High gain switching and detection element for smoke detectors - uses FET which only conducts when voltage detected corresponds to minimal quantity of combustion products

摘要

A high gain switching and detection element for smoke detectors has a F.E.T. whose gate and source are connected to the voltage detection circuit of the measuring chamber. The F.E.T. (80) is chosen so that it only conducts when the detected voltage corresponds to at least a predetermined minimal quantity of airborne combustion products in the chamber (70). The element has also several carcade connected normally non conducting electronic switches (86-90). The first switch (86) being connected to the F.E.T. (80) so that it conducts each time the F.E.T. (80) conducts. All the other carcaded switches being connected to the first so that they conduct when it conducts. The last of these switches (90) is connected to an alarm device (92) which triggers an alarm when the last switch conducts.