Connect LM311 pin 7 to input pin Arduino
The LM311 in your schematic is configured as a open collector output. It will sink current but not source it. This actually makes it very easy to communicate with a system that runs on a different voltage.







simulate this circuit &ndash; Schematic created using CircuitLab

You can omit R1 if you enable the pull-up resistor on the input in software.
Both circuits could probably be ran off a single 5V power supply if this suits your needs better.

I'd recommend adding a 100nF decoupling capacitor on the LM311's power pins, as close as possible to the IC. And perhaps another 100nF capacitor on the TSOP1738's power pins as it is actually a complex IC, not a simple detector.