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Minimum sensor recognition distance:

> Sensor’s minimum-distance does not have to be much smaller than 20cm (8“).

Infrared:

• Binary Output:               detects whether an object is within a threshold distance
• Multiple Bit Output:        detects the more or less accurate distance to an object
                     

Pro:

-       Quality products comparatively cheap

-       In some cases very high resolution (up to 1 mm)

Contra:

-       Problems with sun light

-       Problems with recognition of dark or black objects

-       Small range sizes

Recommendation after some research: 

Sharp GP2Y 0A02 (ca. $11, 20cm – 150cm Range, Analogue Output)

http://acroname.com/products/SHARP-GP2Y0A02YK0F-IR-PACKAGE

(see also: http://acroname.com/articles/sharp-infrared-ranger-comparison for comparison)

Ultrasonic

Pro:

-       Not depending on optical properties of an object -> more reliable in common usage

-       Larger range sizes

Contra:

-       Usually higher prize

-       Lower resolution (2-3 cm)

-       Problems detecting acoustic quieting objects

Recommendation after some research:

Parallax PING Ultrasonic Sensor (ca. $29, 2cm – 300cm Range)

https://www.parallax.com/product/28015

(maybe cheap replacement: http://www.dx.com/p/hc-sr04-ultrasonic-sensor-distance-measuring-module-133696?Utm_rid=37202148&Utm_source=affiliate#.VTsyqa3tmko)