Animated LED Rebar Heart

Inspired by Jiří Praus's beating LED heart and my desire to spend more time welding, I built this heart sculpture.
It's made of 1/2" rebar, with an Adafruit NeoPixel strip driven by a Raspberry Pi.

Is it a little cheesy? Sure, it's a giant heart that lights up with silly animations! But it's a fun beginner-level project in welding, electronics, and programming. I had a great time making it, and I hope this write-up helps you with your own project.

Safety note: Welding, metalworking, and electricity are all dangerous. Please take all necessary safety precautions.

These are the steps:

1. Design and mock up the shape
2. Fabricate it
3. Attach the LED strip
4. Program the animations
5. Mount it

Supplies

This project requires only basic tools for metalworking, welding, and electronics:

• Metalworking: a way to cut and grind steel, like a bandsaw and belt grinder
• Welding: arc welder and safety equipment
• Electronics: soldering iron, proto board, wires, heat shrink tubing

The structure is made with 1/2" rebar, welded with E6011 3/32" stick electrodes.

The lighting is an Adafruit NeoPixel strip driven by a Raspberry Pi with some additional circuitry.

A complete list of tools and materials is at https://github.com/jasonluther/rebar-heart#tools-and-materials.

Step 1: Design

I did not have a concrete design in mind, and the shape evolved as I worked on it.

I started with modeling clay to get a sense of the basic shape I wanted and used aluminum wire to try out different arrangements.

Then I made a slightly larger mockup with bamboo skewers and hot glue to validate the shape.

Once I was happy with the mock-up, I measured each segment with digital calipers to determine the length of the perimeter and guide the fabrication process.

The last big decision to make was the overall size. Aiming for something in the 30-36" range, I chose the final dimensions by scaling the perimeter of the mockup to 10 feet, which is about the length of one piece of rebar stock. With that ratio, I scaled the rest of the caliper measurements to the steel dimensions in a spreadsheet.

Step 2: Fabrication

Welding

Rebar (reinforcing steel) is a great choice for projects like this because it's cheap, it's easy to obtain at big box home centers and building suppliers, and it's easy to stick weld. I also like the look of it. Be sure to use plain carbon steel rebar, not galvanized: welding galvanized metal is hazardous.

Standard welding and metalworking tools are needed:

• Welder, electrodes, safety gear, fire extinguisher
• Grinder:
  • Angle grinder with flap disc to clean up welds
  • Belt grinder to remove sharp edges on each piece. You could use the angle grinder, but a belt grinder is more convenient.
• Wire brush to clean the rebar, like a wire cup brush for a drill
• Something to cut the rebar, like a metal bandsaw or a cutoff wheel in the angle grinder
• Marker and tape measure

I dove right into fabrication, starting with the perimeter.

I marked the width and height on my work table, cut and laid out pieces for one half, and welded them together. Then I assembled the second half alongside the first to achieve rough symmetry. Finally, I joined the two halves.

For the rest of the heart structure, I departed a bit from the mockup and fit pieces as I went.

The process for each piece of rebar is the same:

1. Determine the length from the drawing or by measuring the space for the next piece.
2. Clean the rust from the rebar with a wire brush.
3. Mark the length with a marker.
4. Cut it with a bandsaw, lubricating the cut with a drop of 3-in-1 oil.
5. Grind away the sharp edges.
6. Tack weld into place. Adjust the position as needed.

After tacking everything, I went back and filled in all of the welds fully, sometimes with multiple passes to build up the weld in really large gaps.

I chipped off and cleaned up the slag with a wire brush and then ground off the ugly bits with a flap disc.

I also welded in two small tabs for mounting. In hindsight, I should have drilled holes in the tabs first using a drill press or used small steel rod. It was difficult to drill holes in the tabs after construction was complete.

In the end, I think I used about 60 feet of rebar.

Painting

After applying two coats of rusty metal spray primer, I used a combination of red and burgundy spray paint on the inside and burgundy and brown on the outside.

Then I applied a few coats of clear satin finish to protect the paint and remove some of the gloss.

Step 3: Lighting

I considered several lighting options: spot lights pointed at it, light bulbs hanging inside, a white neon-like LED strip, simple string lights, and more.

I ultimately chose a thin weatherproof Adafruit NeoPixel strip installed around the inside perimeter.

NeoPixel is Adafruit's brand of addressable LED products. Addressable means that each individual multi-color LED package can be controlled independently with just a few wires for data and power. There are many types of addressable LEDs products that vary in cost, quality, compatibility, ease of use, and form factor.

LED Strip

There were a few challenges in attaching the strip to the steel.

First, the strip is encased in a silicone sleeve to keep it weatherproof, and almost nothing sticks to silicone! I ended up wrapping thin magnet wire around the strip to keep it in place.

I also did not plan for the strip when building the structure, so there are a few places where the LED strip bumps out to get around obstacles. It's not a big deal, but it means that the two halves of the heart have a different number of LEDs.

I trimmed the excess strip and filled the open end with clear silicone.

Wiring and Power

The LEDs consume a lot of power, so a large external power supply is required. I could not find a suitable affordable outdoor 5V power supply, so I am using a 12V unit with a 5V converter.

The LED strip requires three connections: 5V power, 5V data, and ground. I soldered the wires on the end of the strip to a 3-conductor cable and covered the joints and the whole connection with heat-shrink tubing.

The addressable LEDs in the strip require signals with precise timing, so the two main choices to drive it are a microcontroller or the Raspberry Pi, a small computer that runs Linux.

The Adafruit CircuitPython NeoPixel library makes it possible to use a Raspberry Pi. I chose the Zero W model because it is much easier than a microcontroller to program and operate remotely, and it's about the same cost as an Arduino-style microcontroller board.

Refer to https://learn.adafruit.com/neopixels-on-raspberry-pi/raspberry-pi-wiring for details on the wiring. The strip requires a 5V data signal, but the Raspberry Pi operates at 3.3V, so I used a Sparkfun level shifter.

The 5V supply also provides power to the Raspberry Pi. To save space, I wired the power directly to the GPIO pins on the Pi instead of powering it through a USB cable and separate power supply. This method bypasses some of the safety circuitry, increasing the likelihood that you will destroy the Pi. To avoid damage from voltage spikes, I used a 25V 1000µF capacitor across the 5V and ground wires.

I wired everything together on a small prototyping circuit board with headers that connect to the Pi's GPIO pins.

The Pi and connecting circuitry are in a small plastic enclosure, and the wires holes are sealed with silicone and hot glue.

If you don't care about custom animations, there are many simpler options, like battery-powered and remote-controlled LED strips or string lights.

Longevity?

I am happy with the cost vs. reliability of this setup, given how I'm using it, and I am prepared to make repairs as needed. To get a more reliable result, you could invest more money for outdoor-rated connectors and more robust components.

Step 4: Programming

There are countless ways to create lighting animations, and I'll make the code I used available, so here are just a few things to consider.

Power Management

At full brightness with all of the LEDs on, the strip may consume too much power. You'll notice this when the Pi crashes or when the LEDs toward the end of the strip get darker or appear more yellow. The Adafruit software library includes a brightness option when initializing the strip, and 0.2 is a great place to start. If your animation won't light up too many LEDs simultaneously, you can increase the brightness.

Libraries

You don't need to worry about the low-level details of driving LEDs because Adafruit provides fantastic software libraries, documentation, and open-source code available to use as a starting point: https://circuitpython.readthedocs.io/projects/neopixel/en/latest/.

Color: RGB and HSV

Each "pixel" is formed by a red, green, and blue LED, with each color set to a value from 0-255. While this RGB color scheme is simple, many operations are easier with the HSV color model, which uses floating point values from 0-1 for hue, saturation, and value. For example, the Value (V) determines the brightness of a pixel, so you can dim a given color just by changing the Value. With RGB, you'd have to adjust all 3 colors. This makes some animations much simpler, like a trail of LEDs where one end fades away. I used the Adafruit FancyLED library to handle HSV to RGB conversion.

This Adafruit guide has a great explanation of more things you can do to improve the display of LEDs for human vision.

Starting your Program

There are many ways to structure, start, stop, and schedule the animation programs. I chose to try to keep the animation programs as simple as possible and use tools provided by the operating system to orchestrate them.

To start your program manually and keep it running after you sign out, use the screen utility.

To start your animation program automatically when the Pi boots, create a .service file that will be used by systemd, the OS's init system. Add a .timer file to run the program at specific times.

Code

The code for my heart is available on GitHub.

The basic framework of each animation is to update the array of pixel values, call pixels.show(), wait some time with time.sleep(...), and repeat.

To turn off the strip automatically when you stop your program, use the atexit module with a SIGTERM handler.

Step 5: Mounting

At last, the time has come to hang the heart!

The heart weighs about 40 pounds and is hung from paracord. To avoid damage to our holly tree, the paracord is attached to wide nylon straps, similar to how you'd hang a hammock on a tree.

The very final step is to secure the power/data cable, Raspberry Pi, and power supply.

I hope you found this information useful. Happy making!