Dryer moisture sensors determine if a tumbling load of clothes is dry by measuring the heat, weight or electrical conductivity of that load. Once the clothes are considered dry, the sensor initiates a cool-down cycle. It then shuts the dryer down.
Dryers have three different types of moisture sensors. One uses a thermostat, which is placed inside an opening near the drum. As the clothes dry, heat is released into that opening. When a set level of heat reaches the thermostat's sensor, the dryer starts its shutdown phase.
The second type of sensor consists of two prongs set near the back of the drum. Because wet clothes are heavier, centrifugal force causes them to spin close to the sides of the drum, which keeps the two prongs touching each other. As the clothes dry, they gravitate toward the center of the drum, which causes the prongs to open. The open prongs indicate the clothes are done.
High-end dryers usually use a conductive sensor that measures the electrical resistance in a load of clothes to determine if they are dry. Because water conducts electricity, wet clothes are less resistant to an electrical charge than dry. When a set resistance level is reached, the moisture sensor initiates the cool-down period and shuts off the dryer.