The circuit is a simple X -band radar detector is capable of indicating changes in rf radiation strength at levels down to 2 mW/cm² . Radiation falling on the detector diode. produces a volt­age at the input of an amplifier whose gain may be adjusted to vary the range at which the warning is given. The amplifier output drives a voltage comparator with a variable threshold set to a level that avoids false alarms. The comparator output is connected in the wired-OR configuration with the open collector output of an oscillator running at a frequency of 2 Hz. In the absence of a signal. the comparator output level is low. inhibiting the oscillator output stage and holding the buffer so the lamp is off. When a signal appears. the comparator output goes high, removing the lock from the oscillator which free-runs, switching the lamp on and off at 2 Hz.  

   Schematic