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A Halloween Sound Trigger with Raspberry Pi and Arduino


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?

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:

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

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:


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)

while True:
        val = ord(
        if (val == 2): 
        if (val == 3):
        if (val == 4):
        val = 0
    except KeyboardInterrupt: 

Save it as 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:


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.

51 thoughts on “A Halloween Sound Trigger with Raspberry Pi and Arduino

  1. Or you could just hook the switches up to GPIO pins, use RPi.GPIO (which makes internal pull-up really easy) and do away with the Arduino…

    1. The WAV files should be in the same directory as the Python script. You could also use an absolute path name in the pygame.mixer.Sound() constructor.

        1. How big are your WAV files? Try it with a sound file under 500k or so. If you need to play larger sound files, you’ll probably need to use something other than Pygame to play them back.

          1. Hmmm…I’d like to see that as well. It looks like you could use PyAudio, though I haven’t tried it. It looks it has to be installed from source, but someone posted these instructions in the forums:

            $ sudo apt-get install git $ git clone $ sudo apt-get install libportaudio0 libportaudio2 libportaudiocpp0 portaudio19-dev $ sudo apt-get python-dev $ sudo python pyaudio/ install

  2. I’m working on an arduino project where I want to be able to play some sound effects so I’ll be using this. Thank you very much for it. Can you answer a question for me though. I didn’t quite follow your description of the hookup (between arduino, Pi, monitors, etc) and there is no diagram so I can’t tell. which Pi board do you have? I ONLY need it to play the sounds so I would like to get the cheaper Model A if possible.

    1. The Arduino is connected to the Pi via USB. If the Ethernet cable is hooked up in this example it was just because I was using that to connect over VNC so I didn’t need a monitor keyboard. So, sure you can use the Model A.

  3. I got everything working like you desribed, but every so often when i hit a button is playing it multiple times in one sec. Any ideas on how to fix this?

  4. I got this working as described above but I’m having an issue. I want the script to execute on startup with no user input. In the process of getting there I logged out of the GUI and tried to run the script from the command line. The issue is that the sounds don’t work from the command line like they do in the GUI. I can tell the script is working though because the “Print” lines are showing up. All I get for audio is a momentary click. Any idea how to fix that?

  5. I am running this program as python but it reports an ImportError: No module named serial. I have installed python-serial and I can import serial within the Python Shell. Do you have any idea why this happening?

    1. For some reason sudo apt-get install python-serial was not sufficient. I had to install it by downloading from pypi, extracting the file and then install it. (I had this problem for the best part of 2 days!)

  6. Sorry I understand the exploration of the integration of the two devices , but why not use just the GPIO of the RPi to control the play of the samples? It doesn’t sound implossible to me , does it ?

    1. well, you’re not WRONG… I’d just think there are a few people out there that wouldn’t want to re-write their entire arduino sketch to accommodate the sound… A raspi at $20-35 that can play multiple channels at once is a better deal than getting an mp3 shield or trigger board for $50 that only does that one thing and is only one channel. A minimum of code-redo on the arduino and this simple-ish setup on the pi, and you’re good to go.

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Shawn Wallace

Shawn Wallace is a MAKE contributor, artist, programmer, and editor living in Providence, R.I. He designs open hardware kits at Modern Device and organized the Fab Academy at the Providence Fab Lab. He makes iPhone synthesizers with the Fluxama collective and is a member of the SMT Computing Society.

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