Computing stuff tied to the physical world

Zeners – success!

In Hardware on Nov 19, 2011 at 00:01

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:

DSC 2783

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:

Screen Shot 2011 11 17 at 03 22 27

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:

DSC 2784

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:

DSC 2786

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…

  1. YES!!!

    Very nice result. And there is no real good reason to go below 100uF, because a 10uf cap is not so much smaller or less expensive.

  2. JC, just to clarify – the latest results are with 27Vdc supply through 33KΩ ? The first trace shows a charging current ~0.46mA

  3. Hmm – actually with this range of reservoir (10-100μF) I expect it looks more like the Loch Ness monster. But I won’t spoil the next episodes… ;-)

    • What do you mean with ‘Loch Ness monster’?

      The whole power supply setup costs less than 50ct and fits easily on 10x10x5mm³.

  4. Sorry, a very English phrase – refers to the shape of the curve I expect to see (http://en.wikipedia.org/wiki/Loch_Ness_Monster)

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