Proximity and Touch Explained
This section deals with detecting objects or measuring their distance from the sensor. This section includes about Touch Screen, Ultrasonic Distance, Optical Distance, Capacitive sensors. .
Touch Screens
Touch screens are commonly used in cellular phones and tablet computers. There
are many different technologies used in touch screens. The most commonly used are
resistive touch screens
Resistive touch screens are one of the older and more readily used touch screen
technologies. They rely on a transparent sheet on top of the display. This sheet is
flexible and also conductive.
Both the top and bottom surfaces are coated with a conductive layer. Insulating
dots are evenly sprayed onto the rigid bottom surface to keep the layers apart, except
when they are pressed together.
To determine the X position of a touch, A is set to 0 V and B to 5 V, establishing a
voltage gradient across the top surface. The voltage measured at C, or for that matter
D, will be proportional to the X position. This is converted into coordinates using an
analog-to-digital converter.
The conductive layer acts just like a potentiometer with C as the slider. If the
device measuring the voltage at C has a very high input impedance, then the resistance
of the track from the surface to the terminal C can be ignored. Most microcontrollers
will have an analog-to-digital converter with a high input resistance, typically
several MΩ. So, the voltage at C will be between 0 and 5 V in direct proportion
to the distance from A of the touch.
When it comes to reading the Y position, some crafty footwork has to take place.
Now C will be set to 0V, D to 5V, and the voltage measured at A or B.
All this processing will be carried out by either a special-purpose controller chip
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Ultrasonic distance-measuring devices are much loved by the developers of hobby
robots. They are also found in commercial equivalents to the tape measure.
These devices send out a pulse of ultrasound and time how long it takes for the
reflection to come back. A simple calculation involving the speed of sound will determine
the distance to the object
These devices have a measurement range of up to around 5 meters (15 ft), but this
depends very much on the size and sound reflecting properties of the object. They
also suffer limits of accuracy due to variations in the speed of sound, which is dependent
on many factors, including atmospheric pressure, humidity, and temperature.
For example, at 0°C, the speed of sound in dry air is 331 meter/second, but this rises
to 346 meter/second at 25°C-a difference of 4.5 percent.
Sometimes separate transducers are used for sending and receiving the pulses of
ultrasound, and sometimes a single transducer is used in both roles.
Some devices rely on a microcontroller to initiate the pulse and time the echo.
Other, more expensive, devices will have their own digital processing that generates
an output that can be read from a microcontroller as one of the following:
* An analog output voltage proportional to the distance
* Serial data
* A train of varying pulse lengths dependent on the distance
In some cases, all three outputs are provided, such as with the SparkFun device
SKU: SEN-00639.
Example: Assuming that in dry air at 20°C, the speed of sound is 340 meter/second,
if the time period between a pulse of ultrasound being sent is 10 milliseconds, how
far away is the reflecting object?
However, that is the distance for the entire round-trip, so the actual distance to the
object is half of that, or 1.7 meters.
Optical Distance
Another useful device for measuring distance at close range is the infrared optical
sensor. This type of device uses the amount of reflection of a modulated
infrared pulse to determine the distance to the object.
Infrared optical sensors are used for closer range measurements than the ultrasonic
devices and produce an analog output. This is not linear or particularly accurate,
so these devices are most useful for simple proximity detection rather than
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Capacitive sensors are frequently used as proximity or touch sensors as a replacement for mechanical push switches. They sense conductors and therefore are ideal for sensing proximity of a hand or finger.
In this case, the sensing is accomplished using two general-purpose input/output
(GPIO) pins of a microcontroller. The Send pin is configured as an output, and the
Receive pin as an input. A single fixed resistor between the two pins forms one half
of an RC arrangement, with the capacitor formed by the object being sensed and a
sensor plate forming the other half of the capacitor.
When a hand moves close to the plate, the capacitance increases. This is sensed
as the control software toggles the state of the Send pin and times how long it takes
for the Receive pin to catch up with the changed state. In this way, it effectively measures
the capacitance and hence the proximity of the object being sensed. The longer
it takes for the Receive pin to change to the same state as the Send pin, the closer the
object is to the plate.
This type of sensing has the advantages that it requires very little special hardware
and can sense through glass, plastic, and other insulators. More advanced
versions of this approach can sense in two dimensions to make a capacitative touch
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