They are often used to convert electricity, hydraulics or pneumatics into motion electric actuators. It is also possible to have actuators that convert motion or movement into electricity.The most common example of an actuator is the switch you use for lighting up your home or office. What you actually do is throw a mechanical lever one way, forcing its spring-laden contacts to come together and close the circuit to power on the electrical appliance. Similarly, solenoid valves allow doors to open automatically when you approach them. The solenoid valve is actuated by a sensor near the door, which senses the presence of a person, either by his or her weight or by the person interrupting an infrared light beam. The actuated solenoid valve pulls up its armature, allowing the compressed air to open the door. When the person has entered, the sensor stops the electrical signal to the solenoid valve, releasing its armature and allowing the spring to close the air entry. The air to the door escapes from another vent, and the door slowly closes.
Another very common example of an actuator is the snap action tactile switch made by TE Connectivity. This PCB mountable switch has a phosphor-bronze dome inside, which is actuated when you push in the plunger with your finger. The phosphor-bronze dome touches two sets of contacts inside, shorting them electrically and making the contact. Releasing the plunger causes the phosphor-bronze dome to spring back, breaking the contact and deactivating the switch. Another actuator from TE Connectivity is the circuit breaker. This is used to direct the flow of current to one of two different circuits pneumatic actuators. This is an SPDT or Single-Pole Double Throw type. The front lever is manually operated to let current flow from a single pole in either direction A or B.