I have tried to build an IR emitter/detector circuit but it does seem like it is working. I have used a matched pair of IR emitter/detector from radio shack Catalog #: 276-142. I have tried two methods to do this.

1) I have used a simple circuit similiar to Andrew's attached here. I connected a 5v supply to an IR emitter in series with 220 ohm resistor for the emitter side.
For the IR detector I have connected the collector side to a 5V supply in series with a 470 ohm resistor. The emitter side of the IR detector is grounded. I have connected basic stamp I/O pin between the 470 ohm resistor and the collector of the IR detector. When the I/O pin is low meaning the IR detector is turned a tested alarm system connected to the basic stamp should not go off. Once an object comes between the emitter and detector the I/O pin should go high and set off an alarm.
The problem I have is the detector never gets turned on when the emitter side is turned on. I measure the voltage across the emitter to 1.3V which is the turn on voltage of the LED. The voltage across the collector and emitter of the IR detector is always at 5V meaning that the detector never gets turned on.

2) The second method I have tried (but in vain) is using 555 timer to produce a modulated square wave signal at about 30 Khz. The output of the timer goes to the emitter. One problem I have seen is once I connect the IR emitter to the output of the timer the output voltage seen with the Oscope goes down to 1V and it gets a little distorted. I finally got a good signal out but does not seem to turn on the IR detector.

Any where I searched seem to indicate that the first method should work at least in doors with less ambient light.

I appreciate any help.
Deq

Are you going to use an interupter of some kind between the two to derive a pulse or do you wish to just turn the LED on and see a resultant effect on the photo transistor thru the I/O pin of the basic stamp?

A few suggestions:
Breadboard the circuit using a LM7555 Cmos version of the 555,(less power consumption). Then for the I/O side,use a LED for testing purposes,(detector side),use a 1K resistor pot instead of the 470 ohm and slowly reduce it till you see a response,but mark the point to which is 470 ohm so that you do not take it lower,use an OHM Meter). Also place the IR Emitter and the IR Photo Transistor in a short small diameter tube pointing at eachother to reduce the ambient light effects.
The matched pairs that Radio Shack sells are not really matched as they say. I have had to make adjustments in order to get them to work consistantly or at all,they were usually about 100-200 Ohms off on the collector side of the detector.