Homework 12: Knock Lock
The Circuit
Parts List
| Name | Quantity | Component |
|---|---|---|
| U1 | 1 | Arduino Uno R3 |
| R1, R2, R3 | 3 | 220 Ω Resistor |
| D1 | 1 | Red LED |
| D2 | 1 | Green LED |
| D3 | 1 | Yellow LED |
| PIEZO1 | 1 | Piezo |
| S1 | 1 | Pushbutton |
| R4 | 1 | 10 kΩ Resistor |
| SERVO1 | 1 | Positional Micro Servo |
| C1 | 1 | 1 uF, 16 V Polarized Capacitor |
| R5 | 1 | 1 MΩ Resistor |
Schematics
Building the Circuit
Building the circuit went by quickly and I don’t really have much more to say about it. I assembled it, loaded the sketch, and it worked fully. One thing I did change in the example sketch was that I added another small delay after the yellow LED went off as more user feedback for accepting a valid knock. It also gave the Piezo time to settle since without the delay, it would register reverberations from the same lock twice, so it could be unlocked with just 1 knock.
Programming
After confirming that the circuit was working, I then unloaded the example sketch and began programming the arduino. The code this time was a bit more complex and I started getting a feel for how more and more complexity stacks with more components, and it was cool to see the piezo having its uses reversed.
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#include <Servo.h>
Servo myServo;
const int piezo = A0;
const int switchPin = 2;
const int yellowLed = 3;
const int greenLed = 4;
const int redLed = 5;
int knockVal;
int switchVal;
// variables for the high and low limits of the knock value
const int quietKnock = 10;
const int loudKnock = 100;
// variable to indicate if locked or not
bool locked = false;
int numberOfKnocks = 0;
void setup() {
myServo.attach(9);
// make the LED pins outputs
pinMode(yellowLed, OUTPUT);
pinMode(redLed, OUTPUT);
pinMode(greenLed, OUTPUT);
// set the switch pin as an input
pinMode(switchPin, INPUT);
// start serial communication for debugging
Serial.begin(9600);
// start with 'door' open
digitalWrite(greenLed, HIGH);
myServo.write(0);
Serial.println("the box is unlocked!");
}
void loop() {
if (!locked) {
switchVal = digitalRead(switchPin);
if (switchVal == HIGH) {
locked = true;
// change the status LEDs
digitalWrite(greenLed, LOW);
digitalWrite(redLed, HIGH);
// move the servo to the locked position
myServo.write(90);
Serial.println("the box is locked!");
delay(1000); // wait for servo to move
}
}
if (locked) {
knockVal = analogRead(piezo);
// if there are not enough valid knocks
if (numberOfKnocks < 3 && knockVal > 0) {
if (checkForKnock(knockVal)) numberOfKnocks++;
// print status of knocks
Serial.print(3 - numberOfKnocks);
Serial.println(" more knocks to go");
}
// if there are three knocks
if (numberOfKnocks >= 3) {
// unlock the box
locked = false;
// move the servo to the unlocked position
myServo.write(0);
delay(20);
// change status LEDs
digitalWrite(greenLed, HIGH);
digitalWrite(redLed, LOW);
Serial.println("the box is unlocked!");
numberOfKnocks = 0;
}
}
}
// checks if detected knock is within range
bool checkForKnock(int value) {
if (value > quietKnock && value < loudKnock) {
// turn the status LED on
digitalWrite(yellowLed, HIGH);
delay(50);
digitalWrite(yellowLed, LOW);
delay(50);
Serial.print("Valid knock of value ");
Serial.println(value);
return true;
}
else {
// print status
Serial.print("Bad knock value ");
Serial.println(value);
return false;
}
}