Giving Digital Calipers Bluetooth

[Fede]’s wife uses a pair of digital calipers to take measurements of fruits, leaves, and stems as part of her field research. Usually this means taking a measurement and writing it down in a log book. All things must be digitized, so [Fede] came up with a way to wirelessly log data off a pair of cheap Chinese calipers with a custom-made Bluetooth circuit.

Most of these cheap Chinese digital calipers already have a serial output, so [Fede] only needed to build a circuit to take the serial output and dump it in to an off-the-shelf Bluetooth module. He fabbed a custom circuit board for this, and after seeing the increased battery drain from the Bluetooth module, decided to add an external battery pack.

In addition to etching his own board for sending the serial output of the calipers to a Bluetooth module, [Fede] also put together a custom flex circuit to connect the two boards. It’s just a small bit of brass glued to a transparency sheet etched with ferric chloride, but the end result looks amazingly professional for something whipped up in a home lab.

28 thoughts on “Giving Digital Calipers Bluetooth

  1. Summery has a minor inaccuracy. The calipers do not put out normal serial so you cannot just hook it to a bluetooth module and go. There is a small micro controller in between that coverts from the scale protocol and normal TTL-232 serial protocol.

    In the article they mention there are only 2 formats for the cheap scales (BCD and 24bit). Well that is not quite correct there are 3 major formats in use on cheap scales BCD, 2x24bit (article covers 2×24), and 1×24 (used by cen-tech scales sold at harbor freight). If anyone is trying to use the cen-tech or related scales and hitting issues (will not work on a 2×24 decoder since half the bits are missing) if you look half way down this page there is a file that will do the conversion from 1×24 to TTL-232.

    1. Valuable data indeed… was thinking of using my Harbor Freight calipers and wondering if there was any possible way they might have messed things up/done things differently! You just saved me a lot of time! Thanks!

      1. No problem that’s exactly why I left the message :)

        There is a DIY mill/lathe DRO called YADRO ( that takes cheap scales + a pc and gives you a full featured DRO readout. I got quite a ways into that project before I figured out that the cen-tech scales did not conform to the “standard” cheap 2×24 protocol. Had to do a bit of revere engineering on them to get the format figured out and built 2 adapters for them. one converted from 1×24 to 2×24 and the other just converted to serial (mostly just for my early debug but put it out there for anyone else who might need it)

        With anything that comes out of harbor freight what you get now may be different from what you got a week ago. If you happen to have a dual channel oscilloscope (just about any speed) the best bet is to just scope the clock and data lines out of the scale before building the adapter. 1×24 and 2×24 are very easy to tell apart (24 bits small gap 24 bits large gap vs 24 bits large gap).

  2. The whole project and write-up is great! A real useful tool for citizen scientists and underfunded institutions.

    What stands out is the custom flex cable. 100% BRILLIANT. What was once often an “if you break it, the whole thing is trash” component, can now be easily replaced with a custom-made replacement. Hurrah!

    1. Why? so you can see how many transcription errors you can introduce into the results? Or to get a smaller sample size (as sampling takes longer).

      I can’t see any reason why you need your results on a bit of paper before you have them in electronic format.

      I’m pretty sure the people at CERN don’t run around writing down particle interactions on bits of paper before typing it into a computer.

      There’s NEVER room for blanket statements!

      1. The scientist a CERN indeed do run around a write stuff down. Weather its data or their lunch order. You have to realize recording data is part of the scientific method. So if they are trained properly they will record their data in more than one place.

        1. Recording data * accurately* is most definitely part of the scientific method.

          Given the atlas detector alone generates one petabyte of data per second, adding humans writing on paper to the loop is left as an exercise for the reader.

          As far as lunch orders and data goes i think you make my point perfectly
          “wait no, it was just some crumbs from my jam doughnut in my notebook”

      2. >>>>
        There’s NEVER room for blanket statements!

        Your blanket statement can be easily disproved as there are times when blanket statements are quite appropriate. Like when the outside temperature drops below 40 degrees F, the furnace goes out and the repair man can’t come until tomorrow. Yep, your likely to make extra blanket statements then.

    2. While using this tool, it becomes impossible to log the data in the notebook first as the data is electronically transmitted to the computer or tablet first. Maintaining the notebook makes sense as it allows for verification of the digital data and also provides the check and balance that is necessary to insure the data that is transmitted via Bluetooth is accurate and not corrupted.

      1. The leading cause of errors in data entry is human error, in the original process you have 4 opportunities for human error, read caliper write notepad (optional drop notepad in bear crap) read notepad write computer.
        If you have the bluetooth system the base layer will automatically checksum and retransmit bad data or failing that a simple checksum algorithm will ensure no data corruption will occur over the life of the universe.
        Any opportunity to remove humans from the process of taking data will generally be a good one, particularly when they are in a tree canopy.

    3. Thats odd because when I was doing my PhD i was sampling data at around 10,000 data points per second. Now, i don’t know about you but I can’t write that fast and with around 200 hours of testing performed that’s 7.2 billion data points. I didn’t think that would fit in my note book so instead I used digital acquisition to record the data then imported it into matlab where I could analyse it.

      But then maybe I’m just lazy and they should have refused to give me my PhD because I didn’t spend hundreds of years continuously writing down data points without a break for food or sleep

      1. No, I would have not given you a PhD because you use an example that is totally useless in the context of this application. The fact that you can trust an automated system in the controlled environment of a lab doesn’t mean you can do the same thing in the forest. Batteries go dead, your touch screen doesn’t respond because its cold, or you can’t see it because of the glare, or it simply starts raining. I’m not saying don’t collect data electronically but your attitude is the reason lab people think they come up with great ideas and they end up being useless in the field.

  3. This sounds like it would be a perfect application for Bluetooth Low Energy – relatively infrequent transmissions of tiny amounts of data from a device that’s powered by a tiny battery. It’s a shame that a lot of hardware doesn’t support it yet.

  4. Your can buy Mahr Federal wireless caliper for $214. 3 year battery life. Waterproof and dustproof (IP67 rating). USB antenna costs $150. Handles up to 8 devices at once. Uses ANT technology, not Bluetooth, hence the long battery life.

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