On Nov 22, 9:21*pm, "DoN. Nichols" wrote:
On 2011-11-22, wrote:

I saw a solar-powered Mitutoyo at a flea market and was intrigued, but
resisted. *Something about having a glass window on a machine tool
close to all that hard steel just didn't sit well--it brought up
memories of scratched and broken watch crystals.

* * * * I've seen them only new, and decided that in my shop conditions,
they would not work very well. :-)

OTOH, I paid $10 for most of my HF calipers. *Those you can take
chances with. *Using the 8x25mm solar panel from a $1 calculator, a
super capacitor for storage, and an LED as a regulator diode was my
notion. $2 in parts, $500 labor ;-).

* * * * O.K. *Do you know the maximum voltage that the solar panel is
likely to produce? *And the voltage drop on the LED? *I know that
silicon diodes are typically between 600 mV and 750 mV. *Also, any clues
as to the maximum voltage that the calipers can tolerate long term?

The max. solar panel voltage doesn't matter--the LED regulator clamps
the solar panel voltage, taking care of that.

Here's a sketch:

D1
.--||--+-----+---- (+) to caliper
+ | | |
.------. + | |
| | --- ---
| ---- | --- \ / ~~
| | C1| ---
| ---- | | | LED (red)
| | | |
'------' | |
- | PV | |
'--------+-----+--- (-) to caliper


C1 - A capacitor to power the caliper during momentary outages.
Optionally a super-cap., e.g. http://www.goldmine-elec-products.co...?number=G13133
(Note1: The PV will take HOURS of bright light to charge a super-
cap, so you might want to precharge the cap, then let the PV just
float it.)
(Note2: Super-caps have significant electrical leakage. If your
cap is too leaky, it'll never charge, and you'll be disappointed. I
haven't measured the above-linked PAS920 to see if this is the case.)

PV - http://www.goldmine-elec-products.co...?number=G17448,
or, stolen from the cheapest solar-powered calculator you can find.
Walmart and the dollar stores have some good candidates.
LED - shunt-regulates the PV output to a safe voltage.
D1 - 1n4148, prevents PV from draining a super-cap C1 when dark,
possibly not needed. (Depends on PV panel's dark leakage current.)


To set the voltage you'd choose an LED with a forward voltage of, say
1.6-1.8v. Three ordinary small-signal silicon diodes in series (e.g.
1n4148) wouldn't be a bad choice either. With surface-mount parts,
the ckt can be tiny.

If the voltage is too high it's not a matter of "long-term," the
caliper will die instantly. I don't know what that voltage is, but
I'd easily wager one of *my* $10 calipers that 2v is okay, and I'd
wager one of *yours* that 2.5v might be okay too. ;-)

--
Cheers,
James Arthur