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[Russpin]

I guess the circuit cannot drive a top that spins faster than 5 x 2 = 10 ms per rotation, or 6,000 RPM.

I think you're right, assuming the base drive generated magnetic field is much greater than the top's magnetic field.

I did a pendulum setup and got two drive pulses, one long (about 20 ms) and one short (about 5 ms).



I started thinking that the regenerative feedback was going into oscillation due to the slowly changing magnetic field of the pendulum. Now I'm not so sure this is correct. Sorry if I'm getting to far off topic.
 

[ta0]

I did a pendulum setup and got two drive pulses, one long (about 20 ms) and one short (about 5 ms).

Mm, could it be the first three pulses are merging in your case?

How did you align the magnet? On my pendulum it's aligned along the arm of the pendulum, North straight down.



I don't recall between which terminals of the transistor I connected the scope.

[Russpin]

The pendulum I'm using is a speaker magnet attached to a bolt with some nuts.



When the length of the pendulum is adjusted so that the magnet is 3 mm above the base at closest approach, I get one long drive pulse that varies from 25 to 30 ms long.  If the length of pendulum is adjusted so that the magnet is 7 mm above the base at closest approach, I get two or three drive pulses. Two pulses in about 25 ms and three pulses in about 30 ms.