I drive it with PWM (Pulse width modulation - skip the next bit if you know what this is)
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PWM is a method used to drive analog devices such as motors or lamps at a variable power from a digital source.
A digital signal is either on or off, so anything connected to a digital signal source will be either fully on or fully off.
This isn't very useful for many applications, as we often require variable power.
This is accomplished by flicking the digital signal on and off very rapidly, so that the attached device is only powered for part of the time.
By varying the ratio of on time to off time, the power (eg speed or brightness) may be varied.
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In the program, I can vary the PWM rate (and thus the speed) of the advance motor, and I can also vary the step rate of the rotation motor.
When I reach the end of the film, I reverse the polarity to the film advance, and apply full power. This automatically flips a gear into place inside the camera to rewind the film. (The wind/rewind changeover part was originally in the camera, so I just left it there and made use of it)