each time the detector output a breaks out of the voltage range defined by the threshold levels of refs a and b the output of differentiator 22 triggers a conventional one shot multivibrator 24 which when triggered provides a pulse of predetermined pulse width the one shot output d serves the dual function of initiating or resetting the timing signal f provided by the timing circuit 52 and of keying a current source 26 to produce a current pulse of the same pulsewidth as the one shot output the amplitude of the pulse produced by the current pulse generator may be adjustable nominally high h medium m and low l to provide a means for adjusting the system sensitivity the output e of the current pulse generator is integrated by integrating means 30 which may comprise a conventional rc network 32 and the integrated output g thereof serves as one input to threshold sensing means 40 the latter may take the form of a differential amplifier a4 as indicated above when the integrator output g exceeds a certain threshold determined by the other input of the threshold sensor i e ref c an alarm relay 60 or the like is energized if however the alarm threshold is not exceeded by the integrator output within the time interval defined by timing circuit 50 the.