I am using transistor 2n3055, diode 1n4001, resistor 680 ohm, bifilar coil with 680 turns of #23 and #26. Rotor is wheel taken from inline skates with four 3/4" round magnets from Home depot. I don't have a core because I could't find R60 welding rods. Before I tried to use bolt but somebody said it is better not to use anything if you don't have R60.