Variable Resistors

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PHOTORESISTOR OPERATION
A photoresistor is placed in the automatic headlight circuit shown below. The photoresistor operates a normally closed headlight relay. During daylight driving light shines on the photoresistor, lowering its resistance, causing current to flow allowing the relay coil to energize, opening the relay contacts, thus preventing the headlights from operating. When the photoresistor is in darkness, its resistance increases preventing enough current flow through the relay coil to keep the relay contacts open. The closed relay contacts allow current to flow to the headlights causing them to turn on. Photoresistors respond slowly to changes in light intensity and may require several minutes to stabilize.