I found an old optical PC mouse, and decided to check what's inside. Everything seemed as usual, but there is a component I've never seen in other mice, and I don't understand its purpose (see photo below).

It looks a lot like a water sensor. However, I don't see the point of having a water sensor inside the mouse: it's not like it is an object which frequently gets some water inside.

The resistance at the edges of each “Ш” is 350 Ω. The first connector goes to left, right and middle buttons of the mouse. The second connector goes to an integrated circuit.

So what is this thing?

enter image description here


2 Answers 2


The other answer was correct in identification.

It's the bottom half of a momentary contact switch.

It uses printed polymer-carbon conductor in a comb pattern to maximize detection from any vector centre force from a flexible conductive membrane push-button contact.


simulate this circuit – Schematic created using CircuitLab

The quality of these switches demands acute attention to the design of low release of particles from the moving carbonized surface attached to an aggressive user. So unlike mechanical switches must experience a wide range of forces with mechanical limiters yet with tactile feedback. It looks like an interdigital piezo filter, or microwave comb filter but it's just TWO carbon resistors in a polymer binder contact switch.

The top mating contact curved elastic smooth conductive-surface is expected to be at least wider than the carbon gaps but anywhere inside the pink zone.

enter image description here

With excess force, it may cover more area ( unnecessarily), unless there is a lack of mating carbon from the moving part which now has shed contaminant carbon particles in the FR4 surface insulation and may eventually fail. So this solution comes only after extreme careful engineering with chemistry and physics to achieve a reliable cheap solution.

The surface shown is usually printed with ink-jet technology or possibly older methods of silk-screen technology but higher maintenance cleaning costs.

But the silicone-carbon composite membrane not shown is also critical but can make for the most reliable devices, with lifetimes greater than 10^7 cycles at 5% strain. ref

But this leaves an ESD path for failure if not properly designed for punch-thru dielectric breakdown and discharge bypassing. enter image description here

is not a trivial design.

The importance of this component to reliable operation and EVERY new design is to verify reliability until failure to establish confidence using every simulation of random robotic keystrokes and also ESD tests to failure. This is mandatory design process is called Design Verification DVT with MTBF tests.

Confidence levels of 95% at rated MTBF is normal practice by accelerated life tests to failure.

  • 4
    \$\begingroup\$ I love how you can "simulate this circuit" that you designed, it is obviously very advanced! \$\endgroup\$
    – pipe
    Mar 27, 2019 at 13:47
  • \$\begingroup\$ it's clever Asian copycat looking symbol \$\endgroup\$ Mar 27, 2019 at 13:49
  • \$\begingroup\$ These aren't common on mice - was it a radio mouse? I've seen them used for the button to connect to the receiver and things like that \$\endgroup\$
    – Chris H
    Mar 27, 2019 at 14:13
  • \$\begingroup\$ The premature failures of a car-wash kiosk in Toronto 10 yrs ago with the membrane bubble switch turned into target practice with pen tips to make contact after not getting their ticket printed after pre-paying reminds me of the consequences of the designer not doing MTBF reliability tests before releasing the product. Designers !! Remember to do DVT with ESD and mechanical life testing before final design release..!! \$\endgroup\$ Mar 27, 2019 at 14:14
  • \$\begingroup\$ The reason it is old technology is this cheaper design cannot possibly be as reliable as the best membrane switch placed on PCB where the OEM has done all the life tests to validate the design MTBF. But it is cheap. \$\endgroup\$ Mar 27, 2019 at 14:28

This is a simple switch (push button type). These were used in older remote controls. Nowadays we've replaced them with tactile push buttons for compactness.

  • 6
    \$\begingroup\$ You forget to mention that this works with a conductive pad which is pressed against this structure. Also it is still used in many modern devices as it is a very cheap solution. \$\endgroup\$ Mar 27, 2019 at 12:30

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.