PRESET (VARABLE RESISTOR)

A preset is a electronics devices that used to varies circuit resistance through which current or voltage can controller . the resistance of circuit can be varies using upper rotary part of preset .preset are available in various valued  in market.i this rotary part can varies using screw driver or other equipment  The resistance does not vary linearly but rather varies in exponential or logarithmic manner. such a preset is commonly used in sensor circuit to increase or decrease the sensitivity of circuit .
for eg .light sensor  ,ir sensor

this component having three terminal .there is back terminal is fixed offering resistance whenever front provide variable resistance .hence when it is used as fixed resistence we can use only back terminal and pin diagra is shown in fig