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Magnetic levitation demonstration One of my friends asked me about magnetic levitation. On reading about it, I came across the Thompson Ring, which is made to levitate by a mainsdriven autotransformer. My design is similar, except it uses a simple circuit driven by a 9V DC 1A plugpack. You can see it working in the video at: siliconchip.au/link/abll The incoming 9V supply is reduced to 5V by the LP2950-5 regulator to power a PIC12F617 microcontroller. When switch S1 is closed, digital input GP2 of IC1 goes high, which triggers the software to produce two square waves from its GP0 and GP1 outputs. They are 180° out of phase with each other. The voltage at analog input GP4 determines the frequency of these square waves. The out-of-phase waveforms are applied to the DRV8871 H-bridge IC, resulting in an 18V peak-to-peak square wave across the coil. As the frequency reduces, more current flows into the coil due to its reactance (inductance), resulting in the continuous aluminium ring rising above the coil by an adjustable distance. To use it, set the potentiometer for the lowest frequency (maximum coil current) with the switch open. When the switch is closed, the ring flies into the air and then oscillates up and down in a damped motion until it becomes stationary at a fixed distance up from the coil. The frequency can then be increased, and the ring will slowly move down until it rests on the coil. Reducing the frequency after that will cause the ring to move up from the coil. If VR1 is a ‘logarithmic’ potentiometer, the height of the ring above the coil is roughly proportional to the rotation angle. The reason that the ring stabilises at a particular position is that the downward force due to gravity is constant but the upward force depends on the strength of the magnetic field emanating from the coil and the distance between the coil and the ring. Thus, for a given field strength, there is a distance at which the upward force from the magnetic field equals gravity, so the ring ‘levitates’. While aluminium is not a magnetic material, the magnetic field from the coil induces a varying current flow in the conductive aluminium, creating an opposing magnetic field. This is similar to how an induction motor induces magnetism in the metal rotor using coils in the stator, which opposes the field from those coils, causing the rotor to rotate. The breakout board for the DRV8871 H-bridge is available from a couple of Australian suppliers for about $15 and also can be purchased directly from Adafruit. The rest of the components are off the shelf. The PCB and programmed micro are available from: siliconchip. au/Shop/8/6866 & siliconchip.au/ Shop/9/6867 You can also download the firmware for the PIC from siliconchip.au/ Shop/6/282 The rod should be made of soft cast iron to enhance the magnetic field. Mild steel should work, but the lift would be less. I have also tested rings made of copper, brass, iron and stainless steel, plus an aluminium ring with a gap cut between the perimeter and the centre and a heavy aluminium ring. The copper ring goes nearly as far as the aluminium one, the brass one rises slightly, while the iron, stainless steel and split rings do not at all. The heavier aluminium ring rises nearly as high as the lighter one. Les Kerr, Ashby, NSW. ($150) POWER+ +9V REG1 LP2950-5 GND 1000F +5V OUT IN 100F 100nF 20F 100nF IRON ROD 100nF ADAFRUIT DRV8871 BREAKOUT MODULE 10k +5V START S1 FROM 9V 1A DC POWER SUPPLY 5 VR1 10k SET FREQUENCY 3 LP2950 GND IN 2 1k OUT 0V 4 1 100nF Vdd 5 GP2 GP4 IC1 PIC12F617 17 –I/P –I/P GP0 GP1 7 6 2 3 4 GP5 VM IC2 DRV8871 IN1 ILIM OUT1 GND 8 30k* POWER– 8 OUT2 IN2 Vss 10k ALUMINIUM RING GP3/MCLR 1 6 COIL PGND 7 SC 2023 * GIVES 2.13A MAXIMUM CURRENT siliconchip.com.au Australia's electronics magazine November 2023  95