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:
- The components you will need and how to set up this simple circuit.
- The concept of operation of this secret knock detector
- A thorough description of each block of code in the Arduino sketch
- 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.
- Place the piezo transducer on the breadboard, with the positive lead and the negative lead on separate rails.
- Connect the positive lead to pin A0 on the Arduino and the other lead to ground.
- Finally, use the 1Mohm resistor to connect the leads of the piezo transducer.
- 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.
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:
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secretKnock[secretKnockLength] = {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:
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/* A simple sketch to detect a secret knock using a piezo transducer Created JUL 2015 by Michael James https://programmingelectronics.com/ 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 |
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