Thread: Repaired Harbor Freight digital caliper
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Winston Winston is offline
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Default Repaired Harbor Freight digital caliper
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!


* 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. 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.


(*) (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)

--Winston
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