Lesson 5 Knock Sensor

Introduction

A knock sensor (as shown below) is similar to a shock switch. Being more sensitive, it can feel slighter vibrations.

Components

– 1 * Raspberry Pi

– 1 * Breadboard

– 1 * Network cable (or USB wireless network adapter)

– 1 * Knock sensor module

– 1 * Dual-color Common-Cathode LED module

– Several jumper wires

Experimental Principle

It’s similar to the shock switch. When you knock the sensor, the two spring leaves will get touched and the circuit will conduct. At the same time, pin S will output Low. In this experiment, we will judge the knock signal by detecting the output voltage. A dual-color LED module is used to indicate knock signals. When the knock switch generates knock signals, the LED will light up.

Experimental Procedures

Step 1: Build the circuit

Knock switch connection: connect pin S of the knock switch module to GPIO0 of the Raspberry Pi; GND to GND; pin + to 3.3V

Dual-color LED module connection: connect pin R of the dual-color LED module to GPIO1 of the Raspberry Pi; GND to GND

Step 2: Edit and save the code (see path/Rpi_SensorKit_code/07_knockSensor/knockSensor.c)

Step 3: Compile

              gcc  knockSensor.c  -lwiringPi

Step 4: Run

              ./a.out

Shake the sensor. Then pin S will output low level and you will see “Detected knocking” displayed on the screen, and the LED will light up.

knockSensor.c 

#include <wiringPi.h>
#include <stdio.h>

#define KnockPin      0
#define LedPin        1

int main(void)
{
	if(wiringPiSetup() == -1){ //when initialize wiring failed,print messageto screen
		printf("setup wiringPi failed !");
		return 1; 
	}
	
	pinMode(KnockPin, INPUT);
	pinMode(LedPin,  OUTPUT);

	while(1){
		if(digitalRead(KnockPin) == LOW){
			printf("Detected knocking!\n");
			//digitalWrite(LedPin, LOW);     //led on
			digitalWrite(LedPin, !digitalRead(LedPin));     //led on
		}
	}

	return 0;
}

Python Code

#!/usr/bin/env python
import RPi.GPIO as GPIO

KnockPin = 11 
LedPin = 12

Led_status = 1

def setup():
	GPIO.setmode(GPIO.BOARD)       # Numbers GPIOs by physical location
	GPIO.setup(LedPin, GPIO.OUT)   # Set LedPin's mode is output
	GPIO.setup(KnockPin, GPIO.IN, pull_up_down=GPIO.PUD_UP)
	GPIO.output(LedPin, GPIO.HIGH) # Set LedPin high(+3.3V) to off led

def swLed(ev=None):
	global Led_status
	Led_status = not Led_status
	GPIO.output(LedPin, Led_status)  # switch led status(on-->off; off-->on)
        print "LED: " + ("on" if Led_status else "off")

def loop():
	GPIO.add_event_detect(KnockPin, GPIO.FALLING, callback=swLed, bouncetime=200) # wait for falling
	while True:
		pass   # Don't do anything

def destroy():
	GPIO.output(LedPin, GPIO.LOW)     # led off
	GPIO.cleanup()                     # Release resource

if __name__ == '__main__':     # Program start from here
	setup()
	try:
		loop()
	except KeyboardInterrupt:  # When 'Ctrl+C' is pressed, the child program destroy() will be  executed.
		destroy()