Found in Exar documentation from the 1980s, this circuit has a 60 dB cutoff at 1800 Hz. The circuit can be adapted to operate at other frequencies.
This simple transistor tester circuit was found in the February 1968 issue of Radio Electronics magazine....
A touch of the X1 sensor causes the motor to be activated for a time that can be adjusted in P1....
This is a traditional flasher configuration with two neon lamps. The frequency is given by the 100 nF...