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Friday, June 12, 2009

Magnetic Levitation

How Levitation Works

If you hold two permanent magnets close together, you see that one of them will jump strongly toward (or away) from the other. In 1842, Samuel Earnshaw expressed the perversity of inanimate magnetic objects in his theorem. It explains this frustrating behavior will always prevent you from suspending one permanent magnet above or below another, no matter how one arranges the two magnets. However, an active control circuit can get around this problem by rapidly adjusting the magnet's strength.

Click this image to see closeup of antigravity relay (38K) The general principle is straight forward: An electromagnet pulls a ball upward while a light beam measures the exact position of the ball's top edge. The magnet's lifting force is adjusted according to position.

As less light is detected, the circuit reduces the electromagnet's current. With less current, the lifting effect is weaker and the ball can move downward until the light beam is less blocked. Voila! The ball stays centered on the detector! It is a small distance across the photodetector, perhaps a millimeter or two, but this is sufficient to measure small changes in position. Of course, if the ball is removed the coil runs at full power. And conversely, if the light beam is blocked the coil is turned completely off.

Click here to see image of breadboard circuit (54K) This device uses two photodetectors: the "signal" detector looks for an interruption in the light beam, and the "reference" detector measures the background light. The circuit subtracts one signal from the other to determine the ball's position. The use of two detectors is my small contribution to advance the art of levitation. This design automatically compensates for changes in ambient light, and eliminates a manually

Magnetic Levitation Parts List


Resistors listed in order by value are 1/4-watt, 5% unless otherwise indicated.

  • 300 ohms R11
  • 500 ohms R2
  • 1,000 ohms R1, R12, R13, R14
  • 1,500 ohms R10
  • 10,000 ohms R4
  • 11,000 ohms R6
  • 22,000 ohms R8
  • 56,000 ohms R3
  • 100,000 ohms R5
  • 150,000 ohms R7
  • 370,000 ohms R9
  • 50K linear taper VR1 (and VR2 opt.)
  • C1,C2 47 uF electrolytic
  • C3 0.1 uF ceramic or tantalum (must not be electrolytic)
  • Q1,Q2 OP505A infrared photo detector, or equivalent
  • Q3 2N3055 NPN power transistor
  • LED1,2,4 Red light-emitting diode
  • LED 3 Infrared LED emitter
  • IC1-4 LM741 op amp, Radio Shack 276-007
  • D1 1N4001 (or 1N4004) silicon diode, 50v (or more) peak inverse voltage

  • +/- 15 vdc power supply, 1 amp
  • 9 vdc power supply, 1 amp
  • Breadboard wiring pad (or printed circuit board by Amadeus)
  • 18-ga stranded wire for power
  • Solid hook-up wire
  • 24-ga (or thicker) magnet wire for lifting coil
  • 6-terminal barrier strip (2 ea.)
  • Wood for base and frame
  • Alternatives for the LM741 Op-Amp

I chose the LM741 op-amp out of nostalgia and convenience. It was an extremely successful and common op-amp about twenty years ago.

There are lots of modern choices for dual- and quad-package op-amps. By using a package with multiple op-amps, you can reduce the number of parts and lower the cost. For example, you could use a single quad-package op-amp instead of four separate 741s. This would allow a very small printed circuit board to contain all the electronics!

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