Repaired Harbor Freight digital caliper

[DoN. Nichols]

"Winston" wrote in message
[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.

 

[Winston]

[email protected] 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

 

[Steve W.]

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



True. A series resistor is needed.


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.


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.

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.

 

[Winston]

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]

(...)


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