How Does A Pull Up Resistor In Parallel Work?
The Red wire powers the receiver (Rx) which drives the base of an open collector NPN to ground out on the White wire when the IR path is blocked.

Then the output goes open circuit when the beam is connected again.  Thus an active edge going low is the leading edge of the blocked IR state.

Putting a resistor in series with the supply would cause the internal PhotoDiode (PD) transimpedance amplifer (TIA using an Op Amp) to reduce Vcc and the open collector would then have no pullup.

Don't worry the Sparkfun site doesn't explain it very well.







simulate this circuit &ndash; Schematic created using CircuitLab