Method and system for providing an audible alarm responsive to sensed conditions

Claim 1

An alarm system for providing an alarm upon the occurrence of a predetermined concentration of gaseous hydrocarbon in an area comprising: a sensor including a gaseous hydrocarbon responsive variable resistance element connected between two electrodes and means for heating said variable resistance element, the resistance between said electrodes being related in value to the resistance of said variable resistance element, the resistance of said variable resistance element assuming a predetermined, relatively stable resistance value upon energization of said heating means beyond an initial heating period, and said resistance value decreasing from said relatively stable value as a function of the concentration of gaseous hydrocarbons in the vicinity of said sensor, and a decrease in said resistance value to an initial action resistance value substantially below said stable value during the initial heating period; a series circuit arrangement comprising first and second switching means connected in series with a power source, each of said switching means being operable between conductive and non-conductive conditions in response to a trigger signal, said switching means producing an alarm signal with both the first and second switching means in the conductive condition; alarm means for sounding an audible alarm in response to said alarm signal; first trigger circuit means for triggering said first switching means into its conductive condition in response to said variable resistance element decreasing at least to a predetermined resistance value, which resistance value is below said stable value by an amount signifying a predetermined concentration of gaseous hydrocarbons in the vicinity of said resistance element, said predetermined resistance value being above said initial action resistance value occurring during said initial heating period; second trigger circuit means including a capacitor having a charging path which includes said first switching means and connected to be charged to at least a predetermined charge level upon energization of said heating means, said second trigger circuit means triggering said second switching means into its conductive condition in response to said capacitor achieving said predetermined charge level; and third trigger circuit means connected to said first switching circuit means for intermittently triggering said first switching circuit means into its conductive condition in response to the occurrence of an abnormal condition of said sensor.