If you look at this blog, you’ll quickly see that many of my projects relate to Halloween. I don’t know if this is a good, bad, or dumb idea, but as I was thinking about my use of Python for controlling Halloween props, I had the thought that it might be nice to have and share a crowdsourced list of python-related resources relating to Halloween, such as useful libraries (e.g., pyAudio, various libraries for using the GPIO pins on a Raspberry Pi), complete software packages (e.g., my own ChatterPi), or even Halloween-themed games written with Python. So, with some help from ChatGPT, I’ve put together a Google sheet, Python Resources for Halloween, with some pre-populated content that anyone can view, along with a Google Form, Submit Python Resource for Halloween, where anyone can submit additional resources for inclusion.
The submissions are moderated, so they won’t show up immediately in the resource sheet. If you have anything to propose adding, please do so using the Google Form. If you have any other feedback, I welcome it as a comment on this post.
There are several different file formats for specifying 3d objects (as Tannenbaum wrote, “The good thing about standards is that there are so many to choose from.” One such standard is the obj or .obj open format. By itself, the .obj file definition does not support coding surface shading properties in the .obj file, but these can be provided in a separate Material Template Library (.mtl) file.
While not part of the official file format, many program support vertex coloring by adding the RGB values for color to the end of the relevant vertex line. The de facto “standard” for this non-standard usage is to code the RGB values as decimals between 0 and 1. However for some reason, Microsoft’s 3D Builder codes them as integers between 0 and 255. As a result, other programs, e.g., Bambu Studio, while capable of using vertex coloring using values between 0.0 and 1.0, won’t read the color information when you import such a file. This simple script converts the RGB values from a range of 0-255 to a range of 0.0 – 1.0 and writes out the modified file. Here’s the code, which is also published as a Gist:
import sys
def convert_obj_vertex_colors(input_file, output_file):
with open(input_file, 'r') as infile, open(output_file, 'w') as
outfile:
for line in infile:
parts = line.strip().split()
if parts and parts[0] == 'v' and len(parts) == 7:
# Convert RGB from [0, 255] to [0, 1]
r, g, b = map(float, parts[4:7])
r, g, b = r / 255.0, g / 255.0, b / 255.0
outfile.write(f"{parts[0]} {parts[1]} {parts[2]} {
parts[3]} {r:.6f} {g:.6f} {b:.6f}\n")
else:
outfile.write(line)
if __name__ == "__main__":
if len(sys.argv) != 3:
print("Usage: python convert_obj_rgb.py input.obj output.obj")
else:
convert_obj_vertex_colors(sys.argv[1], sys.argv[2])
Next Halloween, my setup will include three coordinated skeletons performing together (probably doing “King Tut“). I want to use a single PIR motion detector to trigger three different props at different offsets from the original trigger. And some of the props can only speak or move for short amounts of time, so for a longer performance, they need to be repeatedly triggered. To do this, I have a PIR providing input to a Raspberry Pi Zero W. A python program running on the Pi then sends brief output trigger voltages individually to each of the props, according to the preset schedule for the routine. This same approach could easily be modified to trigger 2, 4, 5, or more coordinated props from a single start trigger. The hardware setup is very simple, and is shown in the figure.
Wiring diagram
Power is provided via the barrel jack on the prototype board. This is also what powers the Pi. Their are four 3-pin female headers on the board. The one on the left is for the PIR sensor input. It has the power and ground connections, and the signal wire is an input that goes to GPIO pin 15 on the Pi. The other three headers are to go out to the three props. The grounds are connected so that the prop controls and this trigger share a common ground. The signal connections are outputs from GPIO pins 23, 24, and 25. There is no need for power for these connections.
In order to test the hardware, I rigged up one LED to each of the three signal outputs, put a resistor in to avoid burning out any of the LEDs, and linked the grounds. The test setup is shown below below:
Test setup to make sure the hardware works and that I got the soldering correct.
I used the GPIO Zero library to write a simple test script for this test setup:
from gpiozero import LED
from gpiozero import MotionSensor
myLED1 = LED(23)
myLED2 = LED(24)
myLED3 = LED(25)
pir = MotionSensor(15)
while True:
if pir.wait_for_motion():
print("motion detected")
myLED3.off()
myLED1.blink(1)
myLED2.blink(2)
pir.wait_for_no_motion()
print("no motion")
myLED1.off()
myLED2.off()
myLED3.blink(3)
This has the pi wait until the PIR detects motion. Then it begins blinking the first 2 LEDs at different rates. When motion stops, those two LEDs are turned off and the third LED begins to blink. This cycle then repeats until the user hits CTRL-C to stop the test script.
Completed electronics in project box
Completed project box with electronics inside and lid on the top
The circuit is mounted in a custom 3d printed project box with slots for the power, sensor, and output wires, as well as a slot for accessing the Pi’s micro SD card. I designed the box using TinkerCad. I also 3d printed the standoffs, and just used hot glue to glue the Pi and the breadboard in place. I put in large holes so that it would be easy to plug and unplug the connectors and also get my fingers in to insert or remove the micro SD card. The top has a large cutout to make it easy to access the 3-pin headers. The finished hardware is shown in the figures on the right.
If you look at this blog, you’ll quickly see that many of my projects relate to Halloween. I don’t know if this is a good, bad, or dumb idea, but as I was thinking about my use of Python for controlling Halloween props, I had the thought that it might be nice to have and share a crowdsourced list of python-related resources relating to Halloween, such as useful libraries (e.g., pyAudio, various libraries for using the GPIO pins on a Raspberry Pi), complete software packages (e.g., my own ChatterPi), or even Halloween-themed games written with Python. So, with some help from ChatGPT, I’ve put together a Google sheet, Python Resources for Halloween, with some pre-populated content that anyone can view, along with a Google Form, Submit Python Resource for Halloween, where anyone can submit additional resources for inclusion.
