Method and apparatus for warning of disconnection of an appliance from a power source

An apparatus for warning of disconnection of an electrical appliance from a power source, comprising:

• an oscillator for producing and injecting an AC signal into the power cord of an appliance to be monitored;
• an impedance bridge circuit having a reference arm including a variable capacitor for producing a reference voltage corresponding to the reference impedance of the power cord and monitored appliance and a sensing arm for producing a present voltage corresponding to the present impedance at the point of signal injection;
• a voltage comparator coupled to both arms of the impedance bridge for monitoring the voltages produced on each arm;
• and
• an alarm responsive to the voltage comparator for warning if the voltage on the sensing arm exceeds the voltage on the reference arm, indicating that the present impedance has increased above the reference impedance because the monitored appliance has been disconnected.

An apparatus for warning of disconnection of an electrical appliance from a power source, comprising:

• a reference voltage;
• means for producing a current;
• means for injecting the current for a predetermined time into the power cord of an appliance to be monitored to produce a present voltage proportional to the present impedance at the point of current injection;
• means for comparing the present voltage against the reference voltage;
• and
• alarm means responsive to the comparing means to warn if the present voltage is no longer in a predetermined relation to the reference voltage, a change in the relation indicating a change in impedance caused by disconnection of the monitored appliance from the apparatus.

A method of warning of disconnection of an electrical appliance from a power source, comprising:

• injecting a signal into a power cord of an appliance to be monitored;
• setting in response to the injected signal a reference voltage corresponding to a reference impedance at the point of signal injection, the reference impedance present with the appliance connected to the power source;
• comparing the reference voltage to a present voltage corresponding to the present impedance at the point of signal injection;
• and
• activating an alarm if the present voltage crosses the reference voltage.