How do touchscreens work?

  • Thread starter rajesh kunhi kambrath
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Sharma, Anand

Mr Rajesh, There are two ways touch screens work that i am aware of.... (1) The technology utilizes non-visible infrared light which is transmitted by the highest quality Light Emitting Diodes (LEDs) and detected by very rugged (military spec) Light Receiving Phototransistors (LRPs). The infrared transmitting and receiving diodes are located on opposite sides, facing each other around the periphery of the touch screen, which is mechanically mounted in front of a display (either a CRT monitor, LCD flat panel, or color plasma display). The infrared light impulses travel from side-to-side and top to bottom in front of the touch screen. When the operator touches the screen, it creates a shade for some of the LRPs. The solid state electronic controller interpolates the locations of the horizontal and vertical shades and automatically determines the center of the operator's finger. This location is assigned specific coordinates, which are transmitted to the computer via a serial cable, similar to a mouse connecting cable. The driver software program installed in the computer, similar to a mouse driver, then indicates the center of the finger by using an arrow, similar to the arrow that shows the pointing location of a mouse. (2)The screen of a touch screen monitor is covered with another layer made up of a hyperfine mesh through each strand of which a small amount of electric current keeps flowing continuously. This leads to the formation of an electric field in the squares formed by the overlapping of horizontal and vertical strands of which the mesh is made up. Once you place your finger on the surface of such mesh you block the free flow of current(and also alter the electric field) through a particular portion of the mesh (which includes some horizontal and some vertical strands). The screen then calculates the serial number of the horizontal and vertical strands where the block in flow of current has happened which is then translated into mouse position, or pushing a button, etc. Hope this helps. Anand Sharma Tata Consultancy Services SEEPZ, Mumbai Ph: 91-22-829 0162 (ext-253) [email protected] ***************************************************************** Before posting, please read Got code? Add it to the PLCArchive at The Automation List is managed by Inc.
There are lots of ways I have seen and there are probably even more: 1. simply bond a see-thru membrane keypad onto the front of the screen. This is dead easy to talk to but you're limited to the "button" on the keypad, so it's hard to make variable size, movable, odd shape etc buttons. Never needs calibrating but can wears quickly (after a while you can see the button). 2. resistive touch screens have two layers of resistive material laid over the front of the screen. When you press on the screen you bring the layers close together and you measure the resistance from the edges of the screen, do a bit of maths and work out where the finger is. Pretty easy to make work but the senser can wear. 3. similar construction to (2) but measures change of capacitance when your finger hits the screen Pretty easy to make work but the senser can wear. Also needs calibrating and the calibration can drift. 4. make a grid of infra red beams across the front of the screen, wherever you put you finger you break a beam. Never needs calibrating and works with gloves etc but you need to be careful not to move your finger around as you touch/release. Also you have to be insensetive to finger size variation. Also can get messed up if you have large particulate contaminants around (if they're big enough to get in the way of the beams). 5. (the oddest one of all, but works with a conventional PC monitor) you sit your monitor on a senser pad. When you press on the screen the force is translated into pressure on the pad that the base of the screen is sitting on. Software then calculates where your finger is!! Obviously this one is completely mad, as the monitor shifts around on the pad and you need to keep re-calibrating it. However, this product was around for a while so somebody must have bought it!! Hope this is useful Regards Geoff Moore ==================================== Straight Forward Solutions Ltd Maynooth Road, Prosperous, Naas, Co.Kildare, Ireland Phone : +353 (0)45 892739 Fax : +353 (0)45 893880 Mobile : +353 (0)86 8179683 email : [email protected] ====================================
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