I think the infrared components are DC components.
The absolute most easiest way you can make a circuit is to do the following:
Hook a super-bright LED (look for a high MCD rating), or better yet, a laser module to a resistor and then to a DC power source. Don't burn the laser or LED. You should have a continuous beam of light.
Now hook up a CDS cell to another power source, an inverter and an indicator all in series.
When the box passes through, the indicator is turned on, provided that you beamed the laser on the cell.
thanks. but my fear with cds circuit is that it could be responding to abient light. I will try it out anyway.
secondly, i think infra-red is A.C. if u use LED to detect infra red u'll see the led blinking
a CDS cell is a photoresistor.