Touch/Capacitive Sensor Patrick Coster s

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2 Touch/Capacitive Sensor Patrick Coster s Overview Touch/Captive sensors have been around for years, but it is only been in recent years where they technology has been applied to consumer products. As such this has been due too recent advances have made it viable alternative too mechanical switches in a wide range of consumer products. In simple terms, they use the human body as a conductor. Thus when the fingers touch the surface of the screen, it will result in a distortion of the body's electrostatic field, and is then measured as a change in capacitance. Applications This type is sensing is used where there is no large amounts of actual human force are required. The sensor can be buried deep with in the plastic or whatever material, and still retain good sensing qualities meaning that it can be visually hidden quite well. Capacitive/Touch sensors are being used in technologies such as Laptop track pads Computer monitors MP3 players Cell phones In addition there are many other types of consumer products. As such they have defined products such as the Ipod with its revolutionary spin wheel, and in a large number of mobile phones. As such capacitive sensors are being chosen to be used to the fact that they have a lot of flexibility and unique human device interface, and when compared to mechanic switches they are far more cost efficient. Moreover capacitive sensing applications have the ability too replace mechanical buttons with capacitive alternatives. Technologies such as multi-touch and gesture-based touch

3 screens are also derived from capacitive sensing technology. Typical capacitive sensor designs can be constructed from many different materials such as copper and Indium tin oxide. Size and spacing of the capacitive sensor are both needed for the sensor performing at a good level. Sensing a finger through an overlay becomes very difficult as the overlay thickness increases. 1) Projected capacitance; Is a capacitive technology which allows very accurate application of use by etching the conductive layer. The X-Y grid that is used, is formed either by etching a single layer to form a grid pattern of electrodes, or by etching two separate, perpendicular layers of conductive material with parallel lines to form the actual grid. This in turn is actually very simular to the pixel grid that is found in many of the LCD displays we use today. Because of the high resolution that it works under it allows use under even screen protectors, or behind weather and vandal-proof glass. Depending upon the use and the situation that is needed with implementation, an active or passive stylus can be used instead or with a human finger. A drawback is that gloved fingers may or may not be sensed. Another disadvantage is that smudges from dirty fingers and moisture can cause disruption of message There are two types of Projected Capacitance: Self Capacitance and Mutual Capacitance. 2) Mutual Capacitance; Is intentional or unintentional capacitance that occurs between two charge-holding objects/conductors. The current then goes onto to pass through one, and then pass into the other. Mutual capacitance only works along short distances. Thus what happens is an object's capacitance increases when another object is brought closer to its proximity. The human body is a great charge-holding object capacitor and

4 sensitive capacitive detectors can be made to function they get closer each others proximity. The capacitive properties of human fingers are also allows the ability to make touch switches, such as the Ipod. Objects such as stylus pens are just as effective. 3) Self or Absolute Capacitance; These sensors actually use the same X-Y grid as mutual capacitance sensors. In turn though the columns and rows are used differently, as they operate independently from each other. The capacitive load of a finger (or whatever object that is being used) is then measured on each individual column or row electrode by a current meter. As such due to the precision of this method it will produce a far stronger signal than mutual capacitance. However though it is unable to resolve accurately more than one finger, essentially meaning that only one finger can be used at a time. Thus it can create "ghosting" or lagging. 4) Surface capacitance; Is a very basic technology. As such its simplicity comes from the fact that only one side of the insulator is coated with a conductive layer. A small voltage is applied to the layer, creating what is known as a uniform electrostatic field. When the conductor such as a human finger or stylus this touches the uncoated surface, a capacitor is dynamically formed. The sensor's controller then can determine the location of the touch indirectly from the change that occurs in the capacitance, which is then measured from the four corners of

5 the panel. It has no moving parts and is not all that durable, in addition it has a limited resolution. Because of its simplicity it is most useful in kiosks, industrial controls and other simple applications. Overview with Arduino Although they are used in consumer goods such as ipods and touch lamps, they can be used as sensors for all kinds of physical interfaces. There is a lot of potential when interfaced with an arduino to be used with many electric devices. As such the process that is highlighted here with an arduino can create a sensor of aluminium foil that can sense a hand about 101mm away. This system requires a 10 M resistor and a piece of wire. The setup steps are as follows; > Connect a 10M resistor between pins 8 and 9 on the Arduino Board > Connect a small piece of alluminum or copper foil to a short wire and also connect it to pin 9 > When using this in an installation or device it's going to be important to use shielded cable if the wire between the sensor is > More than a few inches long, or it runs by anything that is not supposed to be sensed. > Calibration is also probably going to be an issue. > Instead of "hard wiring" threshold values - store the "non touched" values in > If your sensed object is many feet from the Arduino Board you're probably going to be better off using the Quantum cap sensors. The following example was referenced from; Other detailed information can be found there. Or at;

6 Examples of circuits using the touch sensor; References; ch_sensors/cap_touch_sensors.htm

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