Two advantages:
1) At the receiver, the amplifier can be AC-coupled so you don't have to worry about DC offset.
2) The receiver won't be fooled by the ambient light level.
Now, for the high pass filter, should a normal R-C circuit work ?I'd recommend one small change to the basic plan - modulate the LED / laser at a few KHz. This can be as simple as driving it from the output of a 555 oscillator. Then at the receiver, detect that frequency, rather than the average light level. Pick a frequency of at least a few KHz and use a high-pass filter in the receiver so it doesn't get fooled e.g. by flickering fluorescent lights.
Not just you, it really is bizarre.That whole input cct looks a bit bizarre.
Of course it may just be me :0)
From what the OP's previously writtrn, I suspect "K1" is supposed to represent a relay, and I suspect he hopes the circuit will work as follows:
a) When a beam of light falls on the photodiode, it produces current which is used to activate the relay.
b) When the beam of light is broken, the photodiode switches off and the relay contact opens, triggering the 555, which sounds an alarm.
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