I've built a simple little circuit for controlling the run time of a motor. It's based on a 555 timer chip in monostable mode — which means when you press the button, it runs the motor for as long as you set with the knob.
It's can currently range from ≈ 1sec to ≈ 12secs. Modifying this range is as simple as changing a capacitor or resistor. For example, doubling the size of the capacitor would result in a range of ≈ 2-24secs.
The idea is to use this on a model aircraft instead of rubber band power - which can be a bit of a faff.
I'm not sure about the motor yet - it's just been scored from a CD-ROM drive and the power supply here is a 9V battery - which is quite heavy. Happily, the circuit will accept voltages from 2-18V so there's plenty of scope for experimentation without having to alter the circuit.
I translated the circuit from a breadboard test to a stripboard layout and used Pages on the Mac to draw a template. I then printed and stuck the template onto the stripboard which gives a really nice reference as you're adding components. This diagram is version 2 which ironed out a few layout issues.
If it actually gets strapped to a model plane… you'll see it here. :)