Computing stuff tied to the physical world


Can’t be done

In Hardware on Jan 27, 2012 at 00:01

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:

Screen Shot 2012 01 25 at 01 57 45

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:

DSC 2894

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.

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Tektronix 475

In Hardware on Jan 26, 2012 at 00:01

Oh boy, I’ve been bitten by the collector’s bug…

Couldn’t resist an excellent offer on Marktplaats (the Dutch equivalent of eBay) for a fully operational analog scope built in the late 70′s. The Tektronix 475 is, ehm… slightly larger than the Hameg HMO2024:

DSC 2892

Then again, they are quite comparable – both are specified as 200 MHz bandwidth. Check ‘em out:

DSC 2893

The Tek will need to be re-calibrated, but as far as I can tell all the functions work and all the switches, knobs, and indicators are in good order. The previous owner said he had used it for a long time, but not intensively.

First thing I had to try was the analog clock, of course:

DSC 2886

Apart from that? Oh, I don’t know. I’ll refurbish and re-calibrate it one day, there’s lots of info about this classic workhorse out on the web, and I really would like to learn how to diagnose and repair stuff. Even old stuff.

This is a single-beam unit, meaning it can’t show two signals at the same time. Newer and more advanced models are dual-beam, but this one has to either “alternate” between the two beam displays, or “chop” them up and draw bits of one and the other in an interleaved fashion. It’s also not a storage scope, so you’ve got to look very carefully if you want to see one-shot events – the display on the screen shows only as long as the phosphor glow lasts!

Fantastic engineering. Electronics, mechanical, operation, documentation, service – everything!

Would I have bought the Hameg if I’d had this one at the time? You bet – the “S” in DSO is a game changer, especially with microcontrollers and physical computing. But that huge Tek brick is more endearing :)

Heh, I’ve never before collected anything in my life – this is fun!

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The PWR vs the +3V pin

In AVR, Hardware on Jan 25, 2012 at 00:01

JeeNodes and plugs use 6-pin headers with the following pin assignment:

Port

(there’s usually a big printed dot near the PWR pin for orientation)

There has been some confusion about what PWR is w.r.t. +3V and how to use it, so let me elaborate a bit.

First: “+3V” is the main regulated power reference for most chips and circuits. And to make it more complicated: it’s actually 3.3V, not 3.0V. The “+3V” label merely identifies the pin (VCC would be too confusing).

The ATmega on the JeeNode, as well as most (but not all) chips on attached plugs run on this 3.3V voltage level. It is also the reference level for all digital I/O (0 = 0V, 1 = 3.3V), as well as for the ADC when used for analog inputs (1023 steps of 3.22580645161 millivolt each).

As for the other supply pin: “PWR” is usually a higher voltage from which +3V is derived.

There’s a voltage regulator on the JeeNode which can take any input voltage on “PWR” up to about 13V as input and which produces the 3.3V regulated voltage on the “+3V” pin.

The JeeNode regulator has some limits: it’s can’t supply more than 250 mA and it can’t dissipate more than about 600 mW. It’ll shutdown if either of these are exceeded (and it’ll get damaged if you feed it more than 13V). These figures are fairly limiting: if you run a JeeNode off 12V on the PWR pin, then it actually won’t be able to supply more than 70 mA. With 5V on the PWR pin, the limit will be current, not power dissipation, i.e. 250 mA max.

That’s for a JeeNode with through-hole components, i.e. an MCP1702 regulator in a TO-92 package. With the JeeNode SMD and JeeNode USB, you get even less power from the regulator: at most about 250 mW of heat dissipation. That’s 150 mA @ 5V, and only 28 mA @ 12V. In short: the JN SMD and JN USB are not really meant to be powered from more than 5V – or at least not without an extra external regulator (on the plus side: the MCP1703 supports up to 16V on the PWR pin). Note also that the JeeNode USB doesn’t support more than 7V on the PWR pin, due to limitations of the on-board LiPo charger – but that should not be an issue since it’s really only meant to be powered from 5V via USB.

As you can see, there are several limiting factors as to what voltage is supported on the PWR pin. Keep in mind that JeeNodes were designed for ultra-low power consumption, and the regulator was selected for its extremely low idle current draw, not its “high-end” characteristics.

But that’s not all…

One trap for the unwary is with the JeeNode USB: it doesn’t actually feed 5V to the PWR pin when connected to USB but only 4.2V. This is due to the LiPo charger circuit, which allows a LiPo battery to be connected between PWR and GND. With a LiPo battery, the JeeNode USB becomes a very convenient stand-alone unit: low-power untethered operation while not connected to USB, and automatic LiPo charging while connected to USB. It even has a voltage divider on board to monitor the LiPo battery voltage (tied to PC6, i.e. analog 6 in Arduino-speak).

The trouble here is that when you connect plugs to the JeeNode USB, you won’t get the full 5V you might expect. This affects all those plugs which don’t run entirely off +3V but expect some higher voltage on PWR.

And here’s another tricky situation…

With the AA Power Board, you can run JeeNodes off a single AA cell. In this case, there isn’t a voltage higher than 3.3V anywhere in the circuit – so what to do with the PWR line? Well, there’s a solder jumper on the AA board to let you decide: it’s normally open, but you can connect it to either +3V or to the battery “+” (i.e. 1.2..1.5V). Obviously, this setup will be even less suited for plugs which expect a higher voltage than 3.3V to operate.

As you see, the +3V pin is solidly specified as always being 3.3V, but the PWR pin level can be all over the map.

The following plugs are especially affected:

  • the LCD Plug will work, when used with a 3.3V display (as shipped by JeeLabs), but will only be suitable with 5V displays if PWR carries 5V

  • the Graphics Board assumes that PWR is 5V, because the display needs it (it’ll probably still work with 4.2V, but PWR should not be above 5V)

Most plugs will be fine, though, since they only use the +3V supply. The only issue here is to make sure that the total current consumption off +3V is not too high.

Using a JeeNode with an unregulated power supply

The maximum allowed voltage on the PWR pin is about 13V. Unfortunately, many 12V power bricks are fairly badly calibrated, and often deliver substantially more than 12V under no-load conditions. You could easily damage the JeeNode’s on-board regulator with a cheap 12V power brick – better use a 5V or 9V brick.

If you do want to power a JeeNode off 12V (perhaps you need 12V on the rest of the circuit for LEDs, relays, or motor power), then the best approach is to insert a 5V regulator and feed that 5V level to PWR. This can easily be done with a “7805″ regulator chip, which is available from many electronics parts sources.

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