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note that the human body is a good absorber of stray RF fields, so this sensor should be a good people-detector

The Nv neuron circuit may look familiar as it is found in most CMOS data handbooks as an example of a simple edge detector or one-shot or mono-stable circuit application.

The current through a photodiode is directly proportional to the light intensity

The photodiode and phototransistor can be both photovoltaic (generators of potential difference) and photoconductive (modifiers of an electric current), depending on the application.

A reverse-biased photodiode operates in what is called photoconductive mode, since the conduction of the semiconductor junction varies with the illuminating light intensity.  If the reverse-biased voltage is relatively large (i.e. several volts) the reverse-biased photodiode will have a very fast response time (much faster than an LDR) and is suitable for detecting light signals that vary down to a time scale of a fraction of a microsecond.