You only need ONE transistor. A BC547 will do, but there are 1000+ others that also will work.
Collector to Arduino pin, emitter to ground, base to the diodes.
Eight NPN transistors would be the best solution.
NO extra parts needed.
To explain further, the bases are held low via the internal ~1k5 resistor of the PIR (when inactive).
No external resistors needed.
I have built this circuit with the exact same transistor however I have noticed some issues with it. First of all if I connect more than 2 the same way the circuit remains on.
Secondly over the long run (multiple weeks) even with 2 sensors the circuit can go into perma on or off no idea why exactly and it's kinda hard to debug it at this point.
I will trash this whole thing as this is a bad idea, clearly something is wrong with this design.
The reason why I chose this is because it required less wiring, with 3 wire you could basically chain all the sensors instead of pulling 3 x sensor to the Arduino but I regret not going on the normal way.
I just want to understand what was the problem with it. Also I would recommend removing those forum posts as the circuits are not functioning reliably and misleading others.
There are no different sensors here, it's exactly the same. I using standalone PIR sensors with Arduino, Raspberry PI, ESP since many years they run reliably. Nothing like this ever happened.