The dispenser consists of:

- a pump of an ink-printer:

- a geared motor, for driving the pump:

- a plastic bottle, for holding soap:

- a light barrier of a copy machine, it got cut and reconnected with wire:

- two transistors and three resistors, switching the motor and triggered by the sensor:

- a 5V voltage regulator (7805):

- a 12V sealed lead acid battery (had been used in an alarmsystem):

...and some hot glue.

Full view:

Energy consumption:

- 5mA for the transistor circuit

- 5mA for the voltage regulator (the heatsink isn't necessary)

- 10mA ir-LED

 This would work well with a small solar panel. Also could I strobe the irLED for saving energy.