Abstract: An apparatus for detecting turbine blade passing times includes a sensor for producing an input signal each time a blade passes the sensor. Automatic gain control circuitry including a digital multiplier, peak detector and error amplifier regulates the amplitude of the input signal. A zero-crossing detector produces an output signal each time the input signal crosses a reference axis. A pre-trigger comparator produces a gating signal coinciding with the expected arrival time of the blade at the sensor. A gating device is responsive to the gating signal for conducting output signals which occur during the expected arrival time of the blade at the sensor. A monostable vibrator produces a digital pulse coinciding with the time at which the blade passes the sensor.