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People have been asking me about interesting applications for the Raspberry Pi, and whether Raspberry Pi is an Arduino killer of some sort. The answer to the second question is no; in fact it is an Arduino augmenter. This blog post answers the first question with another question: how about a Haunted House sound effects machine?

rc cat A Halloween Sound Trigger with Raspberry Pi and Arduino

A new revision of the Early Release of Getting Started with Raspberry Pi came out last Friday. I read Matt Richardson’s chapter on using Pygame with the GPIO pins on the Pi, which included a simple Sound Sample player. I adapted his example to work with an Arduino that talks to the Pi over a serial connection; this skeletal (ahem) hookup could easily be incorporated into some sort of Halloween installation. I decided to use Arduino for reading the inputs because out of the box it is more robust and can handle a wider variety of inputs. Also, there are many existing Haunted House triggering demos out in the wild that use Arduino.

First, you’ll need to prepare the trigger circuit. The following example uses three toggle switches, but you can replace those with any kind of on/off input. In a haunted house (or porch-based trick-or-treater installation), a PIR sensor would be handy for triggering based on proximity.

Here’s the basic schematic:

arduinorasppi A Halloween Sound Trigger with Raspberry Pi and Arduino

The 10k resistors can be replaced by the Arduino’s own internal pull-up resistors, if you know how to do that.

I connected the Arduino to the Raspberry Pi using a USB cable. While I had the Pi hooked into a monitor, I found that I could just plug the Arduino through my Mac keyboard USB connector and it got enough power from that to work. If you have an older Raspberry Pi with polyfuses limiting the power on the USB port (check to see if you have 2 little green fuses marked “1104” next to the USB ports), you may need an external hub, or run the Pi headless to free up a USB port.

Next, upload the following sketch to the Arduino:

// PiTalk.ino
// Reads three digital inputs. These could be any kind of 
// switch or (for Halloween) PIR sensors.

byte onState[3] = { 0, 0, 0 }; // save the state of each pin

void setup() {
 Serial.begin(9600);   // Open serial connection
 pinMode(2, INPUT);    // Set these three pins for reading
 pinMode(3, INPUT);    // Each have a 10k pullup externally
 pinMode(4, INPUT);    // so a trigger is LOW
}

void loop() {
 for (int i=0; i<3; i++) {         // Iterate over pins
   if (digitalRead(i+2) == LOW) {  // Check if triggered
     if (onState[i] == 0) {        // Just triggered?
       Serial.write(i+2);          // Send the pin number
       onState[i] = 1;             // but just once
     } 
   } else {
     onState[i] = 0;               // Not triggered
   }
 }
 delay(20);
}

You can use your computer to do upload the program, or you can download and install the Arduino IDE directly on the Raspberry Pi (as described in the next release of Getting Started with Raspberry Pi).

Once the Arduino is programmed, open up the Leafpad text editor on the Raspberry Pi and enter this Python program, adapted from Matt’s Sound Sample Player:

# playSounds.py

import pygame.mixer
from time import sleep
from sys import exit
import serial

pygame.mixer.init(44000, -16, 1, 1024) 

soundA = pygame.mixer.Sound("Scream.wav") 
soundB = pygame.mixer.Sound("WilhelmScream.wav")
soundC = pygame.mixer.Sound("CastleThunder.wav")

soundChannelA = pygame.mixer.Channel(1) 
soundChannelB = pygame.mixer.Channel(2)
soundChannelC = pygame.mixer.Channel(3)

print "Sampler Ready."
serialFromArduino = serial.Serial("/dev/ttyACM0",9600)
serialFromArduino.flush()

while True:
    try:
        val = ord(serialFromArduino.read())
        print(val)
        if (val == 2): 
            soundChannelA.play(soundA) 
        if (val == 3):
            soundChannelB.play(soundB)
        if (val == 4):
            soundChannelC.play(soundC)
        val = 0
        sleep(.01)
    except KeyboardInterrupt: 
        exit()

Save it as playSounds.py. Before you run the script you’ll probably need to install the Python serial module. To do that, type:

sudo apt-get install python-serial

If you’re running the latest Raspbian, you probably have everything you need to get this running. Open the LXTerminal and type:

python playSounds.py

If you get an error that Pygame is not installed, type:

sudo apt-get update
sudo apt-get install python-pygame

You’ll also need some sound files to play. I chose three from the Internet Archive: a generic scream, a Wilhelm Scream, and the classic Castle Thunder sample.

When you press the buttons each sound will play once; Pygame’s mixer will even play all three at the same time if you have multiple trick-or-treaters invading your porch.


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