Below, the layout is shown in black without the components. The result will be a correctly scaled layout in high resolution, ready for transfer on UV-light sensitive PCB material. The Switch-on level is adjusted using the pot connected to the 100K resistor. What values you need are determined to some extend by the mechanical nature of your motors. Recorder types have a fairly large diameter, which gives the rotor considerable inertia when compared to for instance a pager motor.
High inertia means a current pulse needs to be relatively long to get the motor going.
On the other hand, a large diameter also tends to give good torque at low rpm. If necessary, you can short one of the three 1N4148 diodes in the chain to get a lower adjustment range. In general, Switch-on should be set at a voltage where the motor accelerates well. Switch-off should occur when the motor is still just doing useful work. The prototypes handle a fairly steep incline and the rather substantial joints between my floor tiles.