On Wed, 23 Nov 2011 06:45:46 -0800 (PST),
wrote:

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

Yeah, but shunt regulators and leaky super-caps are not really
appropriate for micropower devices. They waste power.

In a previous message, James Arthur measured:
Drain: 13.5uA (off), 14.5uA (on)
Battery low threshold (blinking display): 1.37V
Lowest operating voltage: 1.01V

Nominal voltage on a silver oxide battery is 1.5V. Therefore, the
operating power is:
1.5VDC * 15uA = 22.5 microwatts.
From the standpoint of a resistive load, that's about:
1.5VDC / 15 uA = 100K ohms

The first question is whether a small solar cell will product 22.5
microwatts. Testing a somewhat oversized polycrystaline cell that I
found in my junk box (quality unknown), it produces 3.0VDC at 6ma with
a short circuit load (my milliamps guesser). My guess(tm) is that
this cell is about three times as big as will conveniently fit on the
calipers, so I'll just cut the current to 2ma . Delivered power with
my desk lamp is 6 milliwatts. Yeah, it will a 22.5 microwatt load.

The next question is for how long will it run? Assuming the calipers
can handle 3.0VDC without damage, how long will a junk 100UF
electrolytic cap run the calipers?
http://www.kpsec.freeuk.com/capacit.htm
From 1.37V is roughly 50% of full 3.0VDC charge. That's about 80% of
1RC time constant. 1RC is:
0.8 * 100K * 1000uF = 80 seconds
That's probably enough to make a few measurements. Any longer and a
super-cap will probably be needed. Picking 50% of full charge out of
the hat is rather convenient, as it makes the time to charge from zero
to the dropout point the same 80 seconds (yes, I'm lazy). Whether the
user really wants to wait 1.5 minutes under a desk lamp for the
calipers to be usable is dubious. Of course, a longer run time, means
a longer charge time. For example, a 1F 5V 1ua leakage super-cap,
will run the calipers for 80,000 seconds, but will also take 80,000
seconds to charge.

There are low voltage DC-DC boost/buck switching regulator chips
available that can tolerate a wide range of input voltages, and
deliver a constant 1.5VDC.

In my never humble opinion, what makes more sense is to do it exactly
like the typical solar powered calculator. They all have one or two
LR44 batteries inside. However, the solar cell does NOT charge the
battery. When you turn the calculator on, and there's enough light to
run from the solar cell, the battery is essentially disconnected. When
there's not enough light to run the calculator, it runs off the
battery. No waiting to charge a capacitor from the solar cell.

If you're into high tech, there are various energy scavenging devices
that can also power the calipers.
http://en.wikipedia.org/wiki/Energy_harvesting
With only 22.5 microwatts required, it might be possible to power the
device with a wind up key, piezo pressure, body heat, kinetic magnetic
generator, etc. I kinda like the idea of a wind up caliper.

Happy Day of the Turkeys.



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