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The principle of Optically sensors based on either reflections of a lightsource (Source and detector integrated in the same unit)

An alternative method could be a separate lightsource and detector which activated by a object between them.



The electric propertie of a coil (Self-induction coefficient - L) depends of the presence by iron.

A special design electric circuit (based on AC voltage) will be able to detect changes of the self-induction.

The figure presents a system for measurement of speed and motion of a shaft.

Each shaft turn will produce 4 pulses. 

Reed-relays a good alternative to mechanic switches.

A magnet will magnetize the iron-contacts inside and hence forcing then together.

After the magnet removed will a spring-effect of the contacts pull them apart.