If I understand your intention with this project correctly, you want to emit an infrared beam in a continuous manner. Whenever the receiver does not detect the IR-beam you will interpret this as "object detected". Did I get it right?
The IR receiver module TSOP1738 is optimized for reception of a On-Off Key modulated IR signal with 38kHz carrier frequency. This does not mean the 38kHz can be continuously enabled, but only in bursts. After each burst the 38kHz pulses shall be off for a while to prevent the AGC (automatic gain control) in the receiver from saturation. The datasheet recommends each burst to be at least 10 pulses long, but also more pulses may be used per burst. In our case with 38kHz it means 1/38kHz= app 26.3us per pulse, then minimum 10 x 26.3us => minimum 263us/burst. The component test pattern described in the datasheet figure 8 uses app. 23 pulses per burst (600us). Of course this on-off keying also means the IR-receiver output will show a pulse train, rather than a conscious signal. An beam interrupt may be detected when the output has been low longer than the selected burst-cycle time. The decoding of this pulse train could then be done in many different ways:
watchdog circuit, timer functions in a microcontroller, Retriggerable Monostable Multivibrator etc.
Some more details about dimensioning of burst timing: Burst-off time shall be equally long as the burst-on time. If we add the burst-on time with the burst-off time we can call that the burst cycle. Depending of the shortest time you expect the beam to be interrupted, you can decide how short your burst cycles can be at the shortest. To be on the safe side (Nyquist–Shannon sampling theorem), make sure to make the burst-cycles shorter than the shortest possible beam interrupt you expect. For example, if you want to be able to detect beam interrupts as short as 1ms, then the bursts cycles must be shorter than 1ms, with <500us burst-on time and <500us burst-off time. I hope this helps.