Alarm system for sensing smoke and intruders

Claim 1

An alarm system for sensing the presence of a light barrier comprising: a source of light collimated into a narrow parallel beam; a first polarizing means for making the light beam plane polarized; a plurality of sensor stations positioned at places where it is desired to detect a light barrier, each of said stations including a container having an entrance and an exit opening for the passage of the light beam, each of said stations also including a photo-transducer adapted to sense the reduction in intensity of the light beam; an oscillator coupled to each sensor station for generating a distinctive frequency; coupling means between the photo-transducer and the oscillator in each station for starting the oscillator only when the transducer detects a light barrier; a crystal modulator controlled by the oscillator for modulating the light beam as it passes through the sensor container; a control station for receiving the light beam and for interpreting its modulated information, said station including a second polarizing means set at an angle to the first polarizing means for converting the modulated light beam into a variable intensity beam; a photo-transducer for converting the variable intensity beam into an alternating current; a plurality of filter circuits respectively tuned to pass a frequency corresponding to the frequency of one of the oscillators; and an alarm coupLed to each of said filter circuits.