Tag Archive for: touch screens

I am in need of an oddly-shaped, custom projected capacitive multi-touch display that is waterproof, able to work with varying pressure, and provides maximum velocity for input acquisition and transmission to controller. The dimensions of the sensor need to be 460mm long and the widths need to be 42mm and 58mm respectively; the boarder must be narrow at 1-2mm (maybe made of plastic or aluminum). It is also important that the electronics work with Linux.

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I have made mention of how we propeller-heads love the challenges you crazy design engineers give us. But, as exotic requests go, you have thrown us a soft ball. In the old days of resistive and surface capacitive, we have made Cartesian touch sensors that were circular, triangular, spherical, and even a hexagon or two. So your request is quite “doable”.

Recall that multi-touch projected capacitive is a digital technology that generates a high resolution analog coordinate from the row/column intersections. Within reason, the lengths of the rows or columns do not need to be the same. So making either end shorter will not change the output….just TouchGuy_Basicsome of the coordinates will not exist where there is no sensor. Some projected
capacitive electronics work best with different (invisible) ITO patterns, so you will need to pick one that anticipates your strange-dimensions, usually streets-and-alleys, diamonds, or snowflakes…..avoid telephone poles, zigzags, triangles and rings. But odd shapes – they do not trouble us.

So much for non-uniform dimensions, but now to the bigger problem….your one or two
millimeter border. First, I need to understand if you truly have such limited space at the edge. Almost all p-cap touch sensors have a top layer, called a cover glass or cover lens, which usually has an opaque ink on the back to hide the circuitry and replace the bezel, enabling in a flush
design. The opaque ink must be at least as large as the inactive border area, and usually is much wider. For a sensor that is 460mm long, it seems unlikely that you would only be allowed such a tiny edge area. Thus, me thinks you should have a traditional bezel and allow yourself more room.

However, if you persist…..you probably know that all touch screens need a border to deliver
signals to the sensor. In the case of p-cap, there needs to be one signal delivered to each row and column, which, for a 460mm length would be about 65 signal lines in the long dimension. The minimum width of these signal lines is usually 30 microns with a 30 micron separation between the conductive traces. You do the math, but you can see that you simply cannot fit that many signal
lines in 1-2mm space. Touch-International’s solution is to put the signal lines on a flex tail and bond that tail to connecting pads at the edge of the sensor glass. Rarely, we have also drilled holes in the glass and connected via the back, but bring your checkbook for this solution.

You have me wondering what you mean by extreme velocity….here in Texas, everybody is
packing and “extreme velocity” means a bullet is headed your way. As much as I want to say PCAP will last forever… when it comes to target practice, I am lying. However, we can use up to 6mm tempered glass if you are expecting trouble.

Until Next Time,
– Touch Guy

How is TI’s new MulTI-Touch Projected Capacitive Technology able to work with water spray?- Tyler Womack, Wapakoneta, Ohio

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So Tyler,

I guess you know water can be a lot of trouble for touch screens. The most vulnerable is SAW which stops dead in its tracks, to the least vulnerable, resistive touch. Resistive can be sealed so that water does not get in between the PET and glass layers which allows for better performance with water.

The ultimate irony is that most touch screens work because humans are just big bags of salt water and the water in our bodies serve as the primary activating mechanism. The problem is that the touch screen must distinguish between you, the big salt water bag, and the salt water puddle. (There is one rare form of an acoustic touch system than operates in a fully submersible ocean environment, but Touch Guy will ignore that one for now because it is made by a competitor).

So Touch Guy, still the SUPER-CHAMPION of projected capacitive needs to come clean on water and multi-touch. And if you follow Touch Guy, you know his answer to most questions is, “it depends.” In this case, it depends a whole lot on which sensing IC (Integrated Circuit/Chip) you use. Most ICs will invariably yell, “I quit!”, when it gets a lot of water on it – check out your iPhone sometime. The best IC could be saying, “I love water!”, and works really well. The water-loving IC will even work covered in distilled water, but as the salinity increases (as would be the case on the ocean) a salt spray is as far as you can take it. So your projected capacitive fish finder will work on the local lake in a light rain, but bow-busting 40 foot ocean waves are probably a little much.

Still need to survive the salt water wave and keep on multi-touching? Then the Resistive MARS technology is the one for you. Silicone-sealed behind a water tight bezel, this touch screen can take that salt water puddle and keep on touching. Add sunlight readable filters to the MARS sensor and you are ready for a jaunt around the continent on the bridge of your new yacht!

Until next time,
Touch Guy