Low Pass Filter Circuit

In this project, we will show how to build a low pass filter.

This is a filter that passes through low frequency signals without any attenuation and blocks high frequency signals from passing through. High frequency signals will either be completely blocked or greatly attenuated as output.

To build a Low pass filter, we will be using the following components:

Components

A function generator (or any device that can produce AC and DC signals)
0.1µF ceramic capacitor
10KΩ Resistor

This is the schematic of the circuit we will build, shown below:

We input two signals into this circuit to check its output. We first input a DC or very low frequency ac signal into the circuit. The most common source to provide this signal would be from a function generator. If you place the signal to a very low frequency signal such as 0.1Hz (100 mHz) and then take probes from an oscilloscope and measure across the capacitor, you will see that the low frequency signal passes through unimpeded without any attenuation. If you now increase the frequency of the signal to 1KHz, the signal will not go through the route of the capacitor. High frequency signals will not drop down through the capacitor but go straight across. So if you measure the voltage of the high frequency signals, you will either read nothing or very low voltage, since the capacitor does not store high frequency signals.

This is a diagram below showing what happens to low and high frequencies when fed into this low pass filter:

The capacitor has no reactance at high frequency signal and, thus, does not store them. Therefore, these signals do not show up on output. The Low frequency signals are stored by the capacitor and pass to output. This is why it's a Low pass filter. It passes through low frequencies but block high frequencies.

Note that this low pass filter only passes through very low frequencies. The frequency of the signal has to be very close to 0Hz for the capacitor to have effect (for there to be capacitive effect). This means that the frequency should only be a few millihertz, at most 1Hz. Anything above 1Hz and the capacitor stores less and less of the signal. Even at 4Hz, there is no capacitive effect, so a 4Hz signal will not be blocked at all but passes through unimpeded and unattenuated. This Low pass filter only blocks very low frequencies, near 0 Hz.