Justblairs Audio and Electronic's Pages

I recently saw a couple of articles on Hackaday regarding the modification of cheap digital callipers.  It turns out that as well as being able to interface these callipers to a microcontroller such as an Arduino, there are some hidden functions that are simple to unlock.  I was curious because I happen to own a very similar set of callipers already.  Well I could not see a perfectly good tool go unmodified for long. 

Not only that, as impressed as I was by the hacks that have been done already on digital callipers, the execution has been pretty ghetto looking. I had an idea that would produce a more stock looking finish that I think is more durable.

The digital callipers that I modified are of a design that you see all over.  Available from E-bay for modest sums they have served me well for the last couple of years.  Mine was originally purchased from my local LIDL store. 

The callipers are branded “Powerfix(R) ELECTRONIC DIGITAL CALIPER” on the ruler.  On a sticker on the back of the callipers the details read:

“Model No: Z22855
Version: 07/2008
Battery Type: 3v-CR2032
OWIM GmbH & Co. KG
Stiftsbergstraβe 1
D-74172 Neckarsulm”

Although German branded, it’s highly unlikely that the origin of these callipers is anything other than China.
 
What makes these callipers so easy to hack is the provision of a four pad port on the top of the calliper.  This is hidden behind a small pull off tab in the case.  I was lucky in that all of the hard work has been done for me here.  Not only have the tabs been identified by others, but the use of them has also been deciphered.

I have added some links to useful sites at the bottom of this article which give an insight as to how to interface microcontrollers to these and similar electronic digital callipers

The pads provide 1.5v power out, clock out, data out, and ground.  It is using these that I eventually hope to interface the Digital Callipers with an Arduino microcontroller platform.

Disassembly

To begin the modification I had to disassemble the digital callipers.  To do so I removed a  sticker that covered the back of the electronic digital callipers.  This reveals four small screws on which hold the stainless steel calliper jaws to the display and control module.

I noticed that one of the four screws has a finer thread than the rest, its purpose it would appear is to ensure that the steel jaws are connected to the electronic digital callipers circuit ground.

If you are completing this modification on a similar set of callipers, keep an eye out for this as it is likely to affect the operation of the device.

 
With the screws removed the callipers separate into two pieces.  The electronic digital callipers operate by capacitance sensing. 

The circuit boards held in place with some metal screws.  Removing these allowed me to take out the circuit board and have a better look at the pads I was going to add my breakout header and switch to…

Be careful if you remove this  printed circuit board from the callipers plastic casing as the  rubbery buttons that operate the callipers are pretty small and hard to find on the floor beneath your work bench (Well they were in my case).

The pads can be seen in the picture to the right.  There are six in total, but the two fat pads on the left and right we are not going to worry about.  It is the four in the middle that we are going to solder to. 

There is going to be a four terminal header attached to these pads for attaching the electronic digital callipers to out microcontroller. From left to right (as seen on my picture —> ) these pins are:

1. Ground
2. Data
3. Clock
4. 1.5V Power

It has also been discovered however that the same pads when shorted in various ways can control the functions of the device.  Most interesting is when we short pins 2 and 4 of the callipers.  This unlocks new features that have not been supplied originally on the callipers by the manufacturer.

In my modification a push switch is being added that will add max, min and measurement hold features to my callipers, not bad when you consider the extra parts required cost pennies!

The Modification

These are the two parts that I added to the callipers, In previous modifications of similar callipers I have seen RJ11 connectors or surface mount headers added.  These modifications although functional look a little out of place, The headers I chose, being black and roughly the right size to fill aperture left once the the pull out tab is removed.

The header is a five pin header, A four pin header of course is what I really needed, but I could not find one in my parts bin.  Instead I simply removed one of the pins and then filed everything smooth with my diamond coated file set.

The push switch is a through hole two pin part which is perfect for my purpose. 
I had to do a bit of shaping to get my now four pin header to fit to the plastic case of the electronic digital callipers.  This was not especially difficult using a decent quality file set.

I kept offering the header up, shaping, offering up etc. until I got a snug fit.  I was hoping to make the modification look as stock as possible.  The switch is always going to look tagged on, but the header once glued in place looks pretty much part of the original black plastic casing.

The inside of the original plastic housing required a little shaping to take the pins of the header.

I also found that the printed circuit board of the digital calliper was a very snug fit inside the case.  This was going to cause problems as I was going to have to route some cables from the header pins to the pads on the printed circuit board. I found that the diamond coated files  made light work  of shaping the printed circuit board.

 
I wanted to minimise the amount of soldering that I would have to do in situ, so I decided to prepare the header and push switch prior to fitting them in place.  You can see on the picture to the right that I organised the two at right angles to each other.

Before soldering them together in this position, I reshaped the base of the header  with two grooves that take the legs of the push button switch so that when fitted to the electronic digital calliper’s case, there would be as close a fit as possible.

When I was happy with the positioning of the header and push button switch to each other (and had test fitted them again in their final positions!) I soldered them together and trimmed the legs of the switch off.
With this done, I bonded the header/switch assembly into place within the electronic digital calliper’s case.  I used common all garden super glue for this job and it seems to have made a secure and seamless bond

With the assembly  now in place and the glue gone off, I soldered very quickly some fine kynar wire in place on the terminals of the now added header.  Its pretty tight in there and the fine hook-up wire takes very little space..

The kynar wire is also tin coated, making it pretty easy to solder to.

The last thing to do then before reassembling the electronic digital calliper was to add the printed circuit board back in and solder the kynar wire to the pads. 

Then I reassembled the callipers and tested them.After my first go I was finding that the callipers worked mostly, but were a little unreliable, stripping them down and adding some insulating tape over the soldered pins resolved this issue.

The pushbutton now works as hoped.

A single push changes the mode of the electronic digital callipers  from the original measuring mode into  a super sampling mode where the zero button freezes the measurement rather than zeroing the display as it did before.

A double tap on the newly added push button puts the electronic digital callipers into a Max mode.  On this setting the callipers only display the maximum reading they have registered, handy for finding the widest point in an item for instance.

A triple tap of the new push button selects Min mode on the electronic digital callipers.  In this mode the callipers will measure the smallest gap in the jaws to measure say an objects narrowest point. 

Others who have done this modification have found that when entering these hidden modes a previously unused indicator on the display appears, but it would appear that my set of electronic digital callipers are fitted with a more rudimentary display that does not have the indicator

Conclusion

Even if we just consider the additional functions that are now unlocked in this cheap set of electronic digital callipers the completed modification can only be judged a success.  But there is more to uncover.

My next task is to tap into the new header and somehow read the signal coming out with my Arduino.  I have been playing around with this without too much luck so far.  The signal that comes out can be read easily with an oscilloscope and the serial data stream is easy to spot.  However it outputs at 1.5v .

I have experimented with attaching interrupts to the Arduino’s analog comparator to attempt to read the bits being broadcast but without too much luck.  My next attempt is to use some transistors to amplify the signal to 5v and use the interrupt pins on the Arduino to detect the clock signal and read the data. 

I will keep playing with this and update the site with more as I find it out…

In the meanwhile  if you are interested in doing this modification I suggest the following sites for information that I have found useful so far:

• Robocombo - Interfacing TI Launchpad to Digital Calliper
• PCB Heaven - Digital Calliper Protocol
• Hackaday - Hold, fast, and max features on a digital calliper  (The comments are especially worth reading)