A device that is responsive to the presence or absence of a stimulus. 2. In an AM radio receiver, a circuit or device that recovers the signal of interest from the modulated wave. Note: In FM reception, a circuit called a discriminator is used to convert frequency variations to amplitude variations. 3. In an optical communications receiver, a device that converts the received optical signal to another form. Note: Currently, this conversion is from optical to electrical power; however, optical-to-optical techniques are under development.