Repaired Harbor Freight digital caliper

The LED shunt regulator saves the 2uA, so it's simpler, cheaper, and even more efficient.

-- Cheers, James Arthur

When the photocell voltage output too low to power the calipers?

A timer. Caliper runs for 120 seconds and then shuts off. I have a few small battery operated devices like this that have no OFF switch. Just punch any button, and it turns on. Wait a while, and it turns off. My Central Tools "Storm" 3C301 cheapo electronic calipers has this feature.

The unintelligible owners manual proudly highlights this feature on the cover. Apparently, previous versions did NOT turn off automatically. Oddly, it still has an on/off push button, even though it will turn on if the jaws are moved.

Hmmm... the catalog says that it ships with an SR44/357 silver oxide battery. Yet, the one I purchased had alkaline LR44 batteries.

Nope.

I still think that wind up power would be more interesting.

But then the caliper forgets its "zero" every two minutes. That's very annoying.

The problem is that it's a much more complicated design, much more than a metalworking hobbyist could reasonably tackle to upgrade his HF cheapies.

If we're designing calipers from scratch, we'd just design them for

2uA total draw, not add an outboard switcher with Iq=3D2uA, then add a timer to turn it off.

Yep, that's still intriguing. As a practical matter, how do you turn spring tension into 15uA / 1.55V? A generator, an escapement, a coil and a magnet I suppose. Time to call a watchmaker.

-- Cheers, James Arthur

Mine doesn't. The initial position is apparently stored in the circuitry. I can also set it to any position, turn it off, and it will read the same value when turned back on. One of the benefits of having it draw power continuously.

Retrofits are always more complicated. I tend to think in terms of new product designs. If this were for a retrofit or modification, the design philosphy would certainly be different.

Agreed. At 2uA, it probably wouldn't need an on/off switch.

Pancake PM generator which offers the highest velocity at the edge. Basically a magnetized disk and a stator coil. Spring loaded spool parallel to the pancake motor and the same size. Ratchet to control direction. Inertial clutch and some plastic gears between the spool and the pancake motor. Much like a generator powered flashlight. Pull on the string and the spool spins, which causes the generator to run, charging a super-cap. If a pull string is too crude, a spiral mainspring mechanism can be added. If the pull string and generator are too sophisticated, a moving magnet inside a coil, that you shake back and forth to charge the super-cap, much like in some battery-less flashlights. Maybe a thumb wheel for spinning the pancake generator might better. Plenty of options.

I can grind the energy budget numbers (later) if you want. There's plenty of power available in string pull and wind up. However, without a backup battery, the stored zero setting will be lost when the super-cap discharges. Such is the price of entertainment value.

How about a weighted leaf switch in series with the cell? It could take the form of a thicker 'battery door'. The bulk bypass caps in the caliper would hopefully maintain power during vibration glitches and overhead measurements.

Place the caliper 'display down' and power is removed.

--Winston

(...)

Even if you stored the caliper 'face up' for a year, it would cost you no more than if you had no switch at all. Net net, battery life would increase because, the caliper would be turned off for a far longer time than it would be turned on.

I was appalled that I can't use my blender upside down too, until I realized that I never have a need to do that. :)

If you drive your car 24/365, you have much bigger efficiency worries than a $1.00 cell! :)

--Winston

This is what happens when we lose sight of the original problem that we're trying to solve. The problem is that the calipers have a short battery life mostly because they don't really turn off and secondarily because the dropout voltage is too high to use cheap alkaline batteries. Turning the calipers off (actually in standby) for a year won't do much, when the operating current is 14.5uA and the off current is 13.5uA.

You need to contact a product liability attorney, and sue for damages resulting from inverted operation. Be sure to mention that there was no warning in the user manual about inverted operation.

I've used calipers and other measuring instruments in probably every orientation possible. If the calipers turned off while I was making measurements in a contortionists position, I would be rather irritated.

True. Also, if I don't drive the car at all, I get an infinitely large gasoline mileage. Both extremes are worthless. I non-cleverly installed a "shake to unlock" (iShakeLock) application on my iPhone

3G. The standard method of unlocking the phone requires both hands. I figured that one handed unlocking would be better. The problem was that it kept unlocking in my pocket. Battery life deteriorated rapidly. So, I dumped the app.

Er. Jeff?

I was talking about the open circuit represented by a 'position' switch. Off current would be

*far* lower than 13.5 uA.

What proportion of the time do you measure with the display facing down? For me it is a very tiny amount of time. Like seconds-per-decade.

I own seven pairs of calipers, two of which are dial, two of which are solar, two of which are very cheap digitals and one is a vernier. Of them, only the cheap digitals would benefit from this conversion. I would select the proper measuring tool for the job.

You do as well, I trust.

I dunno. The 24/365 commute places an upper limit on the amount of time that the calipers would remain powered up. It probably would cost you say $1.20 a year to leave them 'rightside up' in your toolbox. I don't think of this as a huge toll, somehow.

Perspective!

You keep your iPhone next to your calipers in your pocket?

Don't the pointy 'inside' anvils smart a bit?

:)

--Winston

(...)

I like that! But I can reduce the BOM size. Just a (rotary?) switch in the battery cover.

If you leave it on, you get 10 months out of the battery. If you use the power switch religiously, you get 'shelf life'.

• A switch handling ~40 uA does not have to be nearly as stout as one handling 'short circuit' current into the super cap.

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

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

--Winston

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

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

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.

(*) (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. 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.

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).

-- Cheers, James Arthur

(...)

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!

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.

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. :)

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

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.

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

[big snip...]

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=20 years until I once again tried to use them and found out how to fix = them),=20 it seems that just removing the cell is best, and that guards against=20 possible damage if it leaks. My machine shop is in an unheated building = and=20 the temperature and humidity extremes may make leakage more likely.

Otherwise I think we have established that the normal lifetime of a 175=20 mA-hr cell, even with the typical 10-15 uA of on or off current, still=20 provides well over 1 year of life. Replacing it annually for $0.75 = compared=20 to once every 3-5 years of shelf life does not warrant any expenditure = of=20 time or materials for a high-tech (or even low-tech) solution. I've = already=20 spent an hour or more in this discussion. At my normal consulting rate, = that=20 would buy a hundred batteries!

But if removing the battery is too annoying, you might be able to cut a = thin=20 slot in the battery compartment, and just insert and remove a thin, = stiff=20 piece of mylar or similar insulating material to isolate the negative=20 terminal of the battery from the contact.

Paul

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.

Well, this is a *discussion* group. :-)

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).

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

-- Cheers, James Arthur

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 =3D 100uA @ Vgs =3D 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.

-- Cheers, James Arthur

(...)

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. :)

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:

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