you may drive the relay using the transistor alone. In my case I'm using 2N2222A and having no problem. In this case you can remove the two resistors. In any case, you need one more resistance between the Emitter and ground
You would use R2 or R3 (only need one of them) to make sure that small leakages in the OC1 don't leave the transistor partly on. It can be around 10k, it's not critical. R1 is used to limit the current in case OC1 is too powerful. Although the PC120 is unlikely to need current limiting, R1 is still a good idea.
D1 is required to catch the back EMF from the relay when you want to turn it off.
You should not add a resistance between Emitter and ground.
In this case (omiting R2) doesn't that R1 & R3 act as a voltage divider?ratio will be [12x10]/14.7.will this base voltage burn the transistor?there is no current limiting!
In this case (omiting R2) doesn't that R1 & R3 act as a voltage divider?ratio will be [12x10]/14.7.will this base voltage burn the transistor?there is no current limiting!