Home >> Science & Nature >> Science

How to Build a Low-Pass Filter

Some electronic filters allow part of a signal to pass through while blocking other parts. These filters always have the word "pass" in their names: low-pass, bandpass and high-pass. Low-pass filters allow the lower frequencies in a signal while blocking sound waves above a certain limit. But this "clipping" is not completely clean, as most of the low frequencies get through and all of the high frequencies are greatly reduced. All frequencies are reduced somewhat unless an amplifying device -- such as a transistor -- is included in the filter.

Things You'll Need

Resistor
Capacitor or coil
Transistor (optional)

Instructions

- 1
  Construct a low-pass filter based on a couple of parameters. One of is the cutoff frequency, below which you pass the signal. The other consideration is "passive" or "active." Active low-pass filters include a transistor so signals passing through are not reduced. Passive filters come in two styles: RC and RL. Passive filters always reduce signals coming through, though only by a small amount if the signal is below the cutoff point. Reduction by a passive filter is almost complete if the signal is above the cutoff point.
- 2
  Build a low-pass filter that has cutoff frequency below 100,000 Hertz with a resistor and a capacitor -- an RC filter. The resistor is in series with the signal. The capacitor is across the signal, just before the resistor. High frequencies take a short path through the capacitor and avoid the resistor. The value of the capacitor determines the cutoff frequency. When the frequency is high enough, the capacitor offers little resistance to signals. If the frequency is low enough, the capacitor offers more resistance than the resistor, and the signal will go through the resistor.
- 3
  Build a low-pass filter that has a cutoff frequency above 100,000 Hz with a resistor and a coil -- an RL filter. The coil is in series with the signal. The resistor is across the signal, just before the coil. Low frequencies go straight through the coil because, at low frequencies, it offers little resistance. The value of the coil determines the cutoff frequency. Above the cutoff frequency, the signals find it easier to take a short return path through the resistor.

Science