Smoke detector

Claim 1

A solid state system for monitoring the opacity of a variable density medium and for providing an output signal in response thereto comprising a source of light adapted to be positioned on one side of said medium, means for regulating the current to said source of light to ensure tHat the intensity and color temperature of light produced by said source is a constant value, said means comprising a voltage-controlled, constant-current source including an operational amplifier having an output controlling the conduction of an electronic switch to regulate the magnitude of current provided to said light source, said voltage controlled, constant-current source being provided with a stabilized reference voltage, receiving means comprising photoelectric means disposed opposite said light source, with said medium interposed therebetween, to sense the level of light passing through said medium, indicator means, a balanced currentto-voltage converter comprises a pair of integrated circuit operational amplifiers formed on a common substrate and having an extremely low input resistance to provide a first signal to said indicator means and which is a linear function of the magnitude of light received by said photoelectric means, and an intermediate amplifier having differential inputs, one of said inputs being provided by one of the operational amplifiers of the balanced current-to-voltage converter and the other input being provided by the other of the operation amplifiers of said converter, said intermediate amplifier providing said first signal, said indicator means comprising a meter and a voltage controlled constant current source responsive to said first signal from the balanced current-to-voltage converter for regulating the magnitude of current flowing through said meter, to provide an output signal.