Smoke detectors are designed to sense the products of combustion. They are designed for commercial, industrial or residential applications. There are four basic types of smoke detectors: stand-alone devices, detection systems or components, integrated circuits (ICs), and addressable fire protection devices. Stand-alone smoke detectors include a sensor and detector circuitry. They usually contain a built-in alarm, may instead provide electrical contacts for an auxiliary alarm. Smoke detection systems are networked, programmable products that include a control unit and detector. Individual smoke detection components are also commonly available. Smoke detectors with an IC form factor are semiconductor devices that are designed for smoke-sensing applications. They do not contain an actual smoke sensor, and may be suitable for AC or DC – but not both. Addressable smoke detectors are fire detection products that communicate with each other in a networked system.