Someone drew my attention to a very small PIR sensor on eBay:
The size is great, of course – but the current consumption isn’t: I measured 1.9 mA idle current @ 5V.
The other inconvenience, in the context of JeeNodes, is that this sensor expects a 5..9V supply voltage.
Using my new accurately adjustable power supply, I was able to establish that it actually works all the way down to 3.6V – with current consumption down to 1.3 mA. But that’s still far from the 50 µA current consumption of the PIR sensor used in the Room Board, so this rules out ultra-low power battery nodes. The detection range is specified as 2 to 3 m, not stellar but probably enough for many uses.
Here are the two sides of this really tiny sensor (whoops, I accidentally cracked the lens):
The chip marked 7144-1 appears to be a 4.4V LDO regulator, with excellent < 5 µA idle current and 0.06 V drop-out voltage under light load, but that seems to point to a circuit which really expects to run at 4.4V internally.
I have no idea what this spec means on the eBay page:
It’s definitely not referring to the idle power consumption of this sensor. Too bad!
Should we try and design our own PIR sensor? I wonder what that would take – some way to stabilize on an average detector level, and then detecting changes in that value? Using an ultra-low power op-amp?