> Obviously a wireless sensor will require battery power. So power
management
> is the critial issue. I wanted to get some opinions on the subject.
>
...
> But how would I start to determine the running time of such a setup? Let's
> throw out some numbers that I pulled off the RatShack specifications page.
>
> The nominal Ah rating of a AA alkaline is 1.7 Ah. I'm thinking of using a
> Linear Technology LT1173 switching regulator due to it's extremely low 110
uA
> supply current. My circuit will be driven by a 16F84 (simply because I
have
> them on hand) with the WDT enabled. The nominal current is 7 uA in this
case.
>
> So I envision the basic operation to be:
>
> 1) Sleep until the WDT goes off or get an RB0 interrupt indicating that
the
> door has been opened.
> 2) If the door heartbeat timer goes off, send an IR blip indicating a
heartbeat
> and reset the timer, go back to sleep.
> 3) If an interrupt, send a door open event, go back to sleep.
>
> My preliminary numbers indicate that 1700 mAH/.117 ma -> 14529 hours which
> is about 605 days. That's absolutely fine with me.

Hmmm... this doesn't sound right.  You say that the LT1173 draws only 110uA.
However, that is when the LT1173 is in shutdown.  If this buck regulator is
in shutdown, how can you keep your PIC alive?  The output voltage when the
LT1173 is in shutdown goes down to about 1.4 volts (1.7 volts minus a good
diode's drop) since there is a DC path through the inductor, and diode, to
other devices in your circuit.
I don't know what this switcher draws, as a minimum, when your load is only
the few microamps that a PIC draws when sleeping.  Often, these boost
regulators don't operate efficiently at *all* in the low end of their
current range, typically only providing 10-20% efficiency.  I don't see that
in your computations.  Unfortunately, I don't see a system efficiency curve
for this device in a quick perusal of the data sheet, so I would suggest
that you talk to an FAE, or perhaps do some prototyping to qualify this.

Also, as a "one of" project, you should be aware of the cretinous low ESR
surface mount tantalum capacitor supply crunch that exists right now.  We
are having a tough time getting unit quantities of these capacitors for
*our* buck regulator designs.  This regulator is extremely sensitive to the
quality of the capacitors you bypass your battery and output with.  *Do*
check for capacitor sources before committing artwork to make sure you can
stuff your circuit when you are done.

> BAJ
>

On the other hand, I am using an LT1304 to power my handheld device.  I have
been pleased with its performance thusfar.

My 0x02 cents.

-d

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