Repaired Harbor Freight digital caliper

[Winston]

dagmargoodboat@yahoo.com 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

 

[P E Schoen]

"Winston" wrote in message news:jb11ah02pos@news3.newsguy.com...

[big snip...]

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

I didn't expect my post to generate such a big discussion, but it's been
quite interesting. Since I only use my calipers once in a while (it had been
years until I once again tried to use them and found out how to fix them),
it seems that just removing the cell is best, and that guards against
possible damage if it leaks. My machine shop is in an unheated building and
the temperature and humidity extremes may make leakage more likely.

Otherwise I think we have established that the normal lifetime of a 175
mA-hr cell, even with the typical 10-15 uA of on or off current, still
provides well over 1 year of life. Replacing it annually for $0.75 compared
to once every 3-5 years of shelf life does not warrant any expenditure of
time or materials for a high-tech (or even low-tech) solution. I've already
spent an hour or more in this discussion. At my normal consulting rate, that
would buy a hundred batteries!

But if removing the battery is too annoying, you might be able to cut a thin
slot in the battery compartment, and just insert and remove a thin, stiff
piece of mylar or similar insulating material to isolate the negative
terminal of the battery from the contact.

Paul
www.pstech-inc.com

 

[DoN. Nichols]

On 2011-11-29, P E Schoen <paul@pstech-inc.com> wrote:

"Winston" wrote in message news:jb11ah02pos@news3.newsguy.com...

[big snip...]

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

I didn't expect my post to generate such a big discussion, but it's been
quite interesting. Since I only use my calipers once in a while (it had been
years until I once again tried to use them and found out how to fix them),
it seems that just removing the cell is best, and that guards against
possible damage if it leaks. My machine shop is in an unheated building and
the temperature and humidity extremes may make leakage more likely.

FWIW -- this discussion has prompted me to check out the B&S
Digital calipers which I have (two, only one of which I have run on
batteries, because the proper batteries went to unobtanium before I got
the second one (for free) -- each in its own fitted wooden box. The
first cost me $15.00 at a hamfest.

The original batteries were the 625 Mercury cells -- weird
format and of course a lower voltage than most others -- 1.35 V when
new.

The caliper has a glass optical scale down the groove where the
depth gauge lives and the rack gear is on a dial caliper, so it is one w
which I trust to be accurate.

It used four of those cells. I figured that if the electronics
and LEDs (used to illuminate the glass scale) could tolerate 6 V instead
of the 5.4 V with the original cells, I could likely use a pair of CR 2032
3V cells. Well ... a bit of work on the electronics bench shows that it
will indeed tolerate the higher voltage. It also draws 1.4 mA instead
of the 40 uA range that the newer calipers use) so I really plan to add
a switch to the battery holder.

I think that all I need to do is remove some of the jumper
strips which went between cells, and make a new screw-on cover for the
new cells to hold them in proper alignment.

If so -- it will add two extra calipers to my collection.

Enjoy,
DoN.

--
Remove oil spill source from e-mail
Email: <BPdnicholsBP@d-and-d.com> | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

 

On Nov 28, 8:40 pm, "P E Schoen" <p...@pstech-inc.com> wrote:

"Winston"  wrote in messagenews:jb11ah02pos@news3.newsguy.com...

[big snip...]

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

I didn't expect my post to generate such a big discussion,

Well, this is a *discussion* group.

but it's been
quite interesting. Since I only use my calipers once in a while (it had been
years until I once again tried to use them and found out how to fix them),
it seems that just removing the cell is best, and that guards against
possible damage if it leaks. My machine shop is in an unheated building and
the temperature and humidity extremes may make leakage more likely.

Otherwise I think we have established that the normal lifetime of a 175
mA-hr cell, even with the typical 10-15 uA of on or off current, still
provides well over 1 year of life. Replacing it annually for $0.75 compared
to once every 3-5 years of shelf life does not warrant any expenditure of
time or materials for a high-tech (or even low-tech) solution. I've already
spent an hour or more in this discussion. At my normal consulting rate, that
would buy a hundred batteries!

Of course. The goal is to avoid spending an hour of your time at your
consulting rate finding a new silver oxide cell, after you reach for
the thing and it's dead.

Also, the prevailing rate is $3 per cell (or at least it was, before
we all revealed our fave cheaper sources).

But if removing the battery is too annoying, you might be able to cut a thin
slot in the battery compartment, and just insert and remove a thin, stiff
piece of mylar or similar insulating material to isolate the negative
terminal of the battery from the contact.

That's the ultimate cheap switch. The body, FWIW, is the (+) contact.

--
Cheers,
James Arthur

 

On Nov 28, 5:17 pm, Winston <Wins...@BigBrother.net> wrote:

dagmargoodb...@yahoo.com 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

 

[Winston]

dagmargoodboat@yahoo.com wrote:

(...)

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.

Hopefully! I don't see a gate threshold figure for Id ~100 uA but the
worst case (Id=1 mA) gate threshold for that part is 1.5 V.
http://www.fairchildsemi.com/ds/BS/BSS138.pdf

I like an elastomer carbon button driven by a threaded
knob that makes contact between the '+' side of the
button cell and the caliper cell contact instead:


| <---- Caliper
| '+' contact
----
| /--------- <-- Single sided
.---. | |.--------. Flex PCB
| | | || | interposer
| | | || ..
| P | | || ||
| o | /\/.-.|| ||
| w |-\/\/| ||| ||
| e | | ||| ||
| r | | | | || Caliper
| | | | | Button || '-' contact
| K | | | | ||
| n |-\/\/| | | Cell ||
| o | /\/'-' | ||
| b | | | ||
| | | | ||
| | | | ||
'---' | | ''
| | |
| '--------'


(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


--Winston




--Winston

 

On Nov 29, 10:05 am, Winston <Wins...@BigBrother.net> wrote:

dagmargoodb...@yahoo.com wrote:

(...)

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.

Hopefully!  I don't see a gate threshold figure for Id ~100 uA but the
worst case (Id=1 mA) gate threshold for that part is 1.5 V.  http://www.fairchildsemi.com/ds/BS/BSS138.pdf

Id=1mA @ Vgs=1.3V (typical). That's plenty. There are certainly
others, but Fairchild's search function doesn't work for me.

I like an elastomer carbon button driven by a threaded
knob that makes contact between the '+' side of the
button cell and the caliper cell contact instead:

                       | <----  Caliper
                       |        '+' contact
                    ----
          |     /---------  <-- Single sided
   .---.  |     |.--------.     Flex PCB
   |   |  |     ||        |     interposer
   |   |  |     ||        ..
   | P |  |     ||        ||
   | o |  /\/.-.||        ||
   | w |-\/\/| |||        ||
   | e |     | |||        ||
   | r |     | | |        || Caliper
   |   |     | | | Button || '-' contact
   | K |     | | |        ||
   | n |-\/\/| | |  Cell  ||
   | o |  /\/'-' |        ||
   | b |  |      |        ||
   |   |  |      |        ||
   |   |  |      |        ||
   '---'  |      |        ''
          |      |        |
          |      '--------'

(created by AACircuit v1.28.6 beta 04/19/05www.tech-chat.de)

--Winston

The knob could drive a screw, compressing a leaf spring contact onto
the cell. Or, use a slide mechanism, if you prefer. Viola.

--
Cheers,
James Arthur
> --Winston

 

[Steve W.]

dagmargoodboat@yahoo.com wrote:

On Nov 28, 5:17 pm, Winston <Wins...@BigBrother.net> wrote:
dagmargoodb...@yahoo.com 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

You can get VERY tiny slide switches from many sources. Then you install
the switch in the spot on top of the caliper with the contact pad. This
way you just switch the battery in/out of circuit. They last longer that
way.

--
Steve W.

 

[Winston]

Steve W. wrote:

You can get VERY tiny slide switches from many sources. Then you install
the switch in the spot on top of the caliper with the contact pad. This
way you just switch the battery in/out of circuit. They last longer that
way.

D.C. ratings are really important here.
There are really tiny switches available as you say
but few will tolerate many activations that include
a current spike measuring at multiples of their
D.C. capability.


--Winston

 

[Winston]

dagmargoodboat@yahoo.com wrote:

On Nov 29, 10:05 am, Winston<Wins...@BigBrother.net> wrote:

(...)

I like an elastomer carbon button driven by a threaded
knob that makes contact between the '+' side of the
button cell and the caliper cell contact instead:

|<---- Caliper
| '+' contact
----
| /---------<-- Single sided
.---. | |.--------. Flex PCB
| | | || | interposer
| | | || ..
| P | | || ||
| o | /\/.-.|| ||
| w |-\/\/| ||| ||
| e | | ||| ||
| r | | | | || Caliper
| | | | | Button || '-' contact
| K | | | | ||
| n |-\/\/| | | Cell ||
| o | /\/'-' | ||
| b | | | ||
| | | | ||
| | | | ||
'---' | | ''
| | |
| '--------'

(created by AACircuit v1.28.6 beta 04/19/05www.tech-chat.de)

--Winston

The knob could drive a screw, compressing a leaf spring contact onto
the cell. Or, use a slide mechanism, if you prefer. Viola.

I see the interposer flex PCB as a 'T'- shape in plan view.
The horizontal arm goes between the caliper's '+' contact and
wraps around the body of the cell. The vertical arm is bent
90 and can be enlarged to provide adequate contact for the
elastomer switch element.

I'd call for 2 oz. copper with heavy tin plating so that the
part would keep it's 'bend' and not tear under torque.

Alternative embodiments including one in which the power
knob houses an external cell are included by implication.

The cover would be slid in place without the knob and then
the knob would be screwed in; it'd be removable for cleaning.

A ratcheting torque-limiting clutch
inside the knob is left as an exercise....

--Winston