Review: Contactless sensors

Technology
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This topic list some contactless sensing technology

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Magnet based

Hall effect sensors

Sense the intensity and polarity of a magnetic field:

Magnetic absolute rotary encoder

Same principle sensing a magnetic field:

Require Diametrically magnetised magnets: http://www.first4magnets.com/neodymium-magnets-t137#c34:t177

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Inductance-to-digital converter

A Revolution in Sensing
Inductive sensing is a new, contactless, magnet-free technology that
revolutionizes sensing by delivering better performance, better reliability, and
greater flexibility than existing solutions, at a lower system cost. The LDC1000 is
the world’s first inductance-to-digital converter that enables inductive sensing.

Better Performance and Reliability
The LDC1000 provides 16-bit resonance impedance and 24-bit inductance
values, enabling sub-micron resolution in position-sensing applications. Inductive
sensing is also immune to non-conductive interferences such as dirt and dust
and it supports remote sensor placement, making it well-suited for extreme and
harsh environments.

Lower System Cost
The sensor is virtually free because the sensor can be a wound wire, a coil on a
PCB, a coil printed with conductive ink on a flexible substrate, or even a simple
spring. You also avoid the high costs of magnets, especially those requiring
rare-earth materials.

Component: http://www.ti.com/product/LDC1000
Features: http://www.ti.com/lit/ml/slyb212/slyb212.pdf

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Passive NFC

NFC can power small devices and thus read from many type of sensors provided low power consumption.

The SIC4310 NFC Enabler‘s UART interface can directly drive a piezo speaker to play melodies while its GPIOs (general purpose input/output pins) are able to turn LED lights on and off using only harvested power from an NFC device.

Article: Passive NFC chip can power lights and sounds • NFCW

Components: http://www.sic.co.th/product.php?product=138