Picture of Arduino Frequency Detection
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As a follow up to the Arduino Audio Input tutorial that I posted last week, I wrote a sketch which analyzes a signal coming into the Arduino's analog input and determines the frequency.  The code uses a sampling rate of 38.5kHz and is generalized for arbitrary waveshapes.  I've also turned the LED attached to pin 13 into a clipping indicator, so you know if you need to adjust your signal's amplitude as you send it into the Arduino.

Some project ideas for the code presented here include:

pitch reactive projects- change the color of RGB LEDs with pitch, or make a lock that only opens when you sing a certain pitch or melody
audio to MIDI conversion- get the Arduino to translate an incoming signal into a series of MIDI messages. See my instructable about getting the Arduino to send and receive MIDI for lots of example code to get started
audio effects- use the frequency information to reconstruct an audio signal from the tone() library or with some stored samples to make a cool effects box/synthesizer

The first step of this project is to set up the audio input circuit.  I wrote a detailed Instructable about that here.

Step 1: Detection of Signal Slope

Picture of Detection of Signal Slope
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First I wanted to experiment with peak detection, so I wrote a piece of code (below) that outputs a high signal when the incoming audio signal has a positive slope, and outputs a low signal when the incoming audio signal has a negative slope.  For a simple sine wave, this will generate a pulse signal with the same frequency as the sine wave and a duty cycle of 50% (a square wave).  This way, the peaks are always located where the pulse wave toggles between its high and low states.

The important portion of the code is reproduced below.  All of this code takes place in the ADC interrupt (interrupts and runs each time a new analog in value is ready from A0, more info about what interrupts are and why we use them can be found here)

  prevData = newData;//store previous value
  newData = ADCH;//get value from A0
  if (newData > prevData){//if positive slope
    PORTB |= B00010000;//set pin 12 high
  else if (newData < prevData){if negative slope
    PORTB &= B11101111;//set pin 12 low

I should note here that in this tutorial I use direct port manipulation to turn off and on the output pin (pin 12) of the Arduino.  I did this because port manipulation is a much faster way of addressing the Arduino's pins than the digitalWrite() command.  Since I had to put all the code above inside an interrupt routine that was going off at 38.5kHz, I needed the code to be as efficient as possible.  You can read more about port manipulation on the Arduino website, or see the comments I've written above to understand what each line does.  You'll also notice in the code below that I used some unfamiliar commands in the setup() function so that I could get the Arduino's analog input to sample at a high frequency.  More info on that can be found in my Arduino Audio Input tutorial.

Fig 1 shows the pulse output in blue and the sine wave in yellow on an oscilloscope.  Notice how the pulse output toggles each time the sine wave reaches a maximum or minimum.  Fig 2 shows the pulse output in blue for an arbitrary waveshape in yellow.  Notice here how pulse wave takes on an irregular duty cycle because the incoming signal (yellow) is much more complicated than a sine wave.

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Thanks for the great project info. I've tried it out using an electret microphone with an amplifier, but I'm not getting a clean output on the serial monitor. Basically, the frequencies it spits out are all over the map. For instance, if I play it a note around 70Hz (I'm planning on putting it on my upright bass, so I've been using that to test) it will give me a bunch of frequencies that could be from 10Hz to 7000Hz, then give me several in a row that are about 70Hz, then back to all over, etc. Even when I played a 440Hz note from a tuner it gave me a decent amount of randomness.

My guess is that I need to adjust the slopeTol and timerTol values, since the random frequencies are often (though often not) some multiple of the one I'm looking for, making me think that it's missing some crossings. Am I thinking right? I don't really know which one to change and by how much; I've messed with them both and they're currently at slopeTol = 2 and timerTol = 15. Any ideas on some ballpark ranges for these if I'm looking to measure tones from my bass? Is there some other part that I should tweak? Thanks for the help. I'd really like to use your method instead of FFT or FHT, since it seems much more efficient for my project.

One thing I thought of: right now I'm powering the Arduino through the USB, since I'm using the serial monitor to debug. Would using the USB as a power source be adding noise to the signal? My project will be powered by a 9V battery and I have one hooked up now, just turned off. Would it help to have the 9V source on in addition to being plugged in to the USB? I have to keep it plugged in to use the serial monitor, but if that's what's causing the problems...

Thank you for your sharing Amanda!! I don't have to start from the zero point on my automatic guitar tuner project!

But I have a question.Can I use a piezo to pick up the vibration or the sound instead of using a mic,which enables the tuner to work in noisy environment?

Yes you can. Im using it on a violin. At the moment the algorithm is a little sporadic but if you turn down the time and slope tolerances it works a little better. I think if you restructure the code to get the full 10 bit resolution instead of 8 bits, you should be able to make this code work better. But the extra instruction cycles might hinder any other operations...

I've got a project along these lines as well, and I wondered that same thing about picking up noise from other sources (or instruments, since I plan on using it while playing). Using a piezo came to mind, glad to see someone has done it successfully. I was wondering, what did you use to stick your piezo on your instrument? I don't want do anything that would damage the wood. Did you use some sort of adhesive that's safe for string instruments?

I tried a under the bridge pick up, as well as a bridge stick on pickup; A generic rod piezo and a Shadow SH 3001 respectively. I used it on a dedicated electric so the rod piezo was appropriate and much cheaper, but there are film-type under the bridge pickups like Realist pickups that should work for you. I have found that blending both types of pickups (horizontal and veritcal) through a cheap behringer micromix mx400 gives really good results, although you'll have to contend with two wires instead of just one.

Gotcha, thanks. I'll see if I can find something like that for my upright bass. I'm planning on using it to control an LED strip, so I probably don't need the quality to be quite that high; one pickup should do it. Did you need to wire it up with the whole op amp setup from the Audio Input tutorial, or do some have amplifiers built in?

Yes, you'll need to amplify it a little bit based upon the voltage response characteristics of the element you are using. You'll have to tinker because no two piezo's are the same. There are a lot of projects for arduino out there on how to do this, just search "arduino audio input" if the above schematic isn't descriptive enough for you.

LucasP211 months ago

So with this arduino code I would be able to control a the light's frequency with the frequency of the sound? I want to build a lighting system for my room and car that will react with the sound, but in a way so that bass="cool colors", treble= "warm colors"(or even vice versa). I feel like that would give a better experience to the music

nkuck LucasP22 months ago

Did you manage to get your idea working? I have a similar project idea.

rien.brand8 months ago

Hi Amanda,

Thanks for the very usefull work you have done, nou I can measure the frquency from my agriculture application that use a exciter ring.

I have one qustion how can I change to another analog pin in the ide?


Rien Brand


Not sure if you found the answer to your quesiton, but if you look up ADMUX register in the datasheet for your particular atmega you'll see how to change the analog to digital converter.

Here's a link to understand ADMUX


rjtsh4 months ago

We did it, finally facing problems. We used electric guitar for this purpose. Thanku Amanda. Keep up the good work !

IchbalB5 months ago

Hello Amanda thanks for your sharing. i want to ask about this one..
so i am developing an Arduino human scream detection is it possible to me detect human help scream voice using this kind of Arduino program? because i want to build up the component consist by band pass filter , gain, and rectifier circuit and then look for its pitch detection and voice duration..

Do you have any idea about the circuit that i`ll need to complete my project?
Or is it possible enough to use some Arduino library and write the speech recognition code?

성범강6 months ago

Hi Amanda, thank you very much for this source!! very amazing!!!

but... little problem my arduino...

your source is exellent !!

but.. my arduino is not working

my arduino version...is...

with 256K Bytes


version is problem is different???

byondo7 months ago

Hi Amanda, thank you very much for this instructable! Very useful!

I prefer this simpler code version (integrated with amp treshold), because I want to make a sound analyzer , where you whistle or sing a "oooh" note and then you have the frequency. Making a portable built with a 4 digit display and there it is a perfect gadget for any sound engineer or musician, who wants to know which frequency causes microphones feedbacks.

Does it make any sense collecting a number of measurements and make a mathematical average? as I don't need a fast process, but a slower and more solid (without "garbage" measurements); I tried some "for" cycle with a simple array, but it doesn't seem to add any precision (maybe I don't know where to put it)..

renaissa9 months ago
Hi amanda...your instructables on arduino are great..however i want to know if it is possible to measure a amplitude of sine wave on arduino
shinew9 months ago

Thanks for the instruction! I just put together a circuit with an amplified electret mic signal using opamp feeding into it. However, when I'm looking at the serial output, it only seems to work for the first second or 2, then it stops working with the last line looks like attached image. Any idea what could be the cause? thanks!

Screenshot 2014-11-21 01.45.01.png
weichi.chien9 months ago

Hi, this is an excellent project. It works fine on my arduino uno. However, I need it to run on a arduino mega with ethernet shield. When adding the codes to my ethernet project, the connection to internet is brocken (I'm uploading data onto xively server). Any suggestion?

buddika12310 months ago
How we meaured the frequency ....I want to detect frequncy range 60Hz to 100Hz....can I use this progtame thanks and quick reply from you
Derpancakes10 months ago

This is awesome! I'm looking in to a guitar-MIDI pedal, but a DIY solution is cheaper and way cooler. Props to you for this awesome bit of code!

novaninjas11 months ago

Hi, has anybody completed this project (possibly, with a diff sort of mic) and wouldn't mind sharing about it? Am a beginner and would really like to try this so.. slightly more detailed steps would be most helpful :D

pierattilio12 months ago

Hi amanda, i want to congratulate with you on this instructable, really powerful :D I have a question, that is is it possible to check the frequencies of a playing song in this way, through an aux signal from i.e. an MP3 player or the notebook audio headphones output (after the right managment of the signal like in your instructable on arduino auido input)?

Thanks a lot


aoss1 year ago

Has anyone made this project with ChipKit Uno32. I am really looking for projects like that for my Uno32 developement board because I need to read analog signal frequency coming from optical rotation sensor.

Great work! I'm trying to use this code for an automatic laser oscilloscope I am designing.

I've found that the code stops measuring frequencies as soon as I try analogRead() from a different analog pin. At the moment I don't fully understand the lower level programming of the ADC that you've done. Is there any way to read from the other analog pins without interfering with the pitch measurement?
amandaghassaei (author)  Bokononestly2 years ago
yes, the way this is set up, all of the other analog pins are deactivated. you can read an analog value from a digital pin by using RCTime:
hope that works for what you're doing.

Thank you for the great article. I was just wondering can I use a microphone with a preamp to make the detection.

I have :


Thank you very much!

arimika1 year ago

i tried to build guitar tuner http://www.instructables.com/id/Arduino-Guitar-Tun...

but arduino can't read the input from my guitar.
can you help me to solve this problem ?

amandaghassaei (author)  arimika1 year ago
do you have an oscilloscope?

i don't have oscilloscope.
does become a problem if I use 100nF Cap on the DC offset ?

i see your project use 47nF Cap.

amandaghassaei (author)  arimika1 year ago

no that should be fine. You will need an oscilloscope to debug this, maybe see if there's a hackerspace or school near you where you can use one for free.

what is the main problem that the input from guitar can not reach the Arduino?

whether the project could be completed if there is oscilloscope ?

amandaghassaei (author)  arimika1 year ago

you could try using a computer running audacity as a makeshift oscilloscope, to find out where the signal is getting lost. Do you have an audio input on your computer?

yes, i have.
how to make audio output from amplifier and DC offset circuit into my computer ? there are 3 outputs "A0 , 5v and GND"

I'm new in Arduino, I want to make this but with an Electret Microphone, I think it's very different than the microphone you used. How can I make the circuit with an Electret Microphone?

amandaghassaei (author)  andrew954341 year ago

it shouldn't be too different, but you might need to change the gain on the amplifier going into the Arduino's analog pin. You will probably want an oscilloscope handy so you can see what you're doing.

Do you know if there's a virtual oscilloscope which I could use? Like a PC program.

About the microphone, I'm using an Elecret Microphone with Breakout Board (amplifier included), I connect it directly to the Arduino, is everything alright?

amandaghassaei (author)  andrew954341 year ago

i guess you could use the audio input on your comp and record using Audacity or something.

sweller1 year ago

I'm so happy I stumbled onto your Instructable! This is *exactly* what I'm attempting to do. Unfortunately, I'm trying to do it with a Picaxe. :-(

EthnoChris1 year ago

Hey Amanda! Awesome Instructable. This has been a great learning experience. I managed to get the code ( and circuit from your Arduino Audio Input Instructable ) working fine on an Uno clone, but when uploading the same sketch to an Arduino Micro it doesn't seem to update the output values when I print the variable ( I just get "inf" ).

Is there a difference in the Micro that would prevent this sketch from working? I'm using the same sketch and circuit for both versions ( I've only swapped out the Uno to a Micro ), but have I potentially done something incorrectly?

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