Computing stuff tied to the physical world

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JeeNode goes solar

In Hardware on Sep 3, 2010 at 00:01

Now that ultra low-power options are coming into reach for JeeNodes, lots of new scenarios can be explored.

The most obvious one is probably a solar-powered JeeNode … so meet the latest new node #5 at Jee Labs:

Dsc 1875

It also uses a 0.47 F supercap, same as yesterday, but now hooked up to a small 4.5 V solar cell (which can only deliver a few mA in bright sunlight), and a Schottky diode between the solar cell and the capacitor.

Here’s the “power supply” in more detail:

Dsc 1876

As you can see, the solar cell is tiny. A few square cm’s only. In fact, it takes quite some time for it to charge the supercap to acceptable levels. I had to place the cell in moderately bright sunlight for about half an hour to get to a 4 Volt charge. It was inching along, taking several seconds per 0.01 V increase.

To avoid losing all that charge right away in the power-up cycle, I modified the ATmega’s fuses to start in 258 clock cycles after power down, and to start up within 4.1 msec after reset. That way it will start up as quickly as possible at all times. The 258 CK setting is particularly nice, because it means the ATmega can get out of total power down within about 16 µs, fast enough to respond to a byte RX/TX interrupt from the RFM12B!

Does it work? Check it out: after connecting the JeeNode with the “radioBlip.pde” sketch pre-loaded… away it went – sending one packet every 60 seconds as node 5:

    OK 5 1 0
    OK 5 2 0
    OK 5 3 0
    OK 5 4 0

While exposed to the current partly-sunny / partly-cloudy light levels, the voltage on the supercap is still increasing. This is good – it means there’s a surplus of solar energy, even with these transmissions going on. That extra energy will be crucial if this thing is to last through the night…

If everything works out, this little Arduino-compatible bugger could well be the first JeeNode to become completely autonomous and transmit wirelessly… forever!

Time will tell :)

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Sending packets without battery

In AVR, Hardware, Software on Sep 2, 2010 at 00:01

Here’s a fun experiment…

After yesterday’s improved power-down current results, I wanted to find out how much power it really takes to send out a bunch of small packets. No acks, just periodically sending out a packet and sleeping.

Trouble is, I didn’t want to wait for a battery to run down, since that would take months or even years. Bit long to get results for this weblog, eh?

So instead, I used this little chap:

Dsc 1873

It’s a super-capacitor which can handle up to 5.5V and has a capacity of 0.47 Farad! That’s like putting a thousand 470 µF caps in parallel. Amazing stuff, in an even more amazingly small package.

Here’s a JeeNode, fitted with this new power source, to give you an idea of just how small this thing is:

Dsc 1874

The next step was to design a small sketch to test this. Here’s what I came up with:

Screen Shot 2010 08 29 at 11.57.38

Some of the code left out for brevity. Full source code can be found here.

What this demo does is send out a packet with a 2-byte payload, then sleep for approximately 1 second, then send again, etc. Until power runs out.

Sure enough, packets started coming in every second:

    OK 4 0 0
    OK 4 1 0
    OK 4 2 0
    OK 4 3 0
    [...]

I expected it to send out say 100 packets or so, before the charge in the 0.47 F supercap would run out.

Guess how far it went…

    [...]
    OK 4 178 28
    OK 4 179 28
    OK 4 180 28
    OK 4 181 28

That’s packet number … clickety, clickety … 28 * 256 + 181 = 7349 !!!

In other words, the JeeNode was able to send out well over 7000 packets, i.e. two hours of packets sent once a second.

This is fantastic. I think the secret – apart from the 3 µA powerdown mode – is that the RFM12B + RF12 driver require very little time to start up, send off a packet, and go back to sleep again. There is no ACK involved, the RFM12B is hardly ever in reception mode.

With real-world use, I expect power requirements to be considerably higher. First of all, a Room Board will draw around 50 µA, due to the on-board PIR sensor. And second, I’m going to want to use ACKs for at least the motion detector reports, so that the system has a robust mechanism for reporting motion whenever it is detected. This means putting the RFM12B in receive mode for a few milliseconds, waiting for the ACK. And repeating this process a few times if that ACK isn’t immediately received.

But still … over 7000 packets without a battery!

Update – with one packet per 5 seconds, the charge lasted 4523 packets, i.e. just over 6:15.

Update #2 – with one packet per 60 seconds, 771 packets got sent out, that’s 12:51 hours … looks like the self-discharge of the supercap is eating up the remaining energy.

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Sleep!

In AVR, Software on Sep 1, 2010 at 00:01

The “powerdown_demo.pde” sketch in this recent post draws 20 µA, which surprised me a bit…

A while back, I got it down to a fraction of that, by turning off the brown-out detector (BOD) through a fuse bit on the ATmega. That’s a little circuit which prevents the ATmega from coming out of reset and doing anything if the voltage is too low.

As it turns out, you can turn off the BOD in software, but only for sleep mode. The reasoning being that there’s no point in protecting the processor from going berserk while it’s not doing anything in the first place…

Well, my code to turn off the BOD was wrong. You have to do this right before going to sleep. Here’s the updated powerdown_demo.pde sketch:

Screen Shot 2010 08 28 at 12.50.18

(correction – I mixed up my bits: change “PINB |= …” to “PINB = …” or “PORTD ^= …”)

The result?

Dsc 1872

Now we’re cookin’ again!

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