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How to make a secret knock detector to trigger anything with only an Arduino and a few cheap components

There are a couple good use-case scenarios for making a secret knock detector using an Arduino.

  • You are a spy who needs to authenticate your cohorts
  • You are a super hero who wants a secret knock to open the entrance to your lair

Whatever the reason – by the end of this tutorial you will know how to use an Arduino, a piezo transducer and a couple other cheap components to make secret knock detector.

Here is an overview of exactly what we will talk about in this lesson:

  1. The components you will need and how to set up this simple circuit.
  2. The concept of operation of this secret knock detector
  3. A thorough description of each block of code in the Arduino sketch
  4. Why North American grizzly bears love piezo transducers

For this secret knock detector circuit you need:

  • Arduino (I use the Arduino Uno) [1]
  • Solderless breadboard [1]
  • 1 Mohm Resistor [1]
  • Piezo transducer (aka buzzer) [1]
  • Jumper wires [4]
  • 5.1V Zener diode (for extra protection) [1]
  • No spill stopper for a “to-go” coffee cup

How to set up the Circuit:

This is a really simple circuit to setup, below are step-by-step instructions and a breadboard diagram.

  1. Place the piezo transducer on the breadboard, with the positive lead and the negative lead on separate rails.
  2. Connect the positive lead to pin A0 on the Arduino and the other lead to ground.
  3. Finally, use the 1Mohm resistor to connect the leads of the piezo transducer.
  4. As an additional level of protection, you might consider adding a 5.1V zener diode between the leads to protect against high voltage spikes from frying your input pin – you might call it a cheap insurance policy.

Piezo buzzer circuit with Arduino

An Overview of this Secret Knock Detectors operation

Here is the basic concept of how this will work.

We want something to happen when you tap out a secret code.

We will create a sequence of soft and hard taps – this will be our secret code which will be represented as 0’s and 1’s in an array.

For example:

secretKnock = {0, 0, 1, 0};

The code above represents a secret code of soft , soft , hard, soft .

The piezo transducer will turn the mechanical pressure created by the tap into a signal that the Arduino analog pin can read. The level of the signal will determine whether a tap gets characterized as soft or hard.

The threshold of a soft vs hard tap need to be determined empirically, once you have the circuit built – it will depend on what you have the piezo transducer attached to – I have mine taped to a piece of paper.

You should start with the default threshold values provided in the sketch and change them to suit your specific setup.

Once a tap signal is picked up by the Arduino, the sketch will compare the entered sequence of taps to the secret code, one tap at a time.

If the code is entered correctly, then we will trigger an action on the output pin. In this code below, we trigger an LED to turn on for a couple seconds – but you could trigger a servo arm, a pump, or whatever you might need.

If the code is entered incorrectly – nothing happens.

Here is the code for your hacking enjoyment:

/* A simple sketch to detect a secret knock using a piezo transducer

Created JUL 2015 by
Michael James
Home
This code is in the public domain */ const int outputPin = 6; // led indicator connected to digital pin const int knockSensor = A0; // the piezo is connected to an analog pin const int thresholdHIGH = 150; // threshold value to decide when the detected knock is hard (HIGH) const int thresholdLOW = 120; // threshold value to decide when the detected knock is gentle (LOW) const int secretKnockLength = 4; //How many knocks are in your secret knock /* This is the secret knock sequence * 0 represents a LOW or quiet knock * 1 represents a HIGH or loud knock * The sequence can be as long as you like, but longer codes increase the difficulty of matching */ const int secretKnock[ secretKnockLength ] = {0, 0, 1, 0}; int secretCounter = 0; //this tracks the correct knocks and allows you to move through the sequence int sensorReading = 0; // variable to store the value read from the sensor pin void setup() { //Set the output pin as an OUTPUT pinMode(outputPin, OUTPUT); //Begin Serial Communication. Serial.begin(9600); } void loop() { // read the piezo sensor and store the value in the variable sensorReading: sensorReading = analogRead(knockSensor); // First determine is knock if Hard (HIGH) or Gentle (LOW) //Hard knock (HIGH) is detected if (sensorReading >= thresholdHIGH) { //Check to see if a Hard Knock matches the Secret Knock in the correct sequence. if (secretKnock[ secretCounter ] == 1) { //The Knock was correct, iterate the counter. secretCounter++; Serial.println("Correct"); } else { //The Knock was incorrect, reset the counter secretCounter = 0; Serial.println("Fail - You are a spy!"); }//close if //Allow some time to pass before sampling again to ensure a clear signal. delay(100); //Gentle knock (LOW) is detected } else if (sensorReading >= thresholdLOW) { //Check to see if a Gentle Knock matches the Secret Knock in the correct sequence. if (secretKnock[ secretCounter ] == 0) { //The Knock was correct, iterate the counter. secretCounter++; Serial.println("Correct"); } else { //The Knock was incorrect, reset the counter. secretCounter = 0; Serial.println("Fail - You are a spy!"); }//close if //Allow some time to pass before sampling again to ensure a clear signal. delay(100); }//close if else //Check for successful entry of the code, by seeing if the entire array has been walked through. if (secretCounter == (secretKnockLength) ) { Serial.println("Welcome in fellow Illuminante!"); //if the sececret knock is correct, illuminate the LED for a couple seconds digitalWrite(outputPin, HIGH); delay(2000); digitalWrite(outputPin, LOW); //Reset the secret counter to 0. secretCounter = 0; }//close success check }//close loop

If you enjoyed this lesson, you should join our free Arduino Crash Course – it has 19 Video Training lessons all about using Arduino (you can sign up below).

8 Comments

  1. Avatar elsa on October 19, 2016 at 7:37 am

    hello,

    Thank you so much for this information.

    Altough i have a question according to using this code, and changing it into a code which can detect the length of light signals like long/short (instead of a hard/soft knock)

    Do you know if there is a possibility to change this?

    Thank you in advance

    • Avatar MICHAEL JAMES on October 19, 2016 at 7:49 pm

      Hi Elsa, great question.

      You might change it so that while your input signal is HIGH, (or if it is above a specific threshold for an analog input), than a counter variable is incremented. Like a while loop thats says … while signal HIGH increment a varible.

      Than you use that increment variable as a measure of the relative length of the signals.

      Just a thought – many ways to approach this, best of luck!

  2. Avatar Adi Winchester on September 28, 2017 at 9:42 am

    The article was really great! I both understood the concept and the project. Thank you Programming Electronics! *Brofist*

  3. Avatar Matias Caviglia on November 16, 2017 at 7:58 pm

    exit status 1
    scalar object ‘secretKnock’ requires one element in initializer

    what does it means? its an error. i just copy paste
    thanks

    • Avatar Michael James on July 29, 2019 at 3:09 pm

      Hi Matias – there was an issue with the page not showing the square brackets [ ] . I fixed it here, should work fine now if you copy the code and try again.

  4. Avatar Sam of MESOCODE on June 30, 2019 at 5:41 am

    Great project. Question for u…how to modify the code to add some audio tone for successful knock and other tone for fail knock ? is it possible to get this extra feature using the same piezo ?

    • Avatar Michael James on July 1, 2019 at 11:24 am

      Yes, I think you could do that.

      In the success if statement, you could add the tone generation. For the piezo buzzer, you would have to make sure to change the pinMode to an output (and then back again, to an input once done). You could use the tone() function to make the audio signal. For the fail mode, you would might be able to use an else condition statement to catch the failure, and play the tone.

  5. Avatar eslam nabil on July 26, 2019 at 7:47 pm

    exit status 1
    scalar object ‘secretKnock’ requires one element in initializer

    what does it means? its an error. i just copy paste
    thanks

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