| 摘要 |
A fire alarm system comprises a central control station feeding through a plurality of parallel supply lines a direct current voltage to a corresponding plurality of remote alarm stations. Each station has a fire sensor including a reference ionization chamber and a measuring ionization chamber, both chambers being connected in series between the two supply lines and having a common intermediate electrode. A field effect transistor has a gate electrode connected to the intermediate electrode of the ionization chambers and a source drain conducting channel the conductivity of which is controlled by the gate electrode. The input resistance and the input capacitance of the field effect transistor is connected parallel to the reference ionization chamber and defines therewith a first relatively short time constant of about two seconds. A two-stage switching amplifier is controlled by the channel of the field effect transistor and its output controls a light emitting diode serving as an indicator of the operative condition of the alarm station. The switching amplifier includes an integrating capacitor defining with the input resistance of the amplifier a second relatively low time constant of about 20 milliseconds, for example. The difference between the two time constants is utilized for checking from the central station the operative condition of respective remote alarm stations.
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