In a method and apparatus for detecting a message in a signal the message begins with a preamble consisting of a fixed sequence of symbols, each having a symbol time T, modulated by repetitions of a higher rate code having low autocorrelation at lags anything other than zero. The repetitive code and the fixed sequence of symbols is known at the detector. The method comprises correlating the signal with the repetitive code; autocorrelating the signal with a copy of the signal, which is delayed su…

Apparatus for monitoring gas content on a site, to provide an indication if an undesirable gas content occurs, comprises a transmitter for feeding ultrasonic waves through the gas to a receiver. The ultrasonic waves are frequency-modulated and the phase of the modulation at the receiver relative to that at the transmitter is monitored. Either the relative phase is maintained constant, for example by a phase-locked loop so that the modulation frequency varies as a function of the gas content, or …

A flame detecting apparatus in which a detecting electrode is disposed in a flame, the base of a transistor is connected to the detecting electrode and biased with a predetermined DC voltage derived from a biasing circuit, the biasing circuit including a resistor and the impedance of the flame, an AC signal is applied to the emitter of the transistor, and an amplified AC signal is derived from the collector of the transistor as an output signal; the output signal being used as a control signal f…

A battery-powered fire alarm including a smoke detector, a controllable horn circuit and a battery monitoring circuit. The smoke detector employs a pair of complementary field-effect transistor switches with gates respectively connected to an ionization chamber and a potentiometer of a Wheatstone bridge circuit connected across the battery. The field-effect transistors are biased off to minimize standby power consumption and are connected such that the threshold voltages thereof are offsetting t…

A detection circuitry designed to sense a composite signal of the type such as composite frequency shift keying (FSK) 50% amplitude modulated (AM) signal or pulse code modulated return to zero FSK type signal. The circuitry includes means for separating the composite signal into its constituent components and means for converting the separated signals into an amplitude envelope signal and a differentiated DC component. The circuitry includes a trigger circuitry for providing an output signal whe…

A detection device comprising two spark-gap electrodes connected to a voltage pulse source having an amplitude greater than the striking voltage of the spark-gap, so that each pulse produces a spark between said electrodes. Such a detection device is characterized in that it comprises, connected to at least one of the electrodes, a detection circuit able to supply at its output a signal whose amplitude is representative of a definite condition of the atmosphere present between said electrodes, p…

A conductivity probe unit comprising a chamber with a restricted entrance and an internal button electrode, the chamber being located in the header tank of a radiator.

Apparatus is tested for leaks by enclosing it by an enclosure which includes a flexible bag in its walls, the bag passing through its mouth when the pressure in the enclosure changes as a result of a leak, or comprises a member which flexes as a result of sorption of leaking materials.

A character segment detection system adapted for use with conventional scanning apparatus comprising translating apparatus disposed to illuminate an area of a page, detection means disposed for sensing the level of reflected light within a predetermined aperture from the paper and for producing an electrical signal indicative thereof, peak detector means connected to the detection means for producing the output signal indicative of the peak of the electrical signal, differentiator means connecte…

An alarm system comprising an elongate and flexible pressure line adapted to be charged with a selected line pressure; switching means coupled to the line and responsive to a pressure change in the line to create alarm signal; union means inserted in frictional sealing engagement within said line and having a bore which maintains the fluid continuity of the line; and spring-biased retaining means circumscribing said line and applying a selected clamping force to seal said union means in said lin…