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I’m mounting a projected capacitive touch screen to a 17” monitor and am trying to find out how much of a gap I need to leave between the touch screen and LCD glass. Is the gap dependent upon the type of touch screen used? Also, do you have any suggestions for how to mount it to the LCD or bezel?

Dear Victor:

As they say on the London Underground, “Mind the Gap”.  The separation between the back of the touch sensor and the front of the LCD is important for two reasons—optical and electrical.

In general, one wants the touch sensor to be as close to the display as practical, so that there is no pronounced parallax error.  Parallax occurs when the finger or probe comes in at an angle and is detected “too early”, and thus misses the actual target — such as touching a “z” on the on-screen keyboard, instead of the intended “a”. This is most pronounced in older style infra-red touch technology, because the LED’s are relatively high above the display, but can be a problem with any touch sensor located too far from the display.

But putting the touch panel too close can cause problems if you are able to deflect the touch screen so that it hits the LCD. Your 17” touch panel will probably not have this problem because larger touch screens have a rigid back layer that will not easily bend.  Smaller, thinner, touch panels, however, can be pushed into the LCD and a rainbow (moiré pattern) can occur, which is not good for the life of the LCD.  In a few cases, for LCD’s without a metal frame, you can actually put the touch screen on the LCD, and the moiré pattern will be annoyingly permanent.

The LCD and backlight can also create enough electrical noise to slow or stop the operation of the sensor, so there needs to be a gap between the LCD and the touch sensor.  The required gap size, however, is not a constant.  The needed separation varies with the LCD model and touch technology; those lightning-fast tiny switching transistors do emit some electrical noise, believe it or not.  For projective capacitive technology, the necessary gap also varies with the IC manufacturer.

So, to mind your gap, a separation (from the surface of the LCD) should be the thickness of the metal frame plus 10/1000’s, which is usually the thickness of the adhesive that holds the touch screen to the LCD.  Sometimes, you will be tricked by a particularly noisy LCD and need a larger gap, also handled by better tuning of the electronics, but 40/1000’s (bezel plus adhesive), as  a minimum, will usually work.

Ta ta,

Touch Guy

Do you have a question for Touch Guy? Send him an e-mail at asktouchguy@touchintl.com.

Is projected capacitive technology reliable when used outdoors in a marine environment where it will be exposed to rain, saltwater spray, bright sunlight, etc? What are the likely points of failure? – Charlie, Marblehead, Massachusetts

Hey Charlie:

This is a most appropriate question for national sock burning day because, as is the case with all good sailors, Touch Guy barbequed his socks and pledged to go shoe-to-skin all the way to the last blustery days of Fall.

Ah, but back to your question…Until recently, resistive touch was the only technology that was appropriate for marine applications. It could be fully sealed so that salt water was not able to corrode the protective hard coat, trigger a false touch or become impaired by a layer of salt.

The big problem with resistive touch is that the internal reflections make the display difficult to view in bright sunlight. Typically this problem is overcome by using a very bright backlight, a sun shroud or an expensive sunlight readable filter.

Projected capacitive technology is now the best outdoor solution because it does not have internal reflections and when optically bonded to the display, provides optimal readability. Some projected capacitive electronics are confused by salt water on the sensor but others are totally unaffected with by it – Touch Guy TouchGuy_Basiccan help with this critical ASIC (Application-specific Integrated Circuit) decision. And, if your marine vessel happens to be an aircraft carrier, it is easy to incorporate EMI and night vision filters into the touch sensor.

Other than lowering the boom on the sensor, there are no points of failure, although cold weather sailors will find that projected capacitive technology will not work with thick gloves. It is also important to note that you will need to seal and add a conformal coat to the circuit board to prevent corrosion.

So Charlie, let’s be sock-less ‘til November and get that projected capacitive touch screen on deck!

Until next time,
Touch Guy