Sensors are important in order to build working stuff

Hall Effect
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This kind of sensors use the quanto-mechanic phenomenon to signal the presence of a magnetic field

There are several kind of sensors

 - **latched**: once it is triggered it latches and will not unlatch until a magnetic force of reverse polarity and strength is sensed.
 - **Bipolar** sensors require a positive magnetic field (south pole) to operate them and a negative field (north pole) to release them while **unipolar** sensors require only a single magnetic south pole to both operate and release them as they move in and out of the magnetic field.
 
 ## Microphone
 
  - [Powering microphones](http://www.epanorama.net/circuits/microphone_powering.html)