Re: pic transistor
Transistors (working as a switch) are rated for the maximum collector current, for example, the BC141 Icmax-1A (another parameter is the collector-emitter voltage but in low-voltage applications this rating is not very important) ..
In you case, if you want a transistor to drive a relay, first you have to learn what is the relay’s coil op. current (or coil resistance), then select a transistor that can do it (with a safety margin) ..
For example, if the coils’s current is 30mA you can select a transistor with ICmax>300mA (safety margin) ..
So, 2N2222, BC337, BC338, BC639, ZTX-series … and so on .. should do ..
Rgds,
IanP