Wireless event-recording device with identification codes

DETAILED DESCRIPTION The present invention provides a device that can be interrogated to determine its identity and its state.
The state indicates whether a particular physical or chemical event has taken place.
In effect, the device records the physical or chemical event.
The identity of the device allows it to be distinguished from a number of similar devices.
Thus this invention finds particular usefulness in the context of an array of devices that can be probed by a wireless interrogation unit.
The device tells the interrogator who it is and what state it is in.
The interrogator can thus easily identify particular items in an array that have reached a particular condition.
One aspect of the invention provides a device for reporting a physical or chemical event or state (possibly a time-integrated condition).
The device may be characterized as including the following elements: (a) a sensor for detecting the physical or chemical event or state without using a power source; (b) a recording mechanism integral with or coupled to the sensor for recording that the physical or chemical event or state has occurred; (c) a tag that contains identification information that can distinguish the device from a plurality of similar devices; and (d) a transponder, coupled to the recording mechanism and the tag, configured to transmit a signal indicating the physical or chemical event or state and the identification information when triggered by a wireless interrogation signal.
In one specific example, the sensor is a temperature sensor and the physical or chemical event or state is exceeding a threshold temperature.
A suitable device for this purpose may include a circuit as the recording mechanism and a fuse in the circuit as the sensor.
When the threshold temperature is exceeded, the fuse opens at least one path through the circuit, thus changing the state of the recording mechanism.
In one embodiment, opening the path changes the resonance frequency of the circuit.
One way this can be detected is by probing the device with a swept- or stepped-frequency interrogation signal and detecting the peak frequency of the signal sent from the transponder