Robots that move to music have become popular holiday decorations.
The Hip-Swinging Monster and other dancing figures are popular holiday novelties or gag gifts. You've probably seen many of the different versions at a store in the mall, at the flea market or on television. You activate them by doing one of three things:

  • Making noise
  • Moving in front of them
  • Pressing a button
When activated, the figure begins moving in time to a popular tune that comes from the base of the figure. Instead of having built-in music, some versions actually dance to the music from your own CD player or radio!

Click here to see a short video of the monster dancing.

Now, we will open up the monster and take a look inside. What makes the music? How does he move? How do his eyes light up? We will answer all of these questions as we take a look inside.

Inside the Monster

Figure 1
The dancing monster is an amazing example of elegant engineering. It actually does everything using a single electric
motor and some gears! In the following pictures, we have removed the clothing and outer plastic shell (Figure 1) to look at the underlying framework:

When the button on the base is pressed, the song "Monster Mash" begins to play. After a second or two, the motor (Figure 2) begins to spin. It spins in one direction, then changes direction and spins in the other direction in time with the music. The shaft of the motor is connected by a rubber belt to a larger wheel. Using a larger wheel gears the tiny motor down enough to match the timing of the music.


Figure 2

Figure 3

A metal shaft connects the large belt-driven wheel (Figure 3) to another wheel. As the motor spins the belt-driven wheel back and forth, the motion is transferred to this second wheel. It has a plastic knob (Figure 4) that fits into a long slot on the inside of the monster's back and works like a cam. The monster's back and front body panels are connected together and attached to the frame by two metal pins (Figure 5) that allow them to move back and forth on the frame. As the knob moves from side to side within the slot, this movement translates to sideways movement of the entire body.


Figure 4

Figure 5

The arms each have a vertical plastic tab attached just below the elbow joint. (Figure 6) A metal pin that is connected at a slight angle to the side of the body frame goes through a hole at the top of the tab. As the monster's body moves from side to side, the tab slides up and down on the pin. This makes the forearm move up and down.


Figure 6

The inside bottom of the front and back body panels have molded plastic gear teeth (Figure 7) that fit into similar gears at the top of each leg. (Figure 8) Each leg has a metal pin through the leg where it enters the boot. This allows the legs to move from side to side. As the body moves from side to side, the gears cause the legs to move in the opposite direction.


Figure 7

Figure 8

While the motor and gears are making the monster dance, two green LEDs make his eyes light up. (Figure 9) The LEDs are connected to the circuit board in the base that controls everything. Pulses of current are sent to the LEDs to turn them on briefly. This makes them flicker in time to the music.


Figure 9

Inside the Base


Figure 10

The base of the monster (Figure 10) holds the circuit board. (Figure 11) The circuit board contains a tiny integrated chip (IC) that is the brains of the monster. This chip has the song encoded in it and synchronizes the movement of the motor to the music. It does this by alternating the flow of current to the electric motor so that the motor spins in one direction and then reverses and spins in the other direction.


Figure 11

Five pairs of wires run from the circuit board to all of the other components. They are:


Figure 12

This small speaker in the base reproduces the music sent to it by the IC.

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