Digital Caliper Guide – ElectroSchematics.com

I recently picked up one of my digital calipers from the toolbox to replace its battery, but found the liquid crystal display panel was damaged.

I know that the “oval black patch” issue, making the display unreadable, is common on most passive monochrome LCDs. There are some interesting ideas to revive it.

This dark oval issue can be narrowed down to the reflective layer/film at the bottom of the LCD assembly. This is because the bottom reflective layer has simply lost some of its shiny paint due to age and temperature, hence unable to reflect light properly. Although we can manually replace the reflective film sometimes, it’s probably more practical to replace the display or buy a new device.

Instead of repairing the display panel, I decided to disassemble the device to see the electronics and find ideas for tinkering. This post is about those attempts. Hope you like it.

Digital Caliper – System Overview

First, note that a Digital Caliper is a precision measuring instrument for recording highly accurate measurements. From what I can tell, the dial caliper and vernier caliper, this digital version does not have unlike the traditional rack and pinion mechanism, instead, it takes measurements with the help of a series of capacitive sensors that run along the length of the beam.

The movable jaw (slider) houses the entire electronics of the digital caliper inside a rigid plastic box.

As you can see below there’s a special circuit board on the underside of the movable jaw, ie, at the bottom of the main circuit board housed inside the plastic case. Now, when you look at it, you can easily spot a nice printed pattern there.

There are several rectangular patterns etched onto a copper strip underneath the main scale (the part of the caliper along which the read head/slider moves). With the printed patterns on the bottom of the main circuit board, this forms a grid of capacitors. As the sliding jaw travels along the main scale, the capacitance of the printed patterns changes, the controller (chip-on-board) detects the resulting shifts in electrical charge, processes the signals and finally displays the readings on the LCD.

Obviously, this is not an absolute sensor, it’s relative, this means it needs to be zeroed before each use, but can detect small amounts of travel once calibrated.

Capacitive Sensing System – Chinese Explanation

After some Googling, I found a sensible custom description of how this type of digital caliper works. The snippet below is from here https://glanyi.en.made-in-china.com/company-Anyi-Instrument-Co-Ltd-.html

“Digital calibers contain Grid-Capacitance Linear Synchronous Sensors which are comprised of a set of grid-capacitances and signal processing circuit. The set of grid-capacitances consisted of stator grid-capacitance and sliding grid-capacitance. A pattern of grids is etched directly on the top copper layer of a standard glass-epoxy laminate of the printed circuit board stator. In the slider another printed circuit board contains a set of etched separate grids which are called emitters. The combination of these printed circuit boards forms two variable Capacitors. As the slider moves the capacitance changes in a linear fashion and in a repeating pattern. The two capacitances are out of phase. The signal processing circuit built into the slider counts the grids as the slider moves and does a linear interpolation based on the magnitudes of the capacitors to find the precise position of the slider.

The term “emitters” in the description above intrigued me, so I did a thorough search and quickly found that they’re actually “exciters” creating a multi-phase pulse width modulated signal capacitively coupled to the ground frame (not yet fully confirmed) .

(Also read http://www.yadro.de/digital-scale/working.html)

This is an old patent that describes this technique (translation required) https://patentimages.storage.googleapis.com/7a/1f/59/ee6810e17930e7/EP0184584B1.pdf

Battery & Power Consumption

Below is a close up of the circuit board. The caliper uses a 1.5V coin cell – LR44, but the battery lifetime is shockingly short. If you measure the current consumption, you will find that the circuitry draws a lot of current even in the power off state. As observed, the caliper’s circuitry never fully switches off, actually only the LCD that turns off, leaving the LR44 draining 24×7 (that’s why the battery dies quickly).

At this point, note that a relative-position caliper continues to work even when you switch off it. This is because it needs to be powered on at all times in order to remember the existent position, in order to display a change in position. If it’s to truly power off, you’d need to close the jaws together and re-establish zero each time you powered it on.

Data Output Port & Low Voltage Logic

Another observation is that this generic Chinese digital caliper does have a data output port at top of the enclosure. The data output port of the caliper is in fact a set of closely spaced circuit board traces located at the top right corner of the PCB.

It seems, most of the digital calipers output two low-voltage logic signals – clock and data. Using a multimeter, I easily figured out which one is V+ (+1.5V) and which one is GND (0V), but I could not nail down which line is the clock and which one is the data. I’ve searched the internet about digital caliper hacks and found the Clock line is the one near to the V+ track while Data line is the one near to the GND track (this also coincides with other’s observations on the web).

Data Output Port & Communication Protocols

The data output port is in fact a communication port for interfacing the digital caliper with a computer or other supported electronics. You could connect the data output port to a microcontroller and use the digital caliper as position sensor, for instance.

As I read, there’re basically three types of protocols for interfacing them. The first one is the “Digimatic” from Mitutoyo. The other two communication protocols are both for the Chinese calipers – “Chinese BCD” and “Chinese Binary” (the suffixes come from the way that the data are being transmitted).

(Also read http://pcbheaven.com/exppages/Digital_Caliper_Protocol/)

On a side note, BCD stands for “Binary Coded Decimal”. Using this scheme (a bit less compact than the straight binary representation), each decimal digit is represented using a nibble (4-bit). For example, decimal number 256 (or 2^8) in binary is 10000000. In BCD format it would be 0010, 0101, 0110 (2,5,6).

(Also read https://www.electronics-tutorials.ws/binary/binary-coded-decimal.html).

Data Output Port & Arduino Platform

Reading digital caliper from an Arduino is somewhat easy and straight forward. However, the low voltage logic circuitry (HIGH =1.5V) of the caliper makes it hard to connect its data port to I/Os of a regular 5V microcontroller like Arduino ((HIGH =3.5V). An easy approach here is to use a tricky level-shifter as shown below. Note that this setup will invert the signal so you need to flip it back again in the code.

Coming back to the communication protocols, what I loosely gathered on the internet, most of the generic cheapo digital calipers work like this:

The data is transmitted 8 times in a second. The measurement is transmitted as an integer that is 100 times the measurement in a 24-bit block with data being read on High to Low transitions of the clock signal. The clock is Low in idle state. Before data transmission it goes High, then data line is valid when the clock goes Low, clocking out all data in this fashion and goes back to idle state.

In other words, the Clock signal is taken down on every bit, and the Data is read during that High-to-Low transition period. The Data line is taken High or Low to the signal corresponding the bit. Since the Clock signal is being sent out as series of 24-bit bursts and then a silence (silence between the 24-bit bursts is about 115ms long), we get position updates about 8 times per second.

The first bit is a start bit which’s always High (just ignore it). The next bits, starting from second, form bits of number, starting from less significant bits on left, all up to the 21st bit, which indicates the sign (if it’s high, the number should be negative). The 21st bit is a sign bit, and the last three bits appear to be unused. There’s also a description of the protocol on an online guide that indicates the 24th bit is a flag for mm/inch mode. I didn’t probed this yet.

(Also read http://www.shumatech.com/support/chinese_scales.htm)

Luckily, someone has done the hard work of decoding the digital caliper communication protocol. Below you can find a few helpful resources.

— to be continued

Leave a Comment