Hi!
You should try to make a node with the TCRT5000 IR Barrier Line Track sensor
It emits an infrared light and detects the reflection.
Its used in the water meter: http://www.mysensors.org/build/pulse_water
http://rover.ebay.com/rover/1/711-53200-19255-0/1?icep_ff3=2&pub=5575069610&toolid=10001&campid=5337433187&customid=&icep_item=330924700585&ipn=psmain&icep_vectorid=229466&kwid=902099&mtid=824&kw=lg