Nor gate using photodiodes and transistors doesn't work correctly
I've drawn your original receiver stage on the left, and an improved one on the right.

A photodiode generates current proportional to the light impinging on it, plus a bit of leakage current (called "dark current" in the datasheet).  A transistor will pretty much amplify any base current -- so the circuit on the left will always have some collector current, and in the case of a photodiode that always has some light on it, it will be difficult to control the on/off switch point.

In the circuit on the right, R5 will consume some of the current from D2.  This will hold the base voltage of Q2 down.  Then you can select R5 such that the base voltage of the transistor hits 0.6V at roughly the light level you want to sense.  With carefully controlled lighting, this may be enough to make things work for you.

Note that I suggest that if you use this circuit, you make R5 adjustable -- I'd suggest an audio taper pot, because those are roughly logarithmic (see my comment to this answer for my reason).







simulate this circuit &ndash; Schematic created using CircuitLab