>
> > 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.

According to the data sheet that's not how the part works. It has a comparator
that detects the voltage dropping. When it sees the voltage dropping it then
kicks the inductor/switch/oscillator in to boost the voltage. Then it turns
off again until the voltage drops below the threashold again. It short it
refills the glass when it runs low. There's 50 mV of hysteresis so that
the comparator doesn't go nuts trying to run the engine all the time.

It said at low current draws that it'll operate in quiescent mode for
several seconds, then kick in for a few cycles to refill, then go back to
sleep.

It seems perfect for the application I have. It's lazy and power conserving
when its load is power conserving by can deliver up to 80 mA instantly on
demand running from 2 AA batteries. In their design note 63 they show an
example giving 26 hours run time on a 30 mA load. My design goal is a load
between 150 and 200 uA. This projects to between 160 to 220 days runtime.

I certainly can live with changing batteries once every 6 months.


> 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.

Surface mount? Artwork? As you point out it's a one off. Most likely I'll
wirewrap the unit. I may stick it on a 1/2 RatShack preetched board simply
because they have a board/case combo that seems to fit the bill. Planning on
using a standard radial tantalum. This is definitely hobby work. No
professionalism required, or allowed ;-)

BAJ

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