Fire alarming system

Claim 1

A fire alarm system comprising a plurality of fire sensing units connected in parallel between a pair of conductors, a receiving unit including a voltage source and an alarm device connected to said conductors, each of said fire sensing units including a normally open switch which is closed to short circuit said conductors and energize said alarm device when a fire is sensed, each fire sensing unit further including a band pass filter connected between said conductors and having a characteristic resonant frequency preselected peculiarly to said sensing unit, said receiving unit further including means for generating a range of AC signals having frequencies corresponding to said characteristic resonant frequencies of said filters in the respective sensing units, each of said filters being operated upon actuation of its associated sensing unit to produce its characteristic frequency, a conductor directly connecting the outputs of said filters in said sensing units to said receiving unit and means in said receiving unit and connected with the last said conductor for discriminating the frequencies of the received AC signals and indicating the specific detector activated by the presence of fire whereby each detector upon activation produces both a general alarm and an indicating signal to identify at the receiving unit the specific detector actuated.