A system (100, 300, 500) comprising: a wearable device (110, 210, 310) comprising an antenna (170, 270, 312); and a reader device (180, 340), wherein the wearable device (110, 210, 310) is configured to: receive a level of radiant energy from the reader device (180, 340); detect a change in the received level of radiant energy; determine that the detected change is indicative of a predetermined pattern of received radiant energy, wherein determining that the detected change in the received level of radiant energy is indicative of a predetermined pattern of received radiant energy comprises determining that the detected change in the received level of radiant energy is indicative of at least one body movement; and in response to the determining, operate the antenna (170, 270, 312) to transmit to the reader device (180, 340) an instruction that indicates that the wearable device (110, 210, 310) has determined that the detected change in the received level of radiant energy is indicative of a predetermined pattern of received radiant energy, and wherein the reader device (180, 340) is configured to: receive the instruction from the wearable device (110, 210, 310); and in response to the receiving, operate at least one external device. a wearable device (110, 210, 310) comprising an antenna (170, 270, 312); and a reader device (180, 340), wherein the wearable device (110, 210, 310) is configured to: receive a level of radiant energy from the reader device (180, 340); detect a change in the received level of radiant energy; determine that the detected change is indicative of a predetermined pattern of received radiant energy, wherein determining that the detected change in the received level of radiant energy is indicative of a predetermined pattern of received radiant energy comprises determining that the detected change in the received level of radiant energy is indicative of at least one body movement; and in response to the determining, operate the antenna (170, 270, 312) to transmit to the reader device (180, 340) an instruction that indicates that the wearable device (110, 210, 310) has determined that the detected change in the received level of radiant energy is indicative of a predetermined pattern of received radiant energy, and receive a level of radiant energy from the reader device (180, 340); detect a change in the received level of radiant energy; determine that the detected change is indicative of a predetermined pattern of received radiant energy, wherein determining that the detected change in the received level of radiant energy is indicative of a predetermined pattern of received radiant energy comprises determining that the detected change in the received level of radiant energy is indicative of at least one body movement; and in response to the determining, operate the antenna (170, 270, 312) to transmit to the reader device (180, 340) an instruction that indicates that the wearable device (110, 210, 310) has determined that the detected change in the received level of radiant energy is indicative of a predetermined pattern of received radiant energy, and wherein the reader device (180, 340) is configured to: receive the instruction from the wearable device (110, 210, 310); and in response to the receiving, operate at least one external device. receive the instruction from the wearable device (110, 210, 310); and in response to the receiving, operate at least one external device.