Center coil
Drive coil
Rim coil
Schematics
Drawings
Photos
Center coil
Do not make your center coil to small in diameter. When the pendulum develops an elliptical path it might miss the center coil completely.
The center hole was only used to mount the material on the lathe.
Drive coil
The center hole should accomodate the Center coil with a little bit of pressure.
Determinig the diameter of the drive coil requires knowledge about where in the swing the drive pulse must be given, according to the Schumacher principle. See Ellipse, Schumacher. So, if possible, construct and (temporarely) mount your bob first to do a Q-measurement. Then use the spreadsheet to estimate the radius of the Drive Coil.
I made the drive coil from 15mm thick MDF, but any stiff insulating material can be used.
It is not a bad idea to include an auto-resetting thermal fuse. In case of a defect in the electronics the coil might stay engaged for long time, which at least will smell, and probably may cause a fire hazard. A properly dimensioned PTC (resistor with a steep Positive Thermal Coefficient) may also work.
Rim coil
Dimensions are radii from the centerline rignt, not visible.
The Rim Coil has an effective diameter of 53 cm and 380 turns of 0.125 mm wire.
Measured resistance is 926 Ohm, measured self-inductance is 320 mH.
My rim coil also has 12 led's in a circle which flash at a programmable moment in the swing. In an older version I had a led in the center of the Center Coil, but the current bob needs to be driven so close to the center that the flashes became invisible.
I also prepared for 61 sets of holes to which electronic circuits can be mounted in future. These circuits will contain a Hall sensor and a led which turns on when de bob flies over it and turns off after a few hours. This will be the equivalent of the little poles often seen with (museum) pendulums which are toppled over by the bob and so leave a trace of the bobs path. I use an odd number of these things to prevent that 2 opposite poles will be activated in one swing.
Coil Support
The coil support itself rests on 3 metal pins on the Floorunit. The metal pins are on the same diameter, but not 120 degrees apart, so it will fit only in one orientation and can be removed and replaced very accurately. Washers around the pins allow space for the flatcable to come out. In the upper plate of the floor unit is a hole to have space for the Junction Board.
Assembly
Click for larger
Schematics
Junction Board
The board layout is much the same.
RimLeds
12 Leds in series cannot be supplied from 24 Volts so I made 2 strings. The leds are green, except those on 3, 6 and 9 'o clock which are yellow.
Led 1, North, is a red one and needed less current to be not much brighter than the others.
The series resistors are devided as two in series, so they won't burn when the leds accidenally stay switched on. Normally they are flashed quite shortly.
Photos
The bare coilformers of Center coil and Drive coil
After winding. This is the bottomside where the wires come out.
[more photos will follow]