Combustible gas detection system

Claim 1

In a combustible gas detection system having a central control station and a remotely located combustible gas sensor of the catalytically activated resistance bridge type providing an analog signal indicative of the concentration of combustible gas at the sensor location, a noise immune transmission system for transmitting signals from said sensor to said control station, comprising a two-wire transmission line connecting said control station with said sensor location, means supplying d.c. power to said transmission line; means at said sensor location for converting d.c. power received from said transmission line to a.c. power; means for converting said a.c. power to regulated d.c. operating power for said sensor; an analog to digital converter at said sensor location for converting said analog signal from said sensor into a parallel format binary digital signal; means for converting said parallel format digital signal into a serial format digital signal; a clock oscillator at said sensor providing at least a pair of harmonically related, phase coherent clock signals of different frequency; keying means controlled by said serial format digital signal for selecting one of said clock signals upon the appearance of a logic "1" in said serial digital signal and for selecting the other of said clock signals upon the appearance of a logic "0" in said serial digital signal; and an optically coupled isolator for applying said clock signal selected by said keying means to said transmission line, said isolator including a light emitting diode to which said clock signal selected by said keying means is applied, a load resistor, and a phototransistor connected through said load resistor to said transmission line, said phototransistor responding to light emitted by said light emitting diode to vary current through said load resistor.