> 3-terminal receivers like the Sharp ISU60 are tuned to 38kHz and have
> a built-in daylight filter. A continuous 38kHz stream of IR pulses will
> give a continuous low output. In the absence of a signal the output is
> high. I use this system for a broken beam application.

This is not true for TSOP and SFH 3-pin receivers: a continuous signal
rises the comparison treshold so eventually the output signal will be
noise. Hence a code like RC4 is needed that contains gaps. TSOP and SFH
receivers come in various versions (last 2 digits of the name), ranging
from 30 to 56 kHz.

Wouter

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