Inside a cheap set of eBay digital calipers

Comments · 2.9K

• SH

  @shawnio · 3 years ago · pinned

  another completely pointless video for ad revenue

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• BA

  @badmonkey0001 · 5 years ago

  Has a screwdriver with a changable tip. Has more than one to avoid changing the tip...

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• SY

  @sylviaelse5086 · 5 years ago

  I deal with the lack of a hold function by just pressing zero, and closing it again. Then it shows the negative of what the measurement was.

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• AR

  @arenalife · 5 years ago

  Remember though, just because something displays to more decimal places, doesn't mean it's accurate...

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• SW

  @swagner58 · 5 years ago

  "I wonder if this pops off".  I've found that with (in)appropriate amounts of force and leverage anything will "pop off".  The tricky part is getting it to pop back on again.  : )

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• NO

  @NofewFudtefcity · 5 years ago

  The ending bit where you moved it as fast as possible to prove it wasn't a relative measurement has convinced me to buy one of these. The plastic, even on good models, always felt like cheap junk to me and I was sure there's no way it's actually as accurate as it wants you to believe.

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• PA

  @paulgrimshaw6301 · 5 years ago (edited)

  I looked into this technology a while back. &nbsp;Essentially what you have is a capacitive linear position sensor. &nbsp;It&apos;s not a vernier. &nbsp;The principal is that the teeth of the narrow comb on the sliding PCB are excited by the chip using sine waves. &nbsp; These teeth are in interleaved groups, and each group is fed a sine wave at a different phase. &nbsp;These teeth capacitively couple with the large comb on the measurement rule, which in turn capacitively couples back to the single long receiver bar back on the sliding PCB. &nbsp;The result is that the output from this bar is another sine wave, and critically the phase of this sine wave is determined by the position of the fine teeth relative to the large teeth. &nbsp;So a full 360˚ of phase corresponds with the pitch of the large teeth, 0.2 inch in this case. &nbsp;The chip measures this phase shift to determine the position relative to the set zero. &nbsp;So a 18˚ shift corresponds with 0.01 inch for example.<br><br>In fact in these cheap calipers it&apos;s all done digitally. &nbsp;The excitation is done using PWM coded sine waves, and in the example here they&apos;re fed in differential pairs, so that&apos;s 8 signals, four pairs, 45˚ apart. &nbsp;The output is then very spiky, but with a sine wave envelope which is recovered by digital filtering. &nbsp;Somebody scoped this version a few years ago - see https://web.archive.org/web/20120328090506if_/http://www.yadro.de/pics/hp-logic-analizer.jpg. &nbsp; &nbsp;You can see the four PWM sine wave differential pairs in the 8 digital traces at the bottom, and the single output signal before filtering at the top. &nbsp;Just imagine the peaks of the envelope of this signal are 0.1 inches apart and the signal shifting across with the slider and you&apos;ve got the idea.<br><br>The reason for 56 small teeth rather than just 8 is to multiply capacitance and therefore multiply the signal output, plus averaging of any small physical inaccuracies in and impurities around the teeth. &nbsp; So each set of 8 small teeth in fact precisely line up with each pair of large teeth.

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• JO

  @John_Ridley · 5 years ago

  So the capacitive fingers do work exactly like a traditional vernier scale, with the wave of lined up fingers moving across then restarting at the beginning again.

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• PA

  @paul756uk2 · 5 years ago

  I&apos;ve often wondered how they made these things and managed to make them so cheaply yet with an incredible degree of accuracy. Thanks for making this interesting video.

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• EL

  @ElectroDFW · 5 years ago

  I worked for a company for a a number of months repairing precision measuring equipment including calipers, depth gauges, height gauges, micrometers, you name it. Mostly mechanical, but also electronic versions. <br>What you were able to surmise and figure out &nbsp;from what you saw, without knowing the sorts of things I was taught during training, is amazingly accurate. <br>The main problem with those digicals is once they go wonky, there is no currently available way to repair them. <br>With the analog ones you can always regrind, retension, rerack and realign, but the digital ones go in the recycle bin.

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• MA

  @Manf-ft6zk · 5 years ago (edited)

  When these calipers came up we tested them with some small drops of water under the slider to see that they work with capacitive sensors. The instrument was in fact confused. After little drops had dried out it worked normal again.

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• JU

  @julianwilson5468 · 5 years ago

  After 10 years mine gave up and didn&apos;t switch on anymore. I purchased another one from ebay- a metal one. What arrived was trash. Astonishingly, I was able to transfer the display, pcb and housing to the old chassis. Everything was the same. I&apos;m astonished the parts we&apos;re interchangeable. Thanks Clive for giving me the confidence to even attempt the fix rather than send both to the bin

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