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Build a Solar-Powered Bobblehead

Written by Dev Gualtieri

Using an Electromagnet to Drive the Bobble

Replacing batteries in non-rechargeable devices is expensive. When I built personalized bobbleheads for my grandchildren, I decided to make them solar-powered. Here’s a description of their electromagnetic actuation and simple circuitry.


  • How can I build a bobblehead with a motor at home?
  • What’s a fun solar-powered project I can build?
  • How can I use a CD4011B CMOS logic circuit in a project?
  • CD4011B CMOS logic circuit

Bobbleheads, sometimes called wobbleheads, have been with us for more than 200 years. They were popular in the 1950s, but they surged in popularity in the 1990s with the emergence of precision injection molding techniques that allowed recognizable representations of sports, entertainment, and political figures. Three-dimensional printing technology allowed the creation of personalized bobbleheads starting around the year 2000.

My Internet research revealed that bobblehead and wobblehead as a description for such dolls is a generic term, but “Bobblehead” was briefly trademarked a few years ago for other items, including cat toys and dog toys. Those trademarks have been abandoned, but there is a trademark for a specific type of bobblehead: Royal Bobbles. The alternative spelling, “Bobble Head,” was briefly trademarked, but abandoned. Wobblehead is also the name of a type of fishing lure.

I wanted to make bobbleheads of my grandchildren, but I wanted the heads to be in constant motion without any human interaction. Continuous mechanical actuation of any sort requires a lot of power, and battery power would have required many battery replacements in a year. Using one of the ubiquitous wall transformers for cellphone and tablet computer charging would have been overkill for such a simple toy, and there’s also the problem of a wire attachment. I finally decided to make them solar-powered.

SOLAR POWER

The average intensity of solar energy above the atmosphere directly facing the Sun is about 1,360W per square meter. At Earth’s surface, this intensity varies by latitude, cloudiness, and time of day. At my latitude of about 40.9 degrees, it’s 1,000W/m2 at noon on a clear day around the summer solstice (June 21).

Conversion of this solar energy to electrical energy happens with an energy conversion efficiency. The first photovoltaic cells of the type that powered the Telstar communication satellite in 1962 were fabricated from expensive single-crystal silicon wafers with efficiency of slightly above 10%. While advanced thin film solar cells as used in rooftop systems have efficiencies above 25%, inexpensive polycrystalline photovoltaic cells available to hobbyists have efficiencies of about 10%-15%.

When I was searching for suitable polycrystalline photovoltaic cells, the most frequent comment I saw among customers was that the supplied currents claimed by manufacturers were far higher than what was achieved in practice. At that point, I decided to do a first-principles calculation of how much power should be obtained. Taking 10% as the efficiency, and the theoretical optimum insolation of 1,000W/m2, I calculated that a 5cm x 10cm 5V cell (about the largest acceptable size for my project) would likely give a maximum current of 100mA.

MECHANICS

I created the bobbleheads of my grandchildren from Lego images I found on the internet with image modification by the GNU Image Manipulation Program (GIMP). For the purposes of this article, I needed a public domain image, so I selected the iconic image of Apollo 11 astronaut Buzz Aldrin taken by Neil A. Armstrong. I used GIMP to enhance the image contrast and color saturation. The image was pasted onto foam board and cut to size with an X-ACTO knife.

A current of 100mA at 5V, 0.5W, was not enough power for an up-down bobble action, as my experiments confirmed. However, it was enough for a side-to-side wobble action using a pendulum mechanism that I devised. The actuation was achieved with a rare earth magnet being attracted into a coil of copper wire. The actuation was periodic with about a three-second cycle, and a capacitor was used to store energy between activations to ensure a strong response. A plan view of the pendulum can be seen in Figure 1. An acrylic plastic rod was used as the pendulum with the pivot point slightly above its center. The pivot point can be adjusted to how much wobble you want.

FIGURE 1
Rear view of my bobblehead, left, with a diagram representation on the right. A current pulse in the copper wire coil draws the magnet inwards and rotates the rod. For a larger response, another magnet can be stacked on top of the first magnet.
FIGURE 1
Rear view of my bobblehead, left, with a diagram representation on the right. A current pulse in the copper wire coil draws the magnet inwards and rotates the rod. For a larger response, another magnet can be stacked on top of the first magnet.

A low-friction rotational pivot with limited motion in transverse directions is desired. I made mine from a 4-40 bolt inserted into a block of polyethylene, as shown in Figure 2. I don’t recommend lubrication, as oils will gum with age. Dry graphite powder, as used to lubricate locks, might be suitable, but I didn’t need it for my bobbleheads. I attached the head with a stiff wire that allowed easy adjustment of the head position by bending.

FIGURE 2
Detailed views of the pendulum pivot and attachment to the image head. The pivot should be low-friction with easy rotation but limited-motion in other directions. This was accomplished by using a polyethylene block with a long path for the attachment bolt. Attachment to the image head was with a stiff wire that allowed easy manipulation in all directions.
FIGURE 2
Detailed views of the pendulum pivot and attachment to the image head. The pivot should be low-friction with easy rotation but limited-motion in other directions. This was accomplished by using a polyethylene block with a long path for the attachment bolt. Attachment to the image head was with a stiff wire that allowed easy manipulation in all directions.
COIL

The coil was 1,000 turns of AWG-36 enameled copper wire wound on a coil form made from a 0.5” length of 0.75” outside diameter PVC pipe commonly available at home goods stores. Thin plastic discs were glued to the ends of the PVC pipe section to contain the wire. The large inner diameter of the pipe allowed easy passage of the magnet from one end to the other. The coil resistance is slightly more than 100Ω, and it varied from 103Ω to 112Ω for the five coils I made. This resistance draws about 50mA from the nominal 5V source voltage; so, it’s not too demanding of the solar cell. The magnet used is the commonly available NdFeB rare earth disc magnet, 8mm in diameter by 3mm thickness.

Manually winding 1,000 turns for a single coil is possible, but tedious. Since I needed to make several coils, I assembled a simple winding jig from available scrap materials in my workshop (Figure 3). The jig was mounted in a bench vise, and the copper wire was spooled from a vertical rod. While a manual turns counter would be useful, a precise number of turns is not required. I counted in increments of 100, writing down when each hundred’s interval had passed as a memory aid.

FIGURE 3
My simple coil winding jig (left), and a detailed view of the coil assembly with magnet and pendulum rod. The counterweight, supplied by multiple steel nuts, ensured that the resting state of the magnet was at the entrance of the coil. Careful measurement would center the carrier bolt for the magnet with the lower portion of the coil opening. However, in this case, I needed to use a technique that my colleagues and I humorously called "precision bending" to effect the proper alignment. The connecting wires from the coil are sent to the bottom of the base, where there's a cavity under the solar cell for holding the printed circuit board.
FIGURE 3
My simple coil winding jig (left), and a detailed view of the coil assembly with magnet and pendulum rod. The counterweight, supplied by multiple steel nuts, ensured that the resting state of the magnet was at the entrance of the coil. Careful measurement would center the carrier bolt for the magnet with the lower portion of the coil opening. However, in this case, I needed to use a technique that my colleagues and I humorously called “precision bending” to effect the proper alignment. The connecting wires from the coil are sent to the bottom of the base, where there’s a cavity under the solar cell for holding the printed circuit board.
CIRCUITRY

The circuitry for the bobblehead, as shown in the schematic (Figure 4), is simple and cheap. A CD4011B CMOS logic circuit is used as a square wave oscillator and edge detector. While non-buffered CMOS logic (“A” series) is seldom found today, it’s important to use a buffered (“B” series) chip. Circuit portions, IC1a-IC1c, form a square wave generator, and IC1d is an edge detector. Since we need to use polarized capacitors for the required large capacitance values, two series capacitors with a large value resistor are used in place of a single capacitor to prevent polarity reversal. The nominal oscillator period is three seconds, which gives an aesthetic bobble time for the head. Coil polarity is important. If the magnet is repulsed by the coil rather than being attracted to it, reverse the coil connections.

FIGURE 4
Bobblehead circuit. The circuit provides precaution against polarity reversal of the capacitors. The reverse-biased diode across the coil protects the drive transistor from an inductive kick-back voltage. The circuit was designed with a 1500µF storage capacitor, which worked adequately. In the end, I used a 3300µF capacitor that slightly overflowed the circuit board edges (see Figure 5).
FIGURE 4
Bobblehead circuit. The circuit provides precaution against polarity reversal of the capacitors. The reverse-biased diode across the coil protects the drive transistor from an inductive kick-back voltage. The circuit was designed with a 1500µF storage capacitor, which worked adequately. In the end, I used a 3300µF capacitor that slightly overflowed the circuit board edges (see Figure 5).
FINAL COMMENTS

Children are not typically careful, and I wanted to prevent any damage to the bobblehead figures. For that reason, I enclosed the figures in transparent Lucite plastic cases, 4x4x8” in dimension. I have ceiling spotlights above my workbench, and these have enough light to fully excite the bobblehead into action. LED desk lamps and table lamps work when the solar cell is directly below them, as does an LED flashlight. Direct sunlight through a window will work for a few hours a day; so, the bobblehead would act as a sunlight intensity meter. 

FIGURE 5
Printed circuit board layout (left), and a photograph of the board in the base cavity of the bubblehead figure. As explained in the text, a 1500µF capacitor was originally used to store energy from the solar cell, but a 3300µF capacitor works somewhat better.
FIGURE 5
Printed circuit board layout (left), and a photograph of the board in the base cavity of the bubblehead figure. As explained in the text, a 1500µF capacitor was originally used to store energy from the solar cell, but a 3300µF capacitor works somewhat better.
FIGURE 6
Voltage waveform measured at the bobblehead coil during normal operation. There's a sharp voltage pulse at 2.75-second intervals.
FIGURE 6
Voltage waveform measured at the bobblehead coil during normal operation. There’s a sharp voltage pulse at 2.75-second intervals.

REFERENCES
[1] “Simple inductance formulas for radio coils” in Proceedings of the IRE, Volume 16, Number 10, October, 1908, pp. 1398-1400.

RESOURCES
Texas Instruments | www.ti.com

Code and Supporting Files

PUBLISHED IN CIRCUIT CELLAR MAGAZINE • OCTOBER 2023 #399 – Get a PDF of the issue

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Dev Gualtieri received his PhD. in Solid State Science and Technology from Syracuse University in 1974. He had a 30-year career in research and technology at a major aerospace company and is now retired. Dr. Gualtieri writes a science and technology blog at www.tikalon.com/blog/blog.php. He is the author of three science fiction novels, and books about science and mathematics. See www.tikalonpress.com for details.

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Build a Solar-Powered Bobblehead

by Dev Gualtieri time to read: 6 min