Abstract: A radio-frequency source irradiates a space within which an intruder is to be detected, this space being illuminated by fluorescent tubes. Successive peak levels in the returned signals are compared and a fall in the peak amplitude results in a comparator, arranged to compare the peak amplitude with that of a preceding peak, failing to yield a reset signal to a timer circuit. After a time interval just greater than the periodicity of the a.c. supply to the fluorescent tubes the time yields an alarm control signal. If the peak value recovers during this interval the timer is reset and no alarm control signal is generated. The alarm control signals are applied to an integrator yielding alarm signals in response to predetermined repetition of the alarm control signals. The alarm signal is delayed and used to reset the integrator, as well as initiating an alarm condition.