Abstract: The invention concerns a light-optical detection device for determining the concentration of suspended particles in a gaseous medium within a light-tight housing. The latter essentially is formed by a short cylinder sheath having two front-surface coverings that are formed by a base plate and by an opposite cover plate. Cylinders extend into said housing for receiving, respectively, a light source and a photocell. The cylinders have axes which intersect at right angles at the center of the device. The axis of the cylinder containing the light source essentially runs in the direction towards the diagonally opposite corner of the interior of the device.

Abstract: Several arrangement in which several measurement sources (fire alarms) are connected in parallel to a two-wire line. The supply voltage for the measurement sources is simultaneously fed through this line. Here, a measurement is formed by switching a measurement shunt circuit into the two-wire line, to generate measurement pulses, where the switch-in process is controlled by a local pulse generator. Each measurement source is provided with a pulse generator, and all these pulse generators operate at frequencies of the same order of magnitude. A monitoring circuit, at the far end of the two-wire line, is switched through by a pulse generator whose frequency is significantly lower. The pulse duration of the monitoring pulses is no longer than the pulse pauses of the measurement pulses.