Picture of Passive Filter Circuits

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.


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.
Now, i wonder if there is an easy way to make an ACTUAL bandpass that is variable with a potentiometer?

shaddoty2 months ago
How many db per octave do these filter
ToXiCATOM3 months ago
I made the low pass filter again and it worked, maybe I made a mistake at thr first time. THANKS!
ToXiCATOM3 months ago
Thanks, I made them both nad the low pass one didn't work but the high pass worked perfectly.

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.

nyc640 (author)  patrick.kammerer.569 months ago

Thanks, it should be all fixed. Let me know if you spot any other problems.

There is also another problem. The schematic (under "How to build the circuit") for the high pass filter is identical to the schematic for the low pass filter.

nyc640 (author)  myouknowwho4 months ago

Good catch, I must have mixed up the URLs while fixing the earlier problem.

Yes, you are correct there. I guess I wasn't looking that closely. The C and R should be switched.

In the schematics, it would seem that input is shorted to ground. That's probably not what we want here :)

Electrospark10 months ago

Thanks for all the useful information! :-)

By the way, the image at step 4 is so true! ^_^

reeceado1 year ago

how do we work out which capacitors and resistors we need?

i need to make a 2 pole 1hz high pass filter

FirstSpear1 year ago
Thanks for that. Can I pick your brains a little further? I wanted a passive high-pass filter, and began with a known fcutoff of 300Hz. After much searching I found practical values of C and R as: C = 16nf, R = 33k.
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 "muddies" 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.
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.
BobThAK2 years ago
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!
znorris2 years ago
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.
sleeping2 years ago
Nice and simple, just the way I like it.
rimar20002 years ago
Very useful info, thanks for sharing it.
wilgubeast2 years ago
I always wanted to know how high- and low-pass filters work. Now I know too much. Great write-up, nyc640.
omnibot2 years ago