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How to Build a Band-Pass Filter

A band-pass filter is an electronic circuit that permits a particular signal frequency, called the resonant frequency, to pass through it unchanged, while reducing the strength of higher and lower frequencies. The further from the filter's resonant frequency, the more the filter reduces the signal. Band-pass filters come in several designs. Some are simple, and some are complex. The simplest, called a passive band-pass filter, consists of three components: a resistor, an inductor and a capacitor. Unlike active band-pass filters, this design does not amplify a signal, but it demonstrates every aspect of the band-pass effect.

Things You'll Need

Solderless breadboard
1-millihenry inductor
4 pieces ofwire, 12 inches in length
.1-microfarad 50-volt capacitor
1K-ohm 1/4-watt resistor
Labeling tape
Oscillator or other signal source

Instructions

- 1
  Insert the leads of the inductor into the breadboard. The columns on the board in which you put the leads must be adjacent. Connect the end of one 12-inch wire to one of the inductor's leads by placing it in the same column as the lead. Connect one lead of the .1-microfarad capacitor to the other inductor lead. Set the capacitor's free lead into an unused column on the board.
- 2
  Insert one lead of the 1K-ohm resistor into the same column as the one with the capacitor's free lead. Insert the end of a second wire so it shares this column on the breadboard. Insert the resistor's other lead into an unused column. Insert the two remaining wires into this column.
- 3
  Grasp one of the last two wires you inserted and wrap it a few turns around the wire connected to the inductor. Write "Input" on a tape label and wrap the label around these two wires.
- 4
  Wrap the remaining one of the last two wires a few turns around the wire connected to the resistor and capacitor. Label this pair of wires "Output." Connect a signal source, such as an oscillator, to the pair of wires labeled "Input" and connect the "Output" wires to an amplifier, oscilloscope or other device to monitor the filtered signal. With this circuit's inductor and capacitor values as listed, the filter will pass frequencies near 16,000 hz and progressively remove higher and lower frequencies.

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