The cut-off frequency of the circuit shown in the figure is 750 Hz, with the components used. The circuit applies to almost any operational amplifier and the power supply must be symmetrical.
This traditional two-tone alarm configuration with the 4011 integrated circuit was found in a 1998...
This extremely slow circuit produces one pulse per month and can be used in a timer. We found it...
This circuit is from an 80's documentation, making use of PUTs that are not currently common...