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.

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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.563 months ago

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

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

Electrospark4 months ago

Thanks for all the useful information! :-)

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

reeceado6 months 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.
BobThAK1 year 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 for some more specific information.
awesome thanks!
znorris1 year 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.
sleeping1 year ago
Nice and simple, just the way I like it.
rimar20001 year ago
Very useful info, thanks for sharing it.
wilgubeast1 year ago
I always wanted to know how high- and low-pass filters work. Now I know too much. Great write-up, nyc640.
omnibot1 year ago