This instructable is intended to show you how to make several different filter circuits, in particular, low pass and high pass filters, along with a discussion of notch/trap filters and bandpass filters.

What are Filters?

So what is a filter and why would you ever want to build one? Well, you might not end up building any of these circuits by themselves, but you may find yourself integrating them into more complex circuits. You already know what everyday filters do (e.g. air filters, water filters); electronic filters are no different. They take some signal, which in this case is a voltage signal composed of one or many frequencies, and filter out frequencies in a specific range.

High and Low Pass Filters

High pass filters are circuits used to remove low frequency signals and allow high frequency signals. Low pass filters do the opposite and are used to remove high frequency signals and allow through low frequency signals.


High pass filters are often used in speakers to filter out bass from an audio signal being sent to a tweeter, which could be damaged by the low frequency bass signals. They are also used to remove DC offset or DC bias from a signal, which could otherwise harm amplifiers and other electronic devices. In contrast, low pass filters can be used to filter out high frequency signals in audio being sent to subwoofers that can't efficiently reproduce the high-frequency parts of the audio signal. They are also used in devices such as in the tone knob of an electric guitar (to filter out treble), or in analog synthesizers.

Other Filters

Two other filter circuits that we will briefly discuss are the notch and bandpass filters. Notch filters are used to filter out a very specific range of frequencies, for example to filter out interference of a particular frequency if you happen to live next to a radio station. Bandpass filters do the opposite and will filter out everything but frequencies in a narrow range, and are thus used in radios to tune in to a specific frequency.

Step 1: The High Pass Filter

What You Need

What you put into the circuit:

  • One capacitor with capacitance C (in F).
  • One resistor with resistance R (in Ω).
  • An input signal (ours came from a function generator).
  • An oscilloscope (for testing).

What you get out of the circuit:

  • A filtered output signal.

How to Build the Circuit

high pass circuit diagram

Where do the parts go?

Your input signal is first fed into a capacitor that is connected at its other end to a resistor, which in turn is connected at its other end to ground. Your output signal should be read between the capacitor and resistor.

Test to make sure it works.

If you don't have access to a function generator or oscilloscope, you'll have to trust we tested the circuit for you correctly. We built our circuit as shown:

high pass filter

The red alligator clip carries our input signal (from a function generator), the black alligator clip lead to ground, and the green wire carries our output signal, which we sent to an oscilloscope for testing. As we went from low frequency signals to high frequency signals, the result we read on our oscilloscope looked like this:

high pass filter gif

The yellow curve is our input signal, and the blue curve is our output signal (note that while the yellow curve appears to remain the same, it is because we were changing the frequency scaling on the display of the oscilloscope). At low frequencies, you can see that the entire signal is filtered out and we get almost no output signal. As frequency increases, the output signal becomes larger, until it reaches a point at which it is nearly the same as the input signal. This point is called the cutoff frequency, and we will show you how to find it later. You should also note that the output signal can be phase-shifted from the original input signal, meaning that although the signals have the same frequency, they aren't necessarily "in step", so to speak.

Also, note that while we intentionally inputted signals of a uniform frequency at a time, the circuit will work for compound signals.

The Cutoff Frequency

Calculate the cutoff frequency.

The cutoff frequency is generally considered the frequency at which the signal is attenuated (or filtered). This means that any signal with frequency below the cutoff frequency is considered to be filtered, and any signal with frequency above the cutoff is considered to be "left alone" or unfiltered. So what is the cutoff frequency?

cutoff freq

where R is the resistance of your resistor in Ω and C is the capacitance of your capacitor in F.

What does the cutoff frequency mean for the signal?

If you take a look at the ratio of the amplitude of the output signal to the amplitude of the input signal over a wide range of frequencies, you will get something that looks as so

high pass gain

Note that both axes are log-scaled. This means essentially that if you move up from one light gray line to the next, your value is actually increasing by 10 times. This means that if you looked at this plot without log scaled axes, you would essentially see an almost vertical drop off at the cutoff frequency. Any signal or component of a signal that has frequency higher than the cutoff frequency, however, is unfiltered, to a good approximation.

The output signal is phase shifted from the input.

We said earlier that the input and output signals are not "in step" and are actually shifted. This may be fine for some applications, but there are other applications where this may be important. The phase shift changes as the frequency of the input signal changes, just as with the gain, and the plot of this change looks as so

high pass phase

At low frequencies, the output signal is phase shifted by π/2, and at high frequencies the phase shift is almost zero. The cutoff frequency is important to the phase shift because it is the frequency at which the output signal is phase shifted by exactly half of π/2, or π/4.


If you really want to know, go check out the Theory section for more information about how the circuit works and how we calculated the gain and phase shift.


  1. The high pass filter can be made as follows:
    • Input - Capacitor - (Output) - Resistor - Ground
  2. High pass filters filter out signal with frequencies below the cutoff frequency (1/2πRC).
  3. Since the cutoff is strictly determined by R and C, choose the appropriate resistor and capacitor to cutoff frequencies where you want to
  4. The output signal is phase shifted from the input.
<p>I'm trying to build a tone control for a bass preamp. Preferably a bandpass filter. I have seen circuits that uses a potentiometer sort of backwards, where the center lug is the input and have capacitors on the outer lugs that each go to ground, forming a variable lowpass of sorts. The potentiometer determines how much of each lowpassed signal that should go to the amp. <br>Now, i wonder if there is an easy way to make an ACTUAL bandpass that is variable with a potentiometer?</p>
How many db per octave do these filter
I made the low pass filter again and it worked, maybe I made a mistake at thr first time. THANKS!
Thanks, I made them both nad the low pass one didn't work but the high pass worked perfectly.
<p>Just noticed that you have a problem in your schematics, and you might want to fix it.In both the low pass and high pass schematics, you have the input connected to ground. This would not work, and I think you know that as your breadboard pictures did not have this connection. Just wanted to help.</p>
<p>Thanks, it should be all fixed. Let me know if you spot any other problems.</p>
<p>There is also another problem. The schematic (under &quot;How to build the circuit&quot;) for the high pass filter is identical to the schematic for the low pass filter.</p>
<p>Good catch, I must have mixed up the URLs while fixing the earlier problem.</p>
<p>Yes, you are correct there. I guess I wasn't looking that closely. The C and R should be switched.</p>
<p>In the schematics, it would seem that input is shorted to ground. That's probably not what we want here :)</p>
<p>Thanks for all the useful information! :-)</p><p>By the way, the image at step 4 is so true! ^_^</p>
<p>how do we work out which capacitors and resistors we need?</p><p>i need to make a 2 pole 1hz high pass filter</p>
Thanks for that. Can I pick your brains a little further? I wanted a passive high-pass filter, and began with a known <strong><em>fcutoff</em></strong> of 300Hz. After much searching I found practical values of C and R as: <strong>C = 16nf</strong>, <strong>R = 33k</strong>.<br> The site that gave me the values was outputting the circuit into a preamplifier. I can't do that, I can only place the filter between amplifier and speaker - it's a DAB radio with no tone controls, and a single speaker. I only listen to talk stations, and there is some bass boominess that &quot;muddies&quot; the sound. The amplifier is rated at 4.5 Watts RMS, but I don't know the resistance/impedance of the speaker - this is not supplied with the specifications - and I intend to apply the circuit only when the guarantee expires, and so can't measure anything yet.<br> I'm thinking that the wattage rating of the resistor, and the operating voltage of the capacitor might be important considerations, but are there any other considerations? Any advice or suggestions gratefully received. Cheers.
very nice. very well explained. thanks!
could you make this with a variable resistor to make it search through channels like a regular radio?
Yeah! You can check http://www.instructables.com/id/How-to-Build-and-Tune-an-AM-Radio/ for some more specific information.
awesome thanks!
I'm going to try and use this in conjunction with my cheap software defined radio. Can't wait! I'll let everyone know how it turns out.
Nice and simple, just the way I like it.
Very useful info, thanks for sharing it.
I always wanted to know how high- and low-pass filters work. Now I know too much. Great write-up, nyc640.

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