Industrial remote controls are wireless input devices that are used to control machines and equipment to which they are not directly linked. They use radio frequency (RF) signals, digital pulses, or infrared light to send signals from transmitters to receivers. With RF devices, an antenna is attached to the receiver or installed separately to improve reception. Rugged housing or standard enclosures are used to protect sensitive components and are suitable for indoor or outdoor use. Some industrial remote controls include only a transmitter such as a keypad, paddle, pushbutton, or toggle switch. Others include only a receiver. Complete systems include a matched transmitter-receiver set called a transceiver.