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the simplest way to do it would be to use a transistor with the photodiode attahed to the base of the transistor. if the motor is small it may be ale to be run only one transistor but if the motor is quite large it may pay to use a second transistor that is controlled by the first one.
if you need a schematic i may be able to draw one up for you.
Be careful when switching a motor (or other inductive load) this way - put a flyback diode across the motor, otherwise, when you switch it off, the motor can generate a high voltage (momentarily), and the transistor can be destroyed.
I had a similar issue in a friend's car the other day - his in-car electronic thermometer was resetting itself every time he used his air horn (his new toy...). A 1N4004 across the horn solenoid fixed it 8)