First of all, please excuse my complete lack of knowledge in electronics :)
Let's start with a little bit of context. I am trying to modify a machine that already uses a photo-interrupter to measure distance and speed. My goal is to be able to 'read' the state of the photo-interrupter while letting the original circuit work as intended.
Here is the original circuit, simplified to the extreme. Basically, 'Input' is directly fed to a PIC micro-controller. To be completely honest, I haven't been able to figure out much more by staring at its PCB...
My first thought was to just plug both circuits inputs to the collector pin of the photo-interrupter. I would then power the photo transistor by either power source (both 3.3V) and connect grounds. My circuit's 'input' is an input pit of a micro-controller as well.
Another possibility I thought of: using a emitter follower circuit so that, if I understand correctly, both circuits would be able to draw current from their respective power sources. Only issue I have is the voltage drop due to the emitter follower.
Experimentally, both solutions seem to more or less work, but given my lack of knowledge in the field, I would very much appreciate any comment on what is the 'right' solution.
Should I completely isolate both circuits ? With an optocoupler, for example.
What are the shortcomings of my simplified solutions ?