Illustrations

mirror overall Figure 1. Vibrating mirror project - first prototype.
mirror detail Figure 2. Detail showing electronics sled.
mirror laser sled Figure 3. Closeup of laser sled (rotated).
mirror projected image Figure 4. Image obtained when motor speeds are set about 2x apart.
mirror saddle image Figure 5. The projected image will normally rotate at the beat frequency.

Vibrating Mirror Project

Introduction

This is my first attempt at a more artsy project.

Vibrators jiggle a mirror suspended in the frame by bungie cords. A laser positioned in front of the mirror sends a beam to its surface which is then reflected onto a wall on the opposite side of the room. The pattern on the wall shows the effect of the two vibrators on the motion of the mirror's surface.

These are technically known as Lissajous Patterns (or Curves).

Sub-assemblies

There are four main sub-assemblies:

  • the frame
  • the mirror
  • the laser sled
  • the electronics sled
The frame holds the other three components and in this case is made of plastic. The mirror is made of mirror plastic and is mounted on a plastic holder. The motors are mounted on the other side of this holder (could have been on mirror side) using metal straps. They are mounted at the edge, with a 90 degree angular separation.

The term sled is slang for what is more commonly known as a platform or stage. It's a solid base that holds lighter pieces that would move as a unit.

The electronics sled contains the power source for the laser and the motors. This is a low tech design using linear regulators and potentiometers to adjust motor speed. Each potentiometer drives the gate of a MOSFET transistor that is in series with a motor. As the potentiometer is moved up, the MOSFET is turned more on, and so is the motor. The speed control with this passive setup is not that good, but enough to demonstrate the basic principle.

Materials

All the sheet plastic is from the cutoffs bin at the local plastics products company (TAP). The control panel was made of mirror plastic just as a design flourish. I normally make my panels out of plain white plastic, or use found panels of other materials or colors.

The vibrating motors are from a massage seat pad that I found at Goodwill Outlet.

The laser is from a Radio Shack laser level. Its housing is a cheap LED flashlight housing.

The electronics and cabling is all from junk equipment. The MOSFETS are from switching power supplies. The regulators and faders (linear potentiometers) are from used epuipment.

The pivot mount on the laser sled is from a mini-tripod sold by Radio Shack. I did not machine it myself!

Patterns

The more interesting pattern I could get with this setup is the 2:1 pattern.

However, other patterns are available by tweaking the controls.

When both motors run full on you get a 1:1 pattern that is usually a narrow oval. But by playing around with the controls, you can get a wider oval. When one motor is totally off you also usually get a narrow oval. The stiffness of the suspension system and the strength of the motors will determine how much the mirror vibrates. The electronics' ability to throttle down a motor without it stopping will determine how far beyond the 2:1 speed difference you can get. The addition of a third motor would be possible, but I haven't tested that yet.

The laser color did not photograph well at all. These photos only look red because I enhanced them by taking out most of the blue. I'm not sure if that is common with lasers. They are a narrow-bandwidth coherent light source, and that makes them behave in somewhat peculiar ways.

You must take precautions around lasers to not look into them. Doing so has been known to cause eye damage.

The patterns are normally quite dynamic. Because the motors are electrically separate, they don't stay at a totally constant relative speed. That slow drift is reflected by a rotating effect in the pattern. In radio and piano tuning, that difference is the beat frequency.

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