Battery-powered device having a battery and loud sound detector using passive sensing

1. A communication device comprising:

a processing circuit having at least two modes, a sleep mode and an awake mode;

a wireless communications circuit that can wirelessly send a message as to whether an alarm has been triggered; and

a passive sensor, powered by audio signals impinging on the passive sensor, that provides at least an approximation of an audio signal to the processing circuit so as to cause the processing circuit to switch between the at least two modes, wherein energy for generating the at least an approximation of an audio signal comprises energy generated by the passive sensor.

2. The communication device of claim 1 , further comprising a housing sized to fit into a battery compartment.

3. The communication device of claim 2 , wherein the battery compartment is a battery compartment of a smoke detector.

4. A method of sensing and communicating an alarm condition, the method comprising:

having a sound sensor placed in proximity to an alarm sound generator, wherein the proximity is such that power needed to trigger an alarm signal from the sound sensor is provided by sound waves produced by the alarm sound generator;

triggering a processing circuit to switch from a sleep mode to an awake mode in response to the alarm signal from the sound sensor; and

initiating a wireless communication to send a message as to whether an alarm has been triggered, when the alarm signal from the sound sensor is sent.

5. The method of claim 4 , further comprising enclosing the sound sensor, the processing circuit, a wireless communication circuit and a battery with a housing sized to fit into a battery compartment of a device having the alarm sound generator.

6. The method of claim 4 , wherein the alarm sound generator is part of a smoke detector, and wherein the smoke detector is powered by the battery in the housing.