Quick Post #6: Bears Go Punk!

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I’ve been pleasantly surprised by the uses around the house that I’ve found for the 3d printer my wife got for me last Christmas. This is about one such use. A couple of years back we saw some silhouettes of black bears in a yard while we were on vacation in the Great Smoky Mountains. I found patterns for some online and with plywood, jigsaw, and paint, we made a mama bear and two cubs. We love them, and they drew a lot of attention and positive comments from our neighbors. Unfortunately, the birds also like them, perching on them and doing their business. My wife had the good idea to look into spikes, but then the question was how to attach them. Our son suggested 3d printing clips. I modified that a bit into brackets, and we were in business.
Rubber spike strip

Plastic spike strip, which has three segments and flexes between them.

A still frame of the plastic clips being printed inside the 3d printer

Printing the clips on my printer

Plastic bracket with a slot for plywood to fit between and a base.

The bracket design, a modification of a plywood bracket that I found online.

This is the bracket. I found a plywood stand design online. The base was too large, but I liked the fillet where the supports meet the base and the slot was already pre-sized for 1/2″ plywood.
I couldn’t just scale the design, since it wasn’t parametric and shrinking it would also shrink the slot opening. Instead, I just used TinkerCad to cut off the excess on all four sides.
The 3d printed brackets were a great solution for the question of how to mount the strips. The strips aren’t invisible, but with their black color they don’t jump out too much, either, and I think the silhouettes still look pretty good from a distance. Time will tell what the birds think.😊
What uses around the house have you found for your 3d printer? Please share in the comments.
A plastic spike strip with two brackets glued onto the bottom

One of the plastic spike strips with two of the printed brackets glued onto the bottom, ready for installation.

Black plywood silhouette of a bear, with three spike strips mounted on top.

Mama bear sporting her new punk haircut

Plywood silhouettes of a mama bear and two cubs. Spike strips are mounted along the top of each bear.

The spike strips aren’t invisible, but they don’t stand out either. I think they still look pretty good from a distance.

Quick Post #5: A Utility for Converting .obj Files Created by Microsoft 3D Builder

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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])

Pi-based multi-prop trigger for animatronics, Part 1

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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 for the multi-prop trigger. The left is a Raspberry Pi Zero W with the pins enlarged, and the right is a breadboard. The breadboard has a barrel jack for power (that also is wired to the Pi). There are four 3 pin female headers on the breadboard. One is for the PIR sensor, and has all three connections wired. The other three are to send signals to three props. These have the ground and signal wires connected.
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:

Picture showing a Raspberry Pi Zero W on the right, with soldered connections to a soldered breadboard with a 3 pin header that a PIR sensor is plugged into, and three other 3 pin headers that connect to a solderless breadboard with three LEDs connected.

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.

Black project box without the top. It has a large round opening where the barrel jack for power resids, and a large rectangular opening where the micro SC card can be accessed. Inside are the Pi Zero W and the solderable breadboard.

Completed electronics in project box

Completed project box with electronics inside. The cover is on to top. You can see the solderable breadboard with headers through the top cutout.

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.