A lock solenoid is a type of electric locking mechanism that uses an electromagnetic device containing a tightly wound coil of metal wire called a solenoid to provide the mechanical energy that opens and closes the lock. Solenoid locks are often used as door locks and are also used to lock other things, such as cabinets or drawers. They are especially common for doors that are locked and unlocked remotely, such as the main doors of many apartment complexes or situations where security is a particular concern.
The essential component of a lock solenoid is a length of insulated copper wire connected to a source of electrical current. The wire is coiled around a long, frequently cylinder-shaped core made of iron or some other ferromagnetic material that is positioned so that it can move nearer to or farther away from the center of the coil when force is applied to it. When electrical current passes through the wire, it generates a magnetic field that moves the core, turning electrical energy into mechanical energy and producing linear motion. This allows the solenoid to open or close the bolt in the lock.
Solenoid locks have several features that can make them desirable in many applications. Unlike purely mechanical locks, a lock solenoid can open and close without needing a human to be physically present to operate it, making it useful for locks that receive instructions to open and close from a separate location or that do so automatically according to a preset timer or signals from sensor equipment without direct human input. They can also be designed to be unlocked by things other than a physical key, such as a password; a signal from a radio-frequency identification device or personal digital assistant; or biometric data such as fingerprints, voice prints, or retinal scanning.
Another useful feature is that, depending on the design of the lock solenoid, the core can be positioned so that activating the electrical current either pushes the core outward, throwing the bolt and shutting the lock, or polls the core inward to open the lock. A lock that uses a magnetic field to close the lock will automatically open in the event of failure, or "fail open,” while a lock that uses the magnetic field to open the lock will do the opposite and “fail closed.”
This is a useful attribute of solenoids and an important design consideration, because it determines how the lock will react in the event of equipment failure or loss of electrical power. For example, lock solenoids used on the doors of businesses or residences will usually be designed to fail open so that in the event of a dangerous event that causes power failure, such as a fire in the building, people will be able to escape the building unobstructed. On the other hand, in the case of some solenoid locks, such as locks on a vault, a safe, or a cabinet containing money, valuables, or other potential targets of theft such as sensitive government or business information, it is more desirable for the lock to fail closed so that a power loss does not leave them suddenly vulnerable.