Real-time Audio Frequency Meter (up to 20 kHz)

Arduino Uno Rev3

MAX4466 microphone

Arduino IDE (any version)

Resistors

<html><body>This project monitors sound frequencies in real time by detecting sound zero crossings using inbuilt Arduino UNO Analog Comparator (pin 6/7). To make it robust against noise, a small external circuit (four resistors) is used to create hysteresis in the detection threshold programmatically. A moving average of the detected sound periods can computed to smooth the signal (default window size = 16 periods), but this can also be turned off (when a very quick response time is needed). The default configuration detects frequencies between 16Hz - 20kHz within 0.1% accuracy (if the tone is pure). The frequency range and resolution can be configured (esp. useful if lower frequencies need to be detected, since for higher frequencies code execution times start to become a bottleneck). <h3>External components and wiring</h3><figure><img src="https://projects.arduinocontent.cc/d9fb9fc5-bf40-4504-880d-96ea5b6de5c4.png"/><figcaption></figcaption></figure><ol><li>Pin 6 (AIN0): Audio signal, or other signal (centered at 2.5V) </li><li>Pin 7 (AIN1): Reference voltage (crossing detection threshold) </li><li>Pin 5 (Digital OUTPUT): Feedback line for creating hysteresis in threshold voltage. In the above configuration the hysteresis is about (10k/220k) * 0.5 * Vcc = 0.1 V </li></ol>Here is an example of the signals at pin 6 (AIN0, yellow) and pin 7 (AIN1, pink) for a 247 Hz tone of about 2 Vpp. The pink signal shows the hysteresis in the threshold created by toggling output pin 5 HIGH/LOW when a downwards/upwards crossing is detected. <figure><img src="https://projects.arduinocontent.cc/b30c3e79-3370-411c-92e5-2b6036b57a85.jpg"/><figcaption></figcaption></figure><h3>Example output (shown in Serial Plotter)</h3>Example 1. In the example below I played five open strings on my guitar (5th string down to the 1st string). After each string I damped the sound, since the meter can only detect a single frequency at a time. Here I used a delay of 0.1 s between the printed samples. As a microphone I used my phone (with a "microphone amplifier" app) connected to the circuit via its headphone audio jack (and a small decoupling circuit, capacitor + resistors). <figure><img src="https://projects.arduinocontent.cc/718cc9ab-91cb-4af4-85ba-ee8fa2308275.png"/><figcaption></figcaption></figure>Example 2. Sound from a cheap dog whistle. Gentle blowing gives a frequency of about 3200 Hz, blowing more firmly generates an overtone at about 7500 Hz. <figure><img src="https://projects.arduinocontent.cc/b226e229-6403-431d-8f7b-5c378a3e2903.png"/><figcaption></figcaption></figure>Example 3. Using a tone generator app (from <a href="https://phyphox.org/">phyphox </a>) to play a 5ms "sonar" tjirp (frequency smoothly increasing from 1000-7000Hz in a timespan of 5ms). To capture the fast signal in the Serial Plotter a high baud rate (1M) was used and the loop() was freely running (no delay() and no moving average). The phone headphone output was directly connected to the Arduino pin 6 using a small decoupling circuit.<figure><img src="https://projects.arduinocontent.cc/2d4275ed-72e9-4ee9-9329-7bc3bf4f7958.png"/><figcaption></figcaption></figure> </body></html>

Real-time Audio Frequency Meter (up to 20 kHz)

Monitor sound frequencies in real time using Arduino UNO's inbuilt Analog Comparator (pin 6/7), a microphone and four resistors. Uses fast method of zero-crossing detection (no FFT!) and adjustable threshold hysteresis for noise robustness.