Battery operated surveillance device

Claim 1

A condition responsive alarm device for generating a warning signal upon the occurrence of a preselected degree of smoke density including: continuously running oscillator means for generating timing pulses; first light emitting means connected to receive said oscillator means timing pulses and to emit light pulses in accordance with said timing pulses; light responsive means positioned to receive light pulses from said first light emitting means and to generate electrical signals commensurate in magnitude with the intensity of the received light signals pulses; normally open switch means; control means connected to said switch means and connected to receive said timing signals and said electric signals from said first light responsive means, said control means maintaining said switch means in the normally open condition in response to substantially simultaneous receipt of one of said timing signals and an electric signal of at least a preselected magnitude from said light responsive means, said control means activating said switch means to the closed condition upon substantially simultaneous receipt of a timing signal and an electric signal from said light responsive means having a magnitude less than said preselected magnitude; and alarm means connected to said switch means, said alarm means being energized upon the activation of said switch means to the closed condition commensurate with the partial obscuration of the light responsive means by a preselected smoke density.