The bob of a LEGO pendulum, which is 20 cm long, 3 cm wide, and 2 cm thick, is clamped in between two gears (see Fig. 1).
Fig. 1 Upward-driven pendulum (left), and detail of axis, showing slipping contact (right).
The pendulum is symmetrically suspended in the plane of motion and is always driven upwards by a co-rotating motor. This upward drive implies that at some point the centre-of-mass will rise above the axis that supports the freely rotating motor plus gears (which it sets into motion). Then, when the centre-of-mass is moving obliquely, and is thus no longer exactly above the support, it will be subject to gravitational restoring, and the pendulum will overturn. Once this occurs, it is necessary that the motor switches rotation sense. If not, it would drive the pendulum in the same direction as gravity, out of its suspension. This switch can be chosen to apply either abruptly or smoothly (by taking the motor speed to depend on the angle between pendulum and horizontal; for instance proportional to the sine of this angle). The simple switch used here, the only non-Lego part, leads to rather abrupt changes. It consists of a slipping contact at the axis that guides the electric current from a 12 V battery to the motor hanging in the suspension. This contact consists of a piece of copper, wrapped around the axis, which is cut at two opposing sides, a few milimeter wide each (see Fig. 1b). The two static contact points of the battery, guiding the electric current, switch side once these 'cuts' have passed upon rotation of the axis. There is one switch that controls the output voltage from 0 to the maximum output of 12 V. Due to friction, a minimum voltage of about 8V should be applied in order to get the bob moving.