03/05/2020.

Improvement of a digital caliper

I have owned this digital caliper for about ten years. It’s a very common model, although a quick search shows models lacking the Relative / Absolute measurement function which may come in handy. I don’t use it much. It works well, but it has a rather painful defect: it consumes battery, even display off. Basically, whether I use it or not, I have to change the battery every three months! The ON / OFF button doesn’t change anything.

Digital caliper

The easy way out is to remove the battery after each use, but it’s not practical. I was fed up quickly, that’s why I studied the possibility of installing a switch, a real one!

Specifications

My specifications — a very pompous term here — were quite simple: the least modification possible, the least clutter. The ideal location is behind the small shutter which hides a connector to transmit data, possibility that I don’t use, like most people I suppose. It remained to ensure that the intervention was possible without risk and without too much difficulty.

Which switch to use?

We need a slide model, two positions, unipolar, very small footprint. Here is a possible choice of suitable models.

ALPS SSSS912500 switch diagram

Here are the characteristics of this switch (data sheet in pdf). Its size is (L × W × H) 9 × 4.7 × 5.5. Note: the one I used is only 3.5 mm wide, but I couldn’t find this model again.

C&K OS102011MS2QN1C switch diagram

Here are its characteristics. Its dimensions are almost identical to the previous one. It will only be necessary to cut the body pins. It can be found at TME.

Disassembly of the caliper

First of all, we must remove the battery and the connector cover.

The electronic body is dismantled by five screws hidden behind the rear label, which must be removed. This is done well by sliding a cutter blade.

Showing the body screws

There are four screws for plastic (red) and one for metal (yellow), the last near the wheel.

View of fixing screws

The opening reveals the PCB which is itself fixed with four small screws.

View of the PCB

The push buttons and the display are connected to the circuit by rubber contacts. No problem at this level, but avoid touching these contacts and those of the PCB (comb-shaped) so as not to clog them. These devices must be removed to allow working on the housing. Pay particular attention to the display’s position before removing it.

Body preparation

Depending on the width of the available switch, it will be necessary to enlarge more or less the opening of the connector cover.

Machining of the body

Machining of the body

PCB preparation

The central battery contact must be de-soldered, as it will now be connected via the switch. It is held in place by a piece of double-sided tape, and rotated 90° so that the wire connection is as short as possible and does not pass through the battery compartment.

Displacement of the central battery contact

Switch placement and soldering

The switch is soldered to the edge of the PCB by its extreme pins on the outer tracks of the connector (arrows). Only the one on the left in the photo has an electrical function; the one on the right is only used for mechanical support. This support is nevertheless weak and it will be necessary to handle the switch gently!

Soldering the switch

The central pin is connected to the battery contact by a small wire with the smallest possible cross-section. I used flexible wire with an outside diameter of 0.5 mm. Overview.

View of the modified circuit

Attention: in my switch, the pins are arranged in a triangle. If they are aligned, you have to bend the central pin and isolate the PCB tracks below.

If you see soldering joints on other tracks, it is because of tests that I did before realizing that there was nothing to cut! But it will obviously depend on your caliper’s circuit.

Reassembly and testing

There is nothing special to report. A piece of advice may be obvious, but never know: for the PCB as for the body, do not — moderately — tighten the screws until after having put them all in place and screwed without tightening them.

The battery has been going on for years now…

Device reassembled

Slide switch
Pos: 2; SPDT; 0.1A/12VDC; ON-ON; Mounting: THT
€2,06 € TTC per 5, price 2020
at TME.