Crank-shaft sensor

In case of a petrol engine,it is important to achieve a correct timing between piston and ignition system.So basically we require a sensor.
It is called as:
Crank-shaft sensor :
This sensor reports the crank-shaft speed and piston position to ignition system.The ignition system should know as to when to ignite spark.In other words it must be able to calculate the instant when piston reaches TDC in compression stroke.
It consists of a pulse ring which rotates with the crankshaft.Whenever a notch in the disc aligns with sensor,it produces a voltage pulse.Hence the spark ignites.

There are two such types of sensors.
1.Pick up coil crankshaft sensor:
This arrangement consists of a reluctor i.e; a slotted disc which rotates with crankshaft.As each shaft passes through magnetic field at the end of pickup coil, a voltage is induced in it.
So even if you increase or decrease the speed,The ignition system will know the position of piston as the voltage will also vary.

2.Hall effect crankshaft sensor:
It uses HALL effect:
Def:
The Hall effect is the production of a voltage difference (the Hall voltage) across an electrical conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current.(source of definition is wikipedia)
This crank sensor uses a Hall effect switch to turn voltage on or off.

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