holding at 3.2V.

Can we pull more current out of this circuit? Not really, I’m afraid – see what happens at around 80 mA:

Going full speed now at around 65 KHz, but there’s simply not enough energy: the output collapses to 1.32 V.

With roughly 70 mA @ 3.2 V, input power consumption is about 20 mA @ 30 V. This isn’t stellar (37% efficiency), but also not really indicative of what it will do at 220 V, since I’m running the chip way out of spec.

I’ll need to do some tests at the full 220 VAC to make sure this behavior is similar under real-world conditions, but from what I can tell, 50..65 mA is probably about the limit of what this circuit can supply at about 3.3V. Which would be plenty for a JeeNode in full transmit mode BTW, including some additional circuitry around it.

One problem is the fairly large ripple voltage. It would be better to dimension the circuit for a 5V output, or even 12V, and then add the usual linear regulator to get it down to 3.3V for the logic circuit. This could actually be quite practical in combination with a small 12V relay (which isn’t affected by such voltage fluctuations).

Note that a circuit like this – even if it were to supply only 5 mA – would be plenty to drive a JeeNode which sits mostly in low-power mode and only occasionally needs to activate its RFM12B wireless module.

So all in all: a very interesting (non-isolated) option!

Update – Also ok on 220 V: 65 mA @ 3.0 V (draws 1.25 W, i.e. 15 %). With 2 mA @ 3.7 V, power consumption is 0.40 W (vs. ≈ 8 mW delivered, i.e. 2 % efficiency). At 80 mA, the voltage drops to 2.5 V – above that it collapses.

The first AC-mains powered current node configuration used a resistive transformer-less supply. It took about an hour to charge with no load, and consumed about 0.26 Watt.

This is an improved version, using a capacitor:

(Whoops, I see I forgot to draw the 470 kΩ resistor across the 22 nF cap, to discharge it when disconnected!)

The 4.7 kΩ 1/2W resistor limits inrush current – the worst case being when the cap is empty and plugged in at the top of the AC mains cycle. It’s also a “fusible” resistor, meaning that it’ll act as a fuse when overloaded for any reason. This won’t be enough to protect the circuit, let alone a person touching this circuit, but it will prevent a fire in case of a catastrophic short (which could otherwise pull over 16 amps until the mains fuse blows).

As before, it’s charging the supercap – supplying nearly 1V in this case:

The 470 kΩ resistor right across that yellow 22 nF capacitor quickly drops the residual charge once unplugged.

Charging appears to go a bit faster, but there’s a problem because I’m running the ATtiny with a disabled brown-out detector (BOD). This means the ATtiny isn’t kept in reset as the voltage ramps up. As a result, it’s trying to run even at low voltages (and is bound to malfunction at voltage levels under 1.8V), but more importantly: it’s going to consume current while trying, which will prevent the supercap from charging! Which is is exactly what I see: the supercap voltage is barely rising above 1.34 V.

The BOD is an important hardware feature for circuits like these. It keeps the ATtiny in reset until the power reaches a certain level. That level is configurable for 1.8V, 2.7V, or 4.3V via the ATtiny’s fuses. In this case, 1.8V seems like the proper value to use – it will be too low for the RFM12B module which requires at least 2.2V, but this way the ATtiny can continue to run correctly even at lower levels, and then decide whether it wants to enable the RFM12B or not.

Unfortunately, I’m going to have to improve the sketch first. Right now, it just starts up and tries to do its thing, without consideration for the current voltage. Leaving the unit on for over two hours didn’t lead to a packet transmission, and only got the voltage up to about 1.8 V, whereas it keeps on rising with the ATtiny disconnected. Clearly, the ATtiny needs to become more aware of its current power state before it can act as a reliable AC current sensor. That’s the trouble with ultra-low power systems: it can be tricky to get them right!

Drat, it looks like I just messed up the fuses on the ATtiny, because I can’t reprogram it anymore. That’s the trouble with low pin-count controllers: it’s easy to mess them up!

Time to go back to separate power supply and JNµ test rigs. Let’s not muddle the issues any more than needed.

The good news is that this supply now consumes half of the resistive version, i.e. 0.13 Watt. Note that the power consumption of the resistive version could have been halved as well (see this comment). So in this case, the extra efficiency of the cap seems to be going into supplying a bit more current, i.e. charging the supercap faster.

Progress nevertheless (says the optimist): lower power consumption, faster start-up!

This is a quick experiment to see how this very low-power direct AC mains supply behaves:

Note that I’ve built the 200 kΩ value from two resistors in series. This reduces the voltage over each one, and offers a slight security if one of them shorts out. The max 1 mA or so of current these resistors will let through is not considered lethal – but keep in mind that the other side is a direct connection, so if that happens to be the live wire then it’s still extremely dangerous to touch it!

One idea would be to add a “fusible” 100 Ω @ 0.5 W resistor in series with the 200 kΩ. These are metal-film resistors which will disconnect if they overheat, without releasing gases or causing flames. I can’t insert it in the other wire due to the voltage issue, so I’m not really sure it actually would make things any safer.

Here’s my first test setup of this circuit, built into a full-plastic enclosure:

It took 20 minutes to reach 1.8V, the absolute minimum for operating an ATtiny. This is not a practical operating voltage, because whenever the circuit draws 1 mA or more, that voltage will drop below the minimum again.

The RFM12B wireless module will need over 2.2V to operate, and draw another 25 mA in transmit mode. The only way to make this work will be to keep the transmit times limited to the absolute minimum.

Still, I’m hoping this crude power supply will be sufficient. The idea is to run on the internal 8 MHz RC oscillator with a startup divider of 8, i.e. @ 1 MHz. The brown-out detector will be set to 1.8V, and the main task right after startup will be to monitor the battery voltage until it is considered high enough to do more meaningful work.

With 3.5V power, an ATtiny draws ≈ 600 µA @ 1 MHz in active mode and 175 µA in idle mode, so in principle it can continue running at this rate indefinitely on this power supply. But for “fast” (heh) startup, it’ll be better to use sleep mode, or at least take the system clock down well below 1 MHz.

This might be a nightmare to debug, I don’t know. Then again, I don’t have to use the AC mains coupled supply to test this. A normal low-voltage DC source plus supercap would be fine with appropriately adjusted resistors.

After 35 minutes, the voltage has risen to 2.7V – sure, take your time, hey, don’t rush because of me!

Another 5 minutes pass – we’re at a whopping 3.0V now!

Time for a cup of coffee…

After 45 minutes the charge on the 0.47F supercap has reached 3.3V – yeay! I suspect that this will be enough to operate the unit as current sensor and send out one short packet. We’ll see – it’ll all depend on the code.

After 1 hour: 3.75V, which is about as high as it will go, given the 5.1V zener and the 2x 0.6V voltage drop over the 1N4148 diodes. Update: my test setup tops out at 3.93V – good, that means it won’t need a voltage regulator.

Apparently, supercaps can have a fairly high leakage current (over 100 µA), but this decreases substantially when the supercap is kept charged. In an earlier test, I was indeed able to measure over 2.7V on a supercap after 24 hours, once it had been left charged for a day or so. In this current design the supply will be on all the time, so hopefully the supercap will work optimally here.

Not that it matters for power consumption: a transformerless supply such as this draws a fixed amount of current, regardless of the load. Here’s the final test, hooked up to live mains without the isolation transformer:

Of this energy, over 95% is dissipated and wasted by the resistors. The rest goes into either the load or the zener.

Assuming I can figure out a way to transmit wireless information from the ATtiny, I’d like to make that recent AC current change detector a self-contained and self-powered unit. At minimal cost, i.e. with as few parts as possible.

That’s a bit of a problem. Adding a transformer-based power supply, however feeble, or a ready-made AC/DC converter would probably triple the cost of the setup so far. Not good.

I really only need a teeny bit of power. The techniques to a get a JeeNode into low-power sensing have been well-researched and documented by now. It shouldn’t be too hard to make an ATtiny equally low-power.

First of all, this “power sensing node” really doesn’t have to be on all the time. Measuring power once every few seconds would be fine, and reporting over wireless only when there is a significant change in detected current. So for the sake of argument, let’s say we measure once a second, track the average of three to weed out intermittent spikes, and report only when that average changes 20% or more since the last value. For continuity, let’s also report once every 3 minutes, just to let the system know the node is alive. So that’s one packet with a 2-byte payload every 3 minutes most of the time, and one current measurement every second (with the same ADC sampling and filtering as before).

What this comes down to is that we need perhaps 3.3V @ 10 µA all the time, with a 30 mA peak current draw every couple of minutes.

A battery would do fine. Perhaps 2x AA or a CR123 1/2 AA. But it feels silly… this thing is tied to a power line!

Why not use a transformer-less power supply, as described in this well-known application note from MicroChip?

Well, there’s a problem. These types of supplies draw a constant amount of current, regardless of the load. Whatever the circuit doesn’t use is consumed by the zener diode. So to be able to drive a 30 mA peak, we’d need a power supply which constantly draws 30 mA, i.e. 6.6 watts of power. Whoa, no thanks!

Here’s a basic resistive transformer-less supply (capacitive would also be an option):

There is a way to reduce the current consumption, since we only need that 30 mA surge very briefly, and not very often: use a big fat capacitor on the end, which stores enough energy to provide the surge without the voltage collapsing too far. This might be a good candidate for a trickle-charged small NiMh cell or even a supercap!

Hm, let’s see. If the supply is dimensioned to only supply a very small amount of current, say 1 mA, then it would be more than enough to charge that capacitor and supply the current for the ATtiny while in power-down mode. A 0.47 F supercap (which I happen to have lying around) ought to be plenty. This power supply would draw 0.22 W – continuously. Still not stellar, but not worse than several other power bricks around here.

Alas, such a design comes with a major drawback: with such a small current feeding such a large cap, it will take ages for the initial voltage to build up. I did a quick test, and ended up waiting half an hour for the output to be useful for powering up an ATtiny + RFM12B. That’s a lot a waiting for when you plug in such a system for the first time, eager to see whether it works. It also means that the firmware in the ATTiny has to very careful at all times with the limited energy available to it.

Still, I’m tempted to try this. What’s half an hour in the grand scheme of things anyway?

Yesterday’s post was nice, but it stepped over one teeny weeny little detail: the ATmega’s ADC isn’t sensitive enough to measure AC signals in the millivolts range. With a 3.3V reference, each step is ≈ 3.3 mV, while the signals at low power levels are close to a single ADC step. I could bump the sensitivity slightly by using the 1.1V bandgap as ADC reference voltage, but that still only gets me to 1.1 mV steps. Hardly enough to apply filtering to and set thresholds.

Ah, but there’s a way out… (there always is!)

The ATtiny85 is the smaller brother of the ATmega with a frighteningly low pin count of 8. The key is that it has a differential ADC option, i.e. it’s able to measure the voltage between two points (within 0..VCC) and that it has an optional 20 x analog signal amplifier built-in, when used in differential mode. As reference voltage, there is 1.1V, 2.56V, and VCC – providing a couple of ways to tweak the actual range and sensitivity.

Since I had the right ATtiny85 lying around, as well as TinkerLog’s prototyping board, I thought I’d give it a go:

The problem with an 8-pin chip, of course, is that it only has 5 I/O pins (6 if you’re willing to use the RESET line and lose the ISP programming mode – which I wasn’t). That’s not much to interface with. The ATtiny85 I used has 8 Kb flash and 512 bytes of RAM, so that in itself should be sufficient for all the code I need.

There’s no boot loader. There’s no ready-to-use serial port. And it’s not an Arduino-compatible system, so you can’t just write sketches for it, right?

Not so fast. First of all, there’s the LCD Plug which needs only 2 I/O lines. That leaves me 3 pins: 2 for the differential analog input, and 1 for a debug LED. Plenty!

The LCD needs the Ports library. Which needs the Arduino IDE and runtime. Ah, but there’s more good news: there’s an arduino-tiny project which defines some boards and a set of “core” runtime source files which are intended to run on ATtiny85’s and other non-mega AVR chips. That’s quite a feat, btw, once you consider just how limited an ATtiny85 is (ASCII art by the arduino-tiny project):

                             +-\/-+
    Ain0       (D  5)  PB5  1|    |8   VCC
    Ain3       (D  3)  PB3  2|    |7   PB2  (D  2)  INT0  Ain1
    Ain2       (D  4)  PB4  3|    |6   PB1  (D  1)        pwm1
                       GND  4|    |5   PB0  (D  0)        pwm0
                             +----+

Still, they managed to support a couple of digital and analog I/O pins, with support for the millis() timer, analog in and out, and more. With this installed, I can write sketches and upload them via the AVR-ISP. Pretty amazing!

Sooo… just had to make a few small and obvious tweaks, and the Ports library works. There’s exactly one port (AIO1 = PB2, DIO1 = PB0, IRQ = PB1). Note that I’m using the new Arduino IDE 1.0 (beta).

That leaves me a whopping two pins for the differential analog input, which is what this is all about, after all.

Here’s my setup (hooked up to the safe 20 VAC brick):

I was too excited to tidy up once the LCD hookup worked. It would all have fitted on a single (mini) breadboard.

Tomorrow, I’ll show you how this crazy little setup measures up…

Got quite a scare the other day:

One moment I was cheerily plugging in and unplugging my 100 W light bulb for testing current measurements, and the next it came apart with a huge bang. No fuse tripped anywhere (as it would have in UK households).

It’s a good reminder of the amount of energy an AC power outlet can discharge at any time. In fact, the standard rating for individually-fused 220 V power groups is 16 Amps around here. That’s roughly 3600 W.

To get an idea of what that means – one Horsepower is the sustained power a horse can generate, which is about 740 Watt. So there are almost 5 horses in each power outlet, waiting to charge at you!

If you push that much energy into a liter of water, it’ll boil in 2 minutes flat. Doesn’t sound like much, eh? How about 1000 Amps at 3.3V? You’ll need copper wire with a diameter of 18 mm to handle that much current.

Compare that to a JeeNode in sleep mode drawing 10 µA, i.e. 33 microwatt. That’s 8 orders of magnitude less.

Anyway, I’ve gained some extra respect for 220V mains circuits.

Now, if only I could find a replacement … 100 W incandescent light bulbs are no longer sold in Europe.

Now that direct connection to AC mains is no longer out of the question (at least to establish a baseline), I decided to create a much simpler hookup:

That’s a 0.1 Ω resistor in series with the load and two diodes, all in parallel. The diodes are not that useful for load protection (anything over 1000 W will be a problem), but they may help with transients and spikes:

Note that the sense wires are attached to the shunt resistor. If anything were to get yanked loose by accident, then it would be those wires, not the shunt itself. As usual, this is all hooked up via an isolation transformer.

A quick check tells me that this setup does not perform any better than the previous direct connection with the sketch I’ve been using all along.

Even without anything plugged in, I can affect the readings by moving my hands close to the circuit:

    reading:RF12-868.5.17:oneLong:value = 28250
    reading:RF12-868.5.17:oneLong:value = 28247
    reading:RF12-868.5.17:oneLong:value = 28184
    reading:RF12-868.5.17:oneLong:value = 28637
    reading:RF12-868.5.17:oneLong:value = 33743
    reading:RF12-868.5.17:oneLong:value = 77924
    reading:RF12-868.5.17:oneLong:value = 92240
    reading:RF12-868.5.17:oneLong:value = 50233
    reading:RF12-868.5.17:oneLong:value = 30663
    reading:RF12-868.5.17:oneLong:value = 28476
    reading:RF12-868.5.17:oneLong:value = 28176
    reading:RF12-868.5.17:oneLong:value = 28145
    reading:RF12-868.5.17:oneLong:value = 29082
    reading:RF12-868.5.17:oneLong:value = 34851
    reading:RF12-868.5.17:oneLong:value = 43977
    reading:RF12-868.5.17:oneLong:value = 54547
    reading:RF12-868.5.17:oneLong:value = 62388
    reading:RF12-868.5.17:oneLong:value = 61987
    reading:RF12-868.5.17:oneLong:value = 61366
    reading:RF12-868.5.17:oneLong:value = 62199

So the next step will be to add filtering. Either analog or digital, we’ll see!