When using the TSOP1738 IR Receiver Module in a Beam Break application it is necessary to interrupt the 38KHz signal being generated by the IR Emitter.
An example is a 'People In/Out Counter', where the beam break is used to detect a person entering or leaving a room.

If the TSOP detects a continuous 38KHz pulse if will consider it as a interference source and will not work as expected.
[refer to the attached extract from the TSOP datasheet]

The attached circuit transmits at 38KHz, with a break in the transmission at approx 650Hz.

This gives a group of ~45 IR pulses, with a gap equal to ~ 20 pulses.

The circuit shows two options for the LED IR driver.