Figure 1.  Circuit Diagram for a Tunable Band-Pass Filter

This is a circuit that can be tuned to only allow input signals within a certain range of frequencies to pass to the output, hence the name 'tunable band-pass filter'.  All signals with frequencies lower or higher than this range are attenuated (weakened or decreased in level) by this circuit.

The circuit is an active filter that uses a 741 operational amplifier IC configured to pass a narrow band of frequencies ranging from a few hundred hertz to about 3 kHz. This circuit may be used to detect the presence of a tone in this frequency range.  Variable resistor R2 is used to 'tune' the center frequency of this filter.

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