An astable multivibrator controls a Darlington amplifier to drive a small pager motor. The gently rising and falling motor speed simulates breathing or other natural undulations. The addition of other control stages could add complexity, if that's what you want.
Circuit design by Kelly Heaton, 2014. If you try to build this and it does not work, I'm sorry… I am more artist than engineer. I sometimes struggle to reproduce my own circuit diagrams, but I have learned that tinkering with frustrating electronics is a great way to make new art.
This analog circuit is comprised of two parts: an astable multivibrator that creates a low-frequency square wave (left side) to switch on/off a Darlington amplifier (right side) that drives the pager motor. I have added various capacitors and a resistor to the amplifier stage. These affect the quality of the motor speed, making it turn on and off more naturally. The video shows me removing / replacing the pull-up resistor on the emitter of the second NPN transistor, illustrating its function (to provide a baseline motor speed so that the motor does not come to a complete stop, but instead undulates between a faster and a slower speed).