On Sun, 27 Nov 2011 10:45:00 -0800 (PST),
wrote:

But then the caliper forgets its "zero" every two minutes. That's
very annoying.

Mine doesn't. The initial position is apparently stored in the
circuitry. I can also set it to any position, turn it off, and it
will read the same value when turned back on. One of the benefits of
having it draw power continuously.

The problem is that it's a much more complicated design, much more
than a metalworking hobbyist could reasonably tackle to upgrade his HF
cheapies.

Retrofits are always more complicated. I tend to think in terms of
new product designs. If this were for a retrofit or modification, the
design philosphy would certainly be different.

If we're designing calipers from scratch, we'd just design them for
2uA total draw, not add an outboard switcher with Iq=2uA, then add a
timer to turn it off.

Agreed. At 2uA, it probably wouldn't need an on/off switch.

I still think that wind up power would be more interesting.

Yep, that's still intriguing. As a practical matter, how do you turn
spring tension into 15uA / 1.55V? A generator, an escapement, a coil
and a magnet I suppose. Time to call a watchmaker.

Pancake PM generator which offers the highest velocity at the edge.
Basically a magnetized disk and a stator coil. Spring loaded spool
parallel to the pancake motor and the same size. Ratchet to control
direction. Inertial clutch and some plastic gears between the spool
and the pancake motor. Much like a generator powered flashlight. Pull
on the string and the spool spins, which causes the generator to run,
charging a super-cap. If a pull string is too crude, a spiral
mainspring mechanism can be added. If the pull string and generator
are too sophisticated, a moving magnet inside a coil, that you shake
back and forth to charge the super-cap, much like in some battery-less
flashlights. Maybe a thumb wheel for spinning the pancake generator
might better. Plenty of options.

I can grind the energy budget numbers (later) if you want. There's
plenty of power available in string pull and wind up. However,
without a backup battery, the stored zero setting will be lost when
the super-cap discharges. Such is the price of entertainment value.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558