and 4 121 105 this chamber typically includes a detection chamber dc and a reference chamber rc the chamber connects between opposite polarity lines 43 and 44 these lines also connect to the impedance converter including an fet source follower 46 and associated resistor 48 the common electrode 50 of the chamber couples to the input electrode of the source follower 46 the output from the source follower is taken at one of the two output electrodes on line 52 which couples to both like inputs of comparators 54 and 56 the comparator arrangement and the associated resistive network of fig2 is like the one shown in fig1 c including resistors r1 r2 and r3 connected in series in this arrangement the voltage at the node x between the resistors r1 and r2 is set at a greater value than the voltage at the node y between the resistors r2 and r3 thus the comparator 56 gives the pre alarm output condition while the comparator 54 gives the full alarm output condition any one or more of the resistors r1 r2 and r3 may also include a potentiometer when smoke enters the chamber 42 the voltage at the output of the fet source follower 46 increases to a sufficient value to trigger the comparator 56 this is a pre alarm condition on output a at a higher voltage level determined by the scaling resistors r1 r2 and r3 the comparator 54 becomes triggered providing the output b or full alarm output in another embodiment of the invention the configuration of the chamber 42 may be altered by essentially interchanging the reference and detection chambers with such an arrangement there would be a decrease in voltage at the output.