(This post follows up on Reinhard’s clearvoyant comment yesterday..)

The ARM microcontrollers described in the past few days are a big step up from a “simple” ATmega328, but that’s only if you consider hundreds of kilobytes of flash storage and tens of kilobytes of RAM as being “a lot”.

Compared to notebooks and workstations, it’s still virtually nothing, of course.

But there’s another trend going on, with bang-for-the-buck going off the charts nowadays: small embedded Linux systems with integrated wired and/or wireless Ethernet. These are often based on Broadcom and Atheros chipsets – the same as found in just about every network router and gateway nowadays.

One particularly nice and low-cost example of this is the Carambola by 8devices:

It has 8 MB flash + 32 MB RAM (megabytes!), on-board WiFi, and runs OpenWrt Linux.

There are I2C and SPI interfaces, which can also be used as general-purpose I/O pins, so this thing will interface to a range of things right out of the box.

One gotcha is that the 2×20 pins are on a 2mm grid, not 0.1″. Small size has its trade-offs!

The board will draw up to 1.5 W @ 3.3V (i.e. roughly 500 mA), but that can easily be reduced to about 0.4 W for a blank board with no wired Ethernet attached.

Here are some more specs, obtained from within Linux on the Carambola itself:

As you can see, this unit (like many routers) is based on a MIPS architecture. And it’s actually quite a bit faster that the Bifferboard I described a while back.

Like most low-end ARM chips, these systems often lack hardware floating point (its all implemented in software, just like an Arduino does for the ATmega’s). Don’t expect any number-crunching performance from these little boards, but again it’s good to point out that boards like these are priced about the same as an Arduino Uno.

One of the benefits of Linux is that it’s a full-fledged operating system, with numerous tools and utilities (though you often still need to cross-compile) and with solid full-featured networking built in. The amount of open source software available for Linux (on a wide range of hardware) is absolutely staggering.

Among the drawbacks of Linux in the context of Physical Computing, is that it’s not strictly real time, so programming for it follows a different approach (busy loops for timing are not done, for example). Don’t expect to accurately pulse an I/O pin at a few hundred Hz or more, for example. Linux was also definitely not made for ultra-low power use, such as in remote wireless nodes which you’d like to keep up and running for months or years on a single battery – there’s simply too much going on in a complete operating system.

The other thing about Linux is that it can be somewhat intimidating if you’ve never used it before. Part of this comes from its strong heritage from the “Unix world”. But given the current trends, I strongly recommend trying it out and getting familiar with it – Linux is very mature: it has been around for a while and will remain so for a long time to come. With boards such as the Carambola illustrating just how cheap it can be to have a go at it.

Interesting times. Now if only new software developments would keep up with all this!

A while back, I came across this product, called the “mbed Internet of Things Gateway”:

It’s an ARM microcontroller with an Ethernet port, a µSD storage slot, and an RFM12B wireless transceiver. Very nicely packaged in an extruded-aluminium case with laser-cut front and back panels. Here’s what’s inside:

Not that much circuitry, as you can see – because all the heavy-lifting is done by the MBED board on the left.

That’s a 32-bit microcontroller, with built-in Ethernet and USB, plenty of I/O pins, and lots of features to connect to SPI, I2C, CAN, and other types of devices. Not to mention the 512 KB flash and 32 KB RAM memory – plenty to implement some serious functionality.

MBED comes with an intriguing “cloud-based” compiler and build environment, which is surprisingly effective. Here’s how it works, out of the box:

1. plug the MBED into USB and it’ll present itself as a memory disk with one HTML file on it
2. double-click that file to go to the MBED web site
3. you get a web-based equivalent of a standard Windows IDE, plus a large code sharing community
4. create your project online, enter your own code, and hit the compile button
5. if the code compiles successfully, you end up with a file in your download folder
6. copy that file to the MBED’s USB drive
7. press on the MBED’s reset button, and that’s it … uploaded and running!

This is a very elegant workflow. No need to install any software to develop for MBED. And you can continue work wherever you are, as long as internet works and you have your MBED with you. You do need to sign up and register a (free) account on that MBED site – in return, they’ll do all the compiles for you.

This board is an exciting development. The cost is higher than with just a JeeNode + EtherCard, but there is also a lot more possible when you don’t have to fight the ATmega328’s strict flash and RAM memory constraints.

I’ll have more to say about this hardware and software tomorrow – stay tuned…

This is a follow-up to the Delayed power-up post, this time using some P-MOSFETs (in SOT-23 SMD form, i.e. tiny). The way I’m testing these, is by using a 1 kΩ resistor as simulated load, and hooking things up as follows:

These components were not selected for this purpose, in fact, I picked units with a very low switching threshold voltage, so that they can reliably be switched on from an I/O pin, even if we were to run at just 1.8V.

Here are the characteristics of the Philips BSH203 P-MOSFET:

Just over 1 Ω resistance when driven low by 1.8V, so with a 50 mA load, the voltage drop over this MOSFET will be just over 50 mV.

When placed in the Component Tester, and zooming in on the interesting bit, we get:

Each major horizontal division is 2V, so this thing switches on at about 0.5V.

For comparison, the characteristics of the Vishay SI2333 P-MOSFET:

And in this case, the Component Tester shows this (sorry, can’t zoom in with this setup):

A slightly higher turn-on voltage, but note that the ON resistance is considerably lower at 1.8V: only 0.05 Ω. Not surprising, when you consider that this MOSFET can probably switch over 5 A (without self-destructing from the heat dissipation).

Here’s what these tiny components look like, with wires soldered on for “debugging”:

Looks like either of these will do the trick, when switched from an I/O pin anyway.

Fighting my lab instruments… not good :(

The TG2511 Arbitrary Waveform Generator in my lab is very flexible, and does all the usual things one would expect of a modern signal generator, i.e. setting the shape, frequency, and amplitude of its output signal.

But there’s some weirdness w.r.t. the amplitude. The built-in impedance is quoted as being 50 Ω, no doubt the reasoning being that if you terminate the cable with 50 Ω as well, then a standard coax cable will have the least possible signal degradation (ringing, reflections, and such).

There is a setting to adjust for the assumed load, so that the instrument can adjust its amplitude accordingly. The default is to assume a 50 Ω load at the end of the cable:

But this is where I get mightily confused. Here’s what I see with the scope set in high impedance mode:

That in itself is actually correct: the output amplitude is twice the expected value, since there is no 50 Ω terminating resistor. So let’s add it in (it’s an option inside the scope):

Huh? – Why isn’t the amplitude 3 Vpp, and ends up being only half that value?

But there’s something even stranger, and far more inconvenient about the TG2511. After a few minutes, I often get a message about the output signal being overloaded:

Doesn’t make sense. I’m never loading the output with more than 50 Ω, i.e. well within specs. Sometimes this happens even when I completely disconnect the load. Even more suspect: the green “OUTPUT” doesn’t always match what’s happening. Sometimes the light is off, yet the output signal is still present!

In fact, I get this screen in the most unexpected situations. Maybe the TG2511’s output stage is broken? Yuck.

I have contacted the supplier for advice.

This is an exploration to find out what circuit can be used to switch small electronic loads using a digital I/O pin.

The simplest solution by far, is to use the I/O pin to directly drive the circuit. This works great for low currents, such as an LED with series resistor. But there are limits to how much current you can draw:

As you can see, from the datasheet, drawing 5 mA will already cause a voltage drop of about 0.2V, so the load will not get the full supply voltage. With larger loads, the drop becomes even more pronounced.

Another approach is to use a PNP transistor, as follows:

Here’s how it works, in case you’re not familiar with them PNP transistor thingies:

• pulling the input pin down towards 0V, will turn the PNP transistor on
• when on, the transistor will conduct and supply power to the load
• leaving the pin floating or pulling it up, will cause the PNP transistor to turn off

In my setup, I’m using a very light load for now. The following measurements will be different as the load increases, but not as substantially as with the raw I/O pin drive. A circuit like this could easily drive a 100..250 mA load, if the base resistor has the right value – as we’ll see.

Time to try this out. I am quite interested in how the voltage drop over the transistor depends on the exact voltage placed on the base junction. Or rather: how much current I’ll need (since the resistor passes a known current once we know the voltage drop.

So as input, I’m using a 10 Hz sawtooth signal, which varies from 0 to 3V. Due to the way things are hooked up, it starts at 3V below the 3.3V power rail, i.e. 0.3V, and then rises to the level of the power rail.

Here are the results with a 10 kΩ resistor – both signals have been inverted, as I’m measuring relative to +3.3V:

(oops, ignore the yellow trigger point baseline)

At the start, the transistor is turned on strongly, and then the input voltage falls to zero (yellow trace). As you can see, the voltage drop over the transistor increases non-linearly as it gradually turns off. With only 1V driving it, the voltage drop increases from about 40 mV to 100 mV, i.e. 0.1V. With even less, it starts to switch off and gets the full power supply voltage (so nothing reaches the load).

Now let’s do some math. I used my transistor tester to determine that this particular BC557 PNP transistor had 0.8V drop over its base-to-emitter junction, and that its current amplification factor (hFE) is about 220x.

In other words: with 1.0V on the input, there is 1.0-0.8 = 0.2V on the 10 kΩ resistor. Ohm’s law (E = I x R, or I = E / R) implies that the current through the resistor will be 0.2V / 10kΩ = 20 µA. This current is essentially “wasted”, but given the 220 factor, it will allow the transistor to drive up to 20 µA x 220 = 4.4 mA. Not so great…

Update – not sure what I was smoking at the time I wrote the above paragraph. That’s 1.0V below VCC, roughly the switching threshold of this transistor when used with a 1 kΩ load. I’m not sure what point I was trying to make here, other than that a 20 µA base current is not enough to switch an RFM12B.

But if we pull the input to almost ground level, as would be the case with a digital I/O pin, then the base current increases to (3.3 – 0.8) / 10,000 = 250 µA, supporting a load of up to 250 µA x 220 = 55 mA.

If power waste is not an issue, we could reduce the base resistor to 1 kΩ, and get over 500 mA load switching capability (assuming the transistor is powerful enough). The base current will then be around 2.5 mA, a value which an ATmega I/O pin can still easily supply.

But what if power use is important, i.e. if we can’t afford to waste those 20 µA or more?

Here’s the same circuit, with a 100 kΩ resistor instead:

Note how the emitter-to-collector voltage drop (blue line) rises. With 3V input differential, i.e. almost pulled to ground, the drop over the transistor is still under 0.15V, but we’ll need to keep the input voltage at least 1.5V below the power supply voltage to make this work. So in very low-voltage / low-power scenario’s (i.e. running off two almost-depleted AA cells), this might become tricky.

The current losses are now only 1/10th, i.e. 25 µA when the power supply is 3.3V and the input is tied to ground. Then again, in the ultra-low power world that wasted 25 µA is not really such an impressive figure.

Still, for switching loads which draw up to a few hundred milliamps, this circuit using just a PNP transistor and a resistor is really quite practical. If you use a 1 kΩ resistor, the base current will be well with the I/O pin’s capabilities, and the transistor will usually have enough drive to switch its load.

The only other drawback is that the transistor will always add a small voltage drop of perhaps 50 .. 200 mV.

There is a third solution using MOSFETs, with its own set of trade-offs. To be continued…

To continue where I left off before the summer, let’s examine what a current clamp like this one does:

You put it around a single wire in your AC mains cabling and it’ll generate a voltage proportional with the current going through it. This unit has a built-in burden resistor, which means you get a ± 1V (AC!) output when the current through the wire is 30 A. So let’s have some fun and look at a couple of different loads, eh?

Let’s start off with an old-fashioned 25W incandescent light-bulb, which is a resistive load:

Note the vertical scale – these voltage levels are tiny. The scope caclulates the Root Mean Square (RMS) voltage as being 3.52 mV in this case. That’s the voltage you’d need to draw as direct current to dissipate the same amount of power as this alternating current (let’s ignore phase shift and “reactive” vs “true” power for now).

Sure enough, a 75W light bulb draws three times as much power (note the different vertical scale):

Here’s a 2W LED light bulb, which uses an electronic circuit to pulse small amounts of energy from AC mains:

Again, note how this minute RMS reading corresponds quite accurately to the specified wattage.

Now let’s take a vacuum cleaner, which is an inductive load, and quite a lot beefier too:

The “blips” are switching artifacts from the TRIAC control included in this unit. From the RMS value, my estimate would be that it draws about 1500W. Here’s the same vacuum cleaner, with its power throttled back to minimum:

As with the LED light, you can see the electronics kicking in and pulsing AC mains to throttle power back to around 500W.

Conclusion: a current clamp is a safe way to measure current in an AC mains wire, and it more or less reproduces the measured current as a small output voltage. Very small in fact, for light loads. To accurately determine the RMS value of a load as small as our 2W LED light, we’re going to have to read out this signal in the sub-millivolt range, and do so at perhaps 1000 HZ to collect enough readings per 50 Hz cycle.

For some time now, there have been two of these Tizio lamps in the house. A gorgeous design and very practical:

As so many lamps from last century, they come with a halogen incandescent light bulb:

Lifetimes are great (I don’t think I ever replaced one), but efficiency less so. So I decided to replace them anyway:

These are simple 1.2W warm-white LED lights with built-in rectifier and current limiting resistors. Here’s the result:

The transplant ended up being very simple, the UV filter glass is no longer needed, the power consumption has dropped from 50W 35W to 12W 5W, and – if all is well – these LEDs will never need to be replaced again.

Easy peasy! Now if only all our incandescent lights had equally simple alternatives…