Smoke detector system with electrical operation of ionisation chamber - has step level circuit which includes programmable unijunction transistor

摘要

The smoke detector has an ionisation chamber which includes a radiation source and two separated electrodes. The ionisation chamber has an electrical resistance which varies as a function of the smoke in the free space between the electrodes. A resistive element is connected in series with one of the chambers electrodes so as to form a first voltage divider. The junction of the resistive element and the electrode forms the dividers output. An annunciator is actuated by a signal from a step level circuit to provide an alarm. The step level circuit is connected between the output of the first divider and the annunciator so as to detect a dangerous concentration of smoke in the chambers. The step circuit includes a programmable unifunction transistor (50) and a second voltage divider which establishes the step level. The control electrode of the unifunction transistor (50) is connected to the output of one of the dividers. The anode (17) of the transistor (50) is connected to the other divider (52a).