On Nov 28, 5:17*pm, Winston wrote:
wrote:

(...)

Low-current switches can be a bear--the contacts oxidize. *Gold fixes
that generally, IIRC.

D.C. switches have their own complications, though
I suspect ~40 uA wouldn't be a problem, given the
proper contact plating and wiping action.
I think gold is gonna be problematical if there will
be much in the way of capacitor charging 'inrush'
needed.

TATOO: Look boss, deplate! Deplate!

E.s.r. of the cap I linked was 80 ohms. That just isn't a problem.


* Most super caps don't tolerate high current well.

No problem here. *The low battery voltage and high internal resistance
of these tiny cell-phone super caps limits the current nicely.

The super cap is in parallel with the low-z bulk
bypass caps in the caliper, though.

True. A series resistor is needed.

*Your
pushbutton will have to be properly sized and
snubbed. The folks at Palm found out about that
much too late.

* This mod is dead simple, reversible and does not
* * require access to a ground connection.

I like some sort of dead-man timer better. *It could be just a low
threshold mosfet with a gate cap which you charge, and a *large*
discharge resistor(*) so it times out. That guarantees the battery-
saving feature even if you forget.

I hear LTSpice calling. *
We need a pass element that has a gate voltage
saturation point in the 200-300 mV region.
I just don't see a MOSFET in that role, somehow.

The cell voltage is 1.55v, and it's nearly dead @ 1.2v. A MOSFET with
Id = 100uA @ Vgs = 1.2v would be fine. A BSS138 is in that ballpark;
there might be better choices.




(*) (From the jellybean / junkbox standpoint, the discharge "resistor"
might be a reverse-biased rectifier's leakage.)

But yes, just adding a real switch is a huge improvement over not
having it.

A real switch also lets you hold the zero setting over night if you
want to. *I sometimes do that with the lathe, if, for example, I'm in
the middle of something when it's time to turn in.

You *do* need access to the underside of the cell, since that's where
the (-) contact is.

You can switch the positive side as easily if your
interposer board is thin enough.

Insert a very thin piece of double-sided printed
circuit mat'l, wire a tiny slide switch to both sides of that, and
Bob's yer uncle.

Yup. that is how we do it.
I used very thin double sided stock, though
most of the time the 0.062" stuff worked fine.
It is great for measuring current too.



That's reversible, and if you're a brute, you can even hang the switch
outboard by the wires. *That way there's no modification of the
caliper needed at all.

That's good enough and simple enough that I'll put 'er on the list.
I've got maybe a dozen of these (two on the lathe alone), and it'd be
nice not pulling the batteries (as I do now).

For your 'fixed' installations, you could solder
some small 'earphone' wire to the battery contacts
and use a huge, cheap external cell. (Huge = AA)

Yep. I mentioned that earlier. An Energizer lithium AA or AAA is
well suited, voltage-wise. It still needs strapping somewhere...I'll
be working in the shop today, so I'll cast an eye toward the lathe and
see if there's room.

--
Cheers,
James Arthur